Authors: Shun-Cai Zhao, Liang Luo, Ni-Ya Zhuang
Quantum batteries (QBs) have emerged as promising platforms for microscale energy storage, yet most existing studies assume weak system-environment coupling and Markovian dynamics. Here we explore how physical catalysis can regulate the maximum extractable work (ergotropy) of a spin-chain QB strongly coupled to a cavity environment. We model the system using a Nakajima-Zwanzig-type non-Markovian master equation and simulate the time evolution of ergotropy under various physical parameters. Our results show that increasing the catalyst-spin coupling, spin energy or cavity frequency can effectively suppress ergotropy oscillations and yield quasi-stationary ergotropy regime, while overly strong catalyst, especially when accompanied by increasing system-environment coupling under such conditions, can destabilize work extraction. This study demonstrates how quantum catalysis can serve as a control knob for optimizing battery performance in strongly coupled non-Markovian regimes.
Authors: Italo L. Soares Andrade, Kleber Pirota, Amir O. Caldeira, Francisco Rouxinol
We report the observation of the Purcell effect in a cavity-metallic magnet hybrid system using electric-field-mediated coupling. In this configuration, microwave-induced axial currents in the microwire induce circular magnetic fields that drive the ferromagnetic resonance (FMR) of the magnetized microwire. Field-dependent transmission and reflection spectroscopies reveal a clear cavity perturbation consistent with the Purcell regime, in which the magnetic loss rate exceeds the light-matter coupling strength. Despite the small magnetic volume ($\sim 10^{-13}\,\text{m}^3$), measurements performed at both room temperature and 7 mK show coupling rates as high as 56 MHz, one order of magnitude stronger than expected from conventional coupling at the magnetic antinode. Time-domain ringdown measurements directly show the magnetic-field-dependent modification of the cavity photon lifetime, in agreement with theoretical predictions. These results establish a versatile approach for coupling microwave fields to metallic magnets via geometric and electric-field-mediated interactions, opening new opportunities for hybrid cavity-magnet systems.
Authors: Rahul Gupta, Huaiyang Yuan, Himadri Shekhar Dhar
We present a scheme for generating stable and tunable entanglement between two localized Bloch domain walls in nanomagnetic strips kept inside a chiral optical cavity. The entanglement is mediated by the effective optomechanical interaction between the cavity photons and the two macroscopic, collective modes of the pinned domain walls. By controlling the pinning potential and optical driving frequency, the robust, steady-state entanglement between the two macroscopic domain walls can survive beyond the typical milli-Kelvin temperature range.
Authors: Phoenix M. M. Paing
By performing up to the second-order Magnus expansion on the Jaynes-Cummings Model Hamiltonian governing the interaction of a two-level atom and a single cavity mode electromagnetic field, we find the term involving the squeezing of light beyond the Rotating Wave Approximation. Our results demonstrate that the Magnus expansion provides a systematic framework to capture atom-induced squeezing effects. These findings suggest new avenues for engineering nonclassical states in simple light-matter systems without requiring multi-level structures or strong nonlinear media.
Authors: Pritam Chattopadhyay, Avijit Misra, Saikat Sur, David Petrosyan, Gershon Kurizki
We explore the distribution in space and time of a single-photon excitation shared by a network of dipole-dipole interacting atoms that are also coupled to a common photonic field mode. Time-averaged distributions reveal partial trapping of the excitation near the initially excited atom. This trapping is associated with resonances of the excitation at crossing points of the photon-dressed energy eigenvalues of the network. The predicted photon-induced many-atom trapped excitation (PIMATE) is sensitive to atomic position disorder which broadens the excitation resonances and transforms them to avoided crossings. PIMATE is shown to allow highly effective and accurate sensing of multi-atom networks and their disorder.
Authors: Kosei Hatakeyama, Daisuke Miki, Kazuhiro Yamamoto
We investigate the advantage of using squeezed input light for generating gravity-induced entanglement (GIE) through Fourier-domain analysis. Based on the findings of [1], which demonstrated the feasibility of detecting GIE in optomechanical systems under quantum control, we further demonstrate that squeezed input light can reduce the optical noise in the mechanical conditional state and enhance GIE. Furthermore, we estimate the systematic and statistical errors in the measurement of GIE using the Fourier transformation over a finite measurement time. Based on the error estimations using the signal-to-noise ratio (SNR) in GIE detection, we find that a total measurement time of 10^6 s is required to achieve SNR=1 when using squeezed input light, whereas 10^{6.8} s is needed without squeezed input light. This result highlights the effectiveness of optomechanical systems and the critical role of squeezed input light in enhancing the detectability of GIE.
Authors: Giacomo Sorelli, Manuel Gessner, Frank Schlawin
Coherent anti-Stokes Raman scattering is a widely used imaging technique that provides chemical contrast without the need for labels, making it an extremely valuable tool in physics, chemistry, and biology. In this work, we explore its fundamental precision limits by applying tools from quantum information theory. We identify optimal measurement strategies and show that spatial mode demultiplexing--a technique already accessible in current experimental setups--can achieve these quantum limits and in many situations improve the sensitivity of conventional intensity measurements. Building on this, we introduce an advanced imaging scheme based on vortex beams, which we predict to enhance the image information in the final quantum state of light and thereby lead to even higher resolution and sensitivity. These findings establish a clear path for enhancing nonlinear imaging techniques using concepts from quantum science, bridging the gap between established microscopy methods and the emerging capabilities of quantum technologies.
Authors: Erin Sheridan, Nicholas J. Barton, Richard Birrittella, Vedansh Nehra, Zachary Smith, Christopher Tison, Amos Matthew Smith, Shashank Dharanibalan, Vijit Bedi, David Hucul, Benjamin Kyle, Christpher Nadeau, Mary Draper, John Heinig, Scott Faulkner, Randal Scales, Andrew M. Brownell, Stefan Preble, James Schneeloch, Samuel Schwab, Daniel Campbell, Derrick Sica, Peter Ricci, Vladimir Nikulin, John Malowicki, Jacob Hall, Michael Fanto, Matthew D. LaHaye, Laura Wessing, Paul M. Alsing, Kathy-Anne Soderberg, Donald Telesca
As quantum computing, sensing, timing, and networking technologies mature, quantum network testbeds are being deployed across the United States and around the world. To support the Air Force Research Laboratory (AFRL)'s mission of building heterogeneous quantum networks, we report on the development of Quantum Local Area Networks (QLANs) operating at telecommunications-band frequencies. The multi-node, reconfigurable QLANs include deployed optical fiber and free-space links connected to pristine laboratory environments and rugged outdoor test facilities. Each QLAN is tailored to distinct operating conditions and use cases, with unique environmental characteristics and capabilities. We present network topologies and in-depth link characterization data for three such networks. Using photonic integrated circuit-based sources of entangled photons, we demonstrate entanglement distribution of time-energy Bell states across deployed fiber in a wooded environment. The high quality of the entanglement is confirmed by a Clauser-Horne-Shimony-Holt inequality violation of $S=2.700$, approaching the theoretical maximum of $S=2.828$. We conclude with a discussion of future work aimed at expanding QLAN functionality and enabling entanglement distribution between heterogeneous matter-based quantum systems, including superconducting qubits and trapped ions. These results underscore the practical viability of field-deployable, qubit-agnostic quantum network infrastructure.
Authors: Joseph D. Broz, Jesse C. Hoke, Edwin Acuna, Jason R. Petta
In conventional exchange-only (EO) spin qubit demonstrations, quantum gates have been implemented using sequences of individually pulsed pairwise exchange interactions with only one exchange coupling active at a time. Alternatively, multiple non-commuting exchange interactions can be pulsed simultaneously, reducing circuit depths and providing protection against leakage. We demonstrate high-fidelity quantum control of an always-on exchange-only (AEON) qubit, operated using simultaneous exchange pulses in a triangular quantum dot (QD) array. We use blind randomized benchmarking to characterize the performance of the full AEON single-qubit Clifford gate set, achieving an average Clifford gate fidelity $F_{\rm C1}$ = 99.86\%. Extensions of this work may enable more efficient EO two-qubit entangling gates as well as the implementation of native $i$-Toffoli gates in Loss-DiVincenzo single-spin qubits.
Authors: Himanshu Patange, Kyrylo Gerashchenko, Rémi Rousseau, Paul Manset, Léo Balembois, Thibault Capelle, Samuel Deléglise, Thibaut Jacqmin
We report a two-stage, heterodyne rf-to-microwave transducer that combines a tunable electrostatic pre-amplifier with a superconducting electromechanical cavity. A metalized Si$_3$N$_4$ membrane (3 MHz frequency) forms the movable plate of a vacuum-gap capacitor in a microwave LC resonator. A dc bias across the gap converts any small rf signal into a resonant electrostatic force proportional to the bias, providing a voltage-controlled gain that multiplies the cavity's intrinsic electromechanical gain. In a flip-chip device with a 1.5 $\mathrm{\mu}$m gap operated at 10 mK we observe dc-tunable anti-spring shifts, and rf-to-microwave transduction at 49 V bias, achieving a charge sensitivity of 87 $\mathrm{\mu}$e/$\sqrt{\mathrm{Hz}}$ (0.9 nV/$\sqrt{\mathrm{Hz}}$). Extrapolation to sub-micron gaps and state-of-the-art $Q>10^8$ membrane resonators predicts sub-200 fV/$\sqrt{\mathrm{Hz}}$ sensitivity, establishing dc-biased electromechanics as a practical route towards quantum-grade rf electrometers and low-noise modular heterodyne links for superconducting microwave circuits and charge or voltage sensing.
Authors: Kosei Hatakeyama, Daisuke Miki, Yamamoto Kazuhiro
We investigate the advantage of using squeezed input light for generating gravity-induced entanglement (GIE) through Fourier-domain analysis. Based on the findings of [1], which demonstrated the feasibility of detecting GIE in optomechanical systems under quantum control, we further demonstrate that squeezed input light can reduce the optical noise in the mechanical conditional state and enhance GIE. Furthermore, we estimate the systematic and statistical errors in the measurement of GIE using the Fourier transformation over a finite measurement time. Based on the error estimations using the signal-to-noise ratio (SNR) in GIE detection, we find that a total measurement time of 10^6 s is required to achieve SNR=1 when using squeezed input light, whereas 10^{6.8} s is needed without squeezed input light. This result highlights the effectiveness of optomechanical systems and the critical role of squeezed input light in enhancing the detectability of GIE.
Authors: Nasser Gohari Kamel, Arsalan Mansourzadeh, Ujjwal Gautam, Vinaya Kumar Kavatamane, Ashutosh Singh, Edith Yeung, David B. Northeast, Paul Barclay, Philip J. Poole, Dan Dalacu, Daniel Oblak
The realization of long-distance quantum communication and the envisioned quantum internet relies on coherent hybrid light-matter interfaces connecting quantum light emitters with quantum memory (QM) systems. Unlike probabilistic photon pair sources such as spontaneous parametric down-conversion, deterministic quantum light emitters enable the on-demand production of pure and bright single and entangled photons, essential for scalable quantum networks. In this work, we present the first experimental realization of a coherent hybrid light-matter interface between a chip-integrated InAsP/InP nanowire quantum dot (QD) and a solid-state QM based on Er$^{3+}$ ions doped in a glass silica fiber (erbium-doped fiber, EDF). The emission spectrum of the InAsP/InP nanowire QD aligns with the absorption bandwidth of the EDF at 980 nm at cryogenic temperatures, allowing efficient interaction between the two systems. To demonstrate this, we present a spectroscopic characterization of the $^{4}I_{15/2} \leftrightarrow ^{4}I_{11/2}$ optical transition in EDF at 980 nm. Our measurements reveal substantial inhomogeneous broadening of this optical transition and a long spin population lifetime, underscoring EDFs potential for broadband QM implementation. We implement an 8 GHz bandwidth multimode QM based on the Atomic Frequency Comb protocol, enabling the storage and retrieval of 59 weak coherent pulses. Furthermore, we characterize single-photon emission from an InAsP/InP nanowire QD at 980 nm and demonstrate its deterministic storage and recall in the EDF QM. Notably, this is achieved without spectral tuning of the QD emission, demonstrating its direct compatibility with a solid-state QM.
Authors: Priyanka M. Kannath, Girish S. Agarwal, Ashok Kumar
Recent developments in quantum technologies have enabled significant improvements in the precision of optical sensing systems. This work explores the integration of distributed quantum sensing (DQS) with optical gyroscopes to improve the estimation accuracy of angular velocity. Utilizing bright two-mode squeezed states (bTMSS), which offer high photon numbers and strong bipartite quantum correlations, we propose a novel configuration that leverages continuous-variable entanglement across multiple spatially separated optical gyroscopes. Unlike traditional quantum sensing that enhances a single sensor, our approach focuses on estimating a global phase shift corresponding to the average angular rotation across distributed optical gyroscopes with quantum-enhanced sensitivity. We analyze the phase sensitivities of different bTMSS configurations, including M mode-entangled bTMSS and separable M-bTMSS, and evaluate their performance through the quantum Cramér-Rao bound. The analysis shows that, with 5% photon loss in every channel in the system, the proposed scheme shows a sensitivity enhancement of ~9.3 dB beyond the shot-noise limit, with an initial squeezing of ~9.8 dB. The present scheme has potential applications in quantum-enhanced inertial navigation and precision metrology within emerging quantum networks.
Authors: Haowei Shi, Quntao Zhuang
Optical frequency combs, named for their comb-like peaks in the spectrum, are essential for various sensing applications. As the technology develops, its performance has reached the standard quantum limit dictated by the quantum fluctuations of coherent light field. Quantum combs, with their quantum fluctuation engineered via squeezing and entanglement, are the necessary ingredient for overcoming such limits. We develop the theory for designing and analyzing quantum combs, focusing on dual-comb interferometric measurement. Our analyses cover both squeezed and entangled quantum combs with division receivers and heterodyne receivers, leading to four protocols with quantum advantages scalable with squeezing/entanglement strength. In the spectroscopy of a single absorption line, whereas the division receiver with the squeezed comb suffers from amplified thermal noise, the other three protocols demonstrate a surprising robustness to loss at a few comb lines. Such a unique loss-robustness of a scalable quantum advantage has not been found in any traditional quantum sensing protocols.
Authors: J. Rivera-Dean, P. Stammer, C. Figueira de Morisson Faria, M. Lewenstein
Above-threshold ionization (ATI) is a strong-field-driven process where electrons absorb more photons than required for ionization. While ATI dynamics and outputs are well-understood when driven by classical, perfectly coherent light, the recent development of non-classical light sources for strong-field phenomena has spurred interest in their effect on the involved electron dynamics. In this work, we present a microscopic quantum optical theory describing ATI under the influence of strong squeezed light. We observe that squeezed light significantly enhances the coupling between light and matter, making their mutual backaction more important than under classical driving. This backaction profoundly impacts the electronic ionization times, as well as the non-classical properties of the joint electron-light state. This results in pronounced entanglement features, both immediately after ionization, and at later times. These entanglement features are reflected in the properties of the quantum optical state of the driving field revealing notable non-Gaussian features that depend on both, the amount of squeezing, and the number of ionization events occurring during the interaction.
Authors: Fazilah Nothlawala, Kiki Dekkers, Moslem Mahdavifar, Jonathan Leach, Andrew Forbes, Isaac Nape
Orbital angular momentum (OAM) as both classical and quantum states of light has proven essential in numerous applications, from high-capacity information transfer to enhanced precision and accuracy in metrology. Here, we extend OAM metrology to relativistic scenarios to determine the Lorentz factor of a moving reference frame, exploiting the fact that OAM is not Lorentz invariant. Using OAM correlations of entangled states, we show that their joint OAM spectrum is modified by length contraction, where the rescaling of spatial dimensions alters the orthogonality of the OAM modes themselves. In an emulated experiment, we confirm the predicted broadening of the OAM spectrum and use this to quantitatively infer the Lorentz (contraction) factor, reaching experimentally simulated velocities of up to 0.99c. Our work provides a pathway for novel measurement techniques suitable for relativistic conditions that leverages OAM structured light as a resource.
Authors: Edwin Tham, Min Ye, Ilia Khait, John Gamble, Nicolas Delfosse
We propose an architecture for a quantum memory distributed over a $2 \times L$ array of modules equipped with a cyclic shift implemented via flying qubits. The logical information is distributed across the first row of $L$ modules and quantum error correction is executed using ancilla modules on the second row equipped with a cyclic shift. This work proves that quantum LDPC codes such as BB codes can maintain their performance in a distributed setting while using solely one simple connector: a cyclic shift. We propose two strategies to perform quantum error correction on a $2 \times L$ module array: (i) The cyclic layout which applies to any stabilizer codes, whereas previous results for qubit arrays are limited to CSS codes. (ii) The sparse cyclic layout, specific to bivariate bicycle (BB) codes. For the $[[144,12,12]]$ BB code, using the sparse cyclic layout we obtain a quantum memory with $12$ logical qubits distributed over $12$ modules, containing $12$ physical qubits each. We propose physical implementations of this architecture using flying qubits, that can be faithfully transported, and include qubits encoded in ions, neutral atoms, electrons or photons. We performed numerical simulations when modules are long ion chains and when modules are single-qubit arrays of ions showing that the distributed BB code achieves a logical error rate below $2 \cdot 10^{-6}$ when the physical error rate is $10^{-3}$.
Authors: Dong-Hyun Kim, Seongjin Hong, Yong-Su Kim, Kyunghwan Oh, Su-Yong Lee, Changhyoup Lee, Hyang-Tag Lim
Distributed quantum sensing, which estimates a global parameter across distant nodes, has attracted significant interest for applications such as quantum imaging, sensor networks, and global-scale clock synchronization. $N00N$ states are regarded as one of the optimal quantum resources for quantum metrology, enabling the Heisenberg scaling. Recently, the concept of $N00N$ states has been extended to multi-mode $N00N$ states for quantum-enhanced multiple-parameter estimation. However, the application of multi-mode $N00N$ states in distributed quantum sensing remains unexplored. Here, we propose a distributed quantum sensing scheme that achieves the Heisenberg scaling using multi-mode $N00N$ states. We theoretically show that multi-mode $N00N$ states can reach the Heisenberg scaling by examining both the Cramér-Rao bound and the quantum Cramér-Rao bound. For experimental demonstration, we employ a four-mode $2002$ state to estimate the average of two spatially distributed phases, achieving a 2.74 dB sensitivity enhancement over the standard quantum limit. We believe that utilizing multi-mode $N00N$ states for distributed quantum sensing offers a promising approach for developing entanglement-enhanced sensor networks.
Authors: Francesco Troisi, Hannes Hübener, Angel Rubio, Simone Latini
We propose an all-optical Moiré-like exciton confinement by means of spatially periodic optical cavities. Such periodic photonic structures can control the material properties by coupling the matter excitations to the confined photons and their quantum fluctuations. We develop a low energy non-perturbative quantum electro-dynamical description of strongly coupled excitons and photons at finite momentum transfer. We find that in the classical limit of a laser driven cavity the induced optical confinement directly emulates Moiré physics. In a dark cavity instead, the sole presence of quantum fluctuations of light generates a sizable renormalization of the excitonic bands and effective mass. We attribute these effects to long-range cavity-mediated exciton-exciton interactions which can only be captured in a non-perturbative treatment. With these findings we propose spatially structured cavities as a promising avenue for cavity material engineering.
Authors: Smitarani Mishra, Shaon Sahoo
We revisit the J1-J2 frustrated Heisenberg spin-1/2 chain with dimerization ({\delta}) or modulation in the nearest-neighbor couplings to investigate its thermalization behavior. While the dimerization tends to induce localization, the next-nearest-neighbor interaction J2 generally favors thermalization, making the assessment of the model's compliance with the Eigenstate Thermalization Hypothesis (ETH) particularly subtle. The challenge is further compounded by the model's SU(2) symmetry; the study of ETH compliance is necessarily done for each symmetry sector but separating different sectors of this symmetry is known to be a computationally demanding task. The current study is driven by two main motivations: first, to explore whether the well-known ground-state phases of the model have any bearing on its thermalization properties; and second, to understand how the interplay between two competing factors, namely, the non-uniformity (via {\delta}) and the beyond-nearest-neighbor interactions (via J2) governs the system's approach to thermal equilibrium. A systematic analysis shows that the ETH is most strongly satisfied for intermediate values of {\delta} (~ 0.5) with J2 ranging from intermediate (~ 0.5) to large (~ 1)- a parameter regime falls within the spiral ground-state phase. It is also found that when the system is in the gapless ground-state phase (which falls within the N'eel phase), the ETH is more prone to violation. In the regime of large {\delta} and small J2, the system is seen to enter a localized phase (characterized here by modulation in density-of-states; assessing ETH compliance is less meaningful for this phase.
Authors: Hui-Yu Yang, Hai-Long Shi, Qing-Kun Wan, Kun Zhang, Xiao-Hui Wang, Wen-Li Yang
The Tavis-Cummings (TC) model, which serves as a natural physical realization of a quantum battery, comprises $N_b$ atoms as battery cells that collectively interact with a shared photon field, functioning as the charger, initially containing $n_0$ photons. In this study, we introduce the invariant subspace method to effectively represent the quantum dynamics of the TC battery. Our findings indicate that in the limiting case of $n_0\!\gg\! N_b$ or $N_b\!\gg\! n_0$, a distinct SU(2) symmetry emerges in the dynamics, thereby ensuring the realization of optimal energy storage. We also establish a negative relationship between the battery-charger entanglement and the energy storage capacity. As a result, we demonstrate that the asymptotically optimal energy storage can be achieved in the scenario where $N_b\!=\!n_0\!\gg\! 1$. Our approach not only enhances our comprehension of the algebraic structure inherent in the TC model but also contributes to the broader theoretical framework of quantum batteries. Furthermore, it provides crucial insights into the relation between energy transfer and quantum correlations.
Authors: Ya-Jun Wang, Jun Zhang, Zheng-Yuan Zhang, Shi-Yao Shao, Qing Li, Han-Chao Chen, Yu Ma, Tian-Yu Han, Qi-Feng Wang, Jia-Dou Nan, Yi-Ming Yin, Dong-Yang Zhu, Qiao-Qiao Fang, Chao Yu, Xin Liu, Guang-Can Guo, Bang Liu, Li-Hua Zhang, Dong-Sheng Ding, Bao-Sen Shi
The dynamical trajectory of a dissipative Rydberg many-body system could be flipped under a microwave field driving, displaying an enhanced sensitivity. This is because the intersection of the folded hysteresis trajectories exhibits a sharp peak near the phase transition, amplifying the response to small changes in the microwave field. Here, we demonstrate an experiment of enhanced metrology through flipping the hysteresis trajectory in a cold atomic system, displaying an approach to improve sensitivity near the gap-closing points. By measuring the intersection points of hysteresis trajectories versus Rabi frequency of the microwave field, we quantify the equivalent sensitivity to be 1.6(5) nV cm-1 Hz-1/2. The measurement is also dependent on the interaction time, optical depth and principal quantum number since the long-range interaction between Rydberg atoms could dramatically change the shape of hysteresis trajectories. The reported results suggest that flipping trajectory features in cold Rydberg many-body systems could advance sensing and metrology applications.
Authors: Panagiotis Dorlis, Nick E. Mavromatos, Sarben Sarkar, Sotirios-Neilos Vlachos
We propose, in (3+1)-dimensional spacetimes, a novel astrophysical source of squeezed graviton states, due to superradiant axionic clouds surrounding rotating (Kerr-type) black holes (BH). The microscopic origin of these axions is diverse, ranging from the Kalb-Ramond (model-independent) axions and compactification axions in string theory, to \cm contorted geometries exemplified by a totally antisymmetric component of torsion in Einstein-Cartan theory. The axion fields couple to chiral gauge and gravitational Chern-Simons (CS) anomaly terms in the effective gravitational actions. In the presence of a Kerr BH background, such axions lead, upon acquiring a mass, to superradiance and the production of pairs of entangled gravitons in a squeezed state. The specific microscopic origin of the axions is not important, provided they are massive. This multimode squeezed-graviton state is examined through a Takagi-like decomposition, used in quantum optics. In the effective action it is shown that squeezing effects associated with conventional general relativity (GR) dominate, by many orders of magnitude, the corresponding effects due to the CS gravitational anomaly terms. For a sufficiently long lifetime of the axionic cloud of the BH, we find that significant squeezing (quantified through the average number of gravitons with respect to the appropriate vacuum) can be produced from the GR effects. It is also demonstrated explicitly that the structure of the entangled states (when the latter are expressed in a left-right polarization basis) depends highly on whether the GR or the anomalous CS effects produce the entanglement.
Authors: Michael Davidovich
The Van Kampen method is used to calculate the Casimir force for two dielectric layers. Several terms of Lorentz oscillators are used in the permittivity model. A conductive dielectric (metal) with the Drude model is considered as a special case. The dependence of strength on thickness has a complex character with saturation at thicknesses of the order of 10 nm. At low thickness, the force density is proportional to the square of the thickness, but this is the case at low thicknesses, when the continuum model is no longer applicable. The correspondence between the method of the Casimir model and the Lorentz model is shown, as well as its applicability for an arbitrary configuration of layers and for a finite temperature.
Authors: H. Solki, Ali Motazedifard, M. H. Naderi, A. Youssefi, R. Roknizadeh
We propose an experimentally feasible optomechanical system (OMS) that is dispersively driven and operates in the reversed dissipation regime (RDR), where the mechanical damping rate far exceeds the cavity decay rate. We demonstrate that coherent, fast-time modulation of the driving laser frequency-on time scales longer than the mechanical decoherence time-allows for adiabatic elimination of the mechanical mode, resulting in strong parametric amplification of quantum vacuum fluctuations of the intracavity field. This mechanism, known as the parametric dynamical Casimir effect (parametric-DCE), leads to the generation of Casimir photons. In the dispersive RDR, we find that the total system Hamiltonian-including the DCE term-is intrinsically modified by a generalized optomechanical Kerr-type nonlinearity. This nonlinearity not only saturates the mean number of radiated Casimir photons on short time scales, even without dissipation, but also induces oscillatory behavior in their dynamics and quantum characteristics. Remarkably, the presence of the Kerr nonlinearity causes the generated DCE photons to exhibit nonclassical features, including sub-Poissonian statistics, negative Wigner function and quadrature squeezing which can be controlled by adjusting the system parameters. The proposed nonclassical microwave radiation source possesses the potential to be applied in quantum information processing, quantum computing as well as microwave quantum sensing.
Authors: Diego Fernández de la Pradilla, Esteban Moreno, Johannes Feist
Theoretical accounts of ultrastrongly coupled light-matter systems commonly assume that it arises from the interaction of an emitter with propagating photon modes supported by a structure, understanding photons as the excitations of the transverse electromagnetic field. This description discards the Coulomb interaction between the emitter and structure charges. Here, we show with a general argument based on electromagnetic constraints that the emitter-photon coupling strength is fundamentally limited. Accordingly, we conclude that the ultrastrong coupling regime cannot be reached with photons. Instead, it must originate from the Coulomb interactions between charges. A further corollary is that the so-called polarization self-energy term does not need to be included. We illustrate our claims by solving an analytical model of the paradigmatic case of an emitter next to a metallic nanosphere. These findings shed light on the fundamental processes underlying ultrastrong coupling, clarify the role of the polarization self-energy term and compel a reevaluation of previous literature.
Authors: Frieder Lindel, Carlos J. Sánchez Martínez, Johannes Feist, Francisco J. García-Vidal
Periodically structured surfaces (metasurfaces), i.e., periodic light, have evolved as a powerful tool for manipulating electromagnetic fields both in classical and quantum regimes. However, no general approach for quantizing the electromagnetic fields and treating quantum light-matter interactions in such structures exists. Here, we construct an ab initio few-mode quantization scheme for metasurface resonances based on macroscopic quantum electrodynamics. We use our approach to propose a framework for strong light-matter coupling in which collective excitations of periodic arrays of quantum emitters are strongly coupled to the light modes supported by the metasurface, leading to the formation of crystal polaritons. As a proof-of-principle example of their potential, we show that interactions between crystal polaritons can lead to an efficient and directional generation of entangled photon pairs.
Authors: Alberto Miguel-Torcal, Alejandro González-Tudela, F. J. García-Vidal, Antonio I. Fernández-Domínguez
In recent years, Born-Markov master equations based on tracing out the electromagnetic degrees of freedom have been extensively employed in the description of quantum optical phenomena originating from photon-mediated interactions in quantum emitter ensembles. The breakdown of these effective models, built on assumptions such as ensemble spectral homogeneity, an unstructured photonic density of states, and weak light-matter coupling, has also recently attracted considerable attention. Here, we investigate the accuracy of this well-established framework beyond the most conventional, and extensively explored, spontaneous emission configuration. Specifically, we consider a system comprising two coherently driven and detuned quantum emitters, embedded within a hybrid photonic-plasmonic cavity, formed by a metallic nanorod integrated into a high-refractive-index dielectric microresonator. The local density of photonic states in this structure exhibits a complex frequency dependence, making it a compelling platform for exploring photon-mediated interactions beyond the assumptions above. We benchmark this modeling approach for the quantum dynamics of the emitter pair against exact calculations based on a macroscopic field quantization formalism, providing an illustrative assessment of its validity in significantly structured and dispersive photonic environments. Our analysis reveals four distinct regimes of laser driving and frequency splitting that lead to markedly different levels of accuracy in the effective model.
Authors: Victor Gondret, Clothilde Lamirault, Rui Dias, Charlie Leprince, Christoph I. Westbrook, David Clément, Denis Boiron
We study the entanglement properties of two-mode bosonic Gaussian states based on their multi-mode counting statistics. We exploit the idea that measuring high-order correlations of particle numbers can reveal entanglement without making any assumptions about the coherence of the fields. We show that the two- and four-body number correlations are sufficient to fully characterize the entanglement of two-mode bosonic Gaussian states for which each mode exhibits a thermal distribution. In addition, we derive an entanglement witness based on two-body correlations alone. Our findings are of great importance because it becomes possible to reveal entanglement in a series of recent experiments.
Authors: Oleksandr Gamayun, Murtaza Ali Mir, Oleg Lychkovskiy, Zoran Ristivojevic
Quantum observables of generic many-body systems exhibit a universal pattern of growth in the Krylov space of operators. This pattern becomes particularly manifest in the Lanczos basis, where the evolution superoperator assumes the tridiagonal form. According to the universal operator growth hypothesis, the nonzero elements of the superoperator, known as Lanczos coefficients, grow asymptotically linearly. We introduce and explore broad families of Lanczos coefficients that are consistent with the universal operator growth and lead to the exactly solvable dynamics. Within these families, the subleading terms of asymptotic expansion of the Lanczos sequence can be controlled and fine-tuned to produce diverse dynamical patterns. For one of the families, the Krylov complexity is computed exactly.
Authors: Ya-Nan Lu, Dong Yuan, Yixuan Ma, Yan-Qing Liu, Si Jiang, Xiang-Qian Meng, Yi-Jie Xu, Xiu-Ying Chang, Chong Zu, Hong-Zheng Zhao, Dong-Ling Deng, Lu-Ming Duan, Pan-Yu Hou
Understanding and controlling non-equilibrium dynamics in quantum many-body systems is a fundamental challenge in modern physics, with profound implications for advancing quantum technologies. Typically, periodically driven systems in the absence of conservation laws thermalize to a featureless "infinite-temperature" state, erasing all memory of their initial conditions. However, this paradigm can break down through mechanisms such as integrability, many-body localization, quantum many-body scars, and Hilbert space fragmentation. Here, we report the experimental observation of dynamical freezing, a distinct mechanism of thermalization breakdown in driven systems, and demonstrate its application in quantum sensing using an ensemble of approximately $10^4$ interacting nitrogen-vacancy spins in diamond. By precisely controlling the driving frequency and detuning, we observe emergent long-lived spin magnetization and coherent oscillatory micromotions, persisting over timescales exceeding the interaction-limited coherence time ($T_2$) by more than an order of magnitude. Leveraging these unconventional dynamics, we develop a dynamical-freezing-enhanced ac magnetometry that extends optimal sensing times far beyond $T_2$, outperforming conventional dynamical decoupling magnetometry with a 4.3 dB sensitivity enhancement. Our results not only provide clear experimental observation of dynamical freezing -- a peculiar mechanism defying thermalization through emergent conservation laws -- but also establish a robust control method generally applicable to diverse physical platforms, with broad implications in quantum metrology and beyond.
Authors: Twesh Upadhyaya, Zacharie Van Herstraeten, Jack Davis, Oliver Hahn, Nikolaos Koukoulekidis, Ulysse Chabaud
Majorization theory is a powerful mathematical tool to compare the disorder in distributions, with wide-ranging applications in many fields including mathematics, physics, information theory, and economics. While majorization theory typically focuses on probability distributions, quasiprobability distributions provide a pivotal framework for advancing our understanding of quantum mechanics, quantum information, and signal processing. Here, we introduce a notion of majorization for continuous quasiprobability distributions over infinite measure spaces. Generalizing a seminal theorem by Hardy, Littlewood, and Pólya, we prove the equivalence of four definitions for both majorization and relative majorization in this setting. We give several applications of our results in the context of quantum resource theories, obtaining new families of resource monotones and no-goes for quantum state conversions. A prominent example we explore is the Wigner function in quantum optics. More generally, our results provide an extensive majorization framework for assessing the disorder of integrable functions over infinite measure spaces.
Authors: Heba A. Labib, Goksu Can Toga, J. K. Freericks, A. F. Kemper
Pump-probe spectroscopy is a powerful tool for probing response dynamics of quantum many-body systems in and out-of-equilibrium. Quantum computers have proved useful in simulating such experiments by exciting the system, evolving, and then measuring observables to first order, all in one setting. Here, we use this approach to investigate the mixed-field Ising model, where the longitudinal field plays the role of a confining potential that prohibits the spread of the excitations, spinons, or domain walls into space. We study the discrete bound states that arise from such a setting and their evolution under different quench dynamics by initially pumping the chain out of equilibrium and then probing various non-equal time correlation functions. Finally, we study false vacuum decay, where initially one expects unhindered propagation of the ground state, or true vacuum, bubbles into the lattice, but instead sees the emergence of Bloch oscillations that are directly the reason for the long-lived oscillations in this finite-size model. Our work sets the stage for simulating systems out-of-equilibrium on classical and quantum computers using pump-probe experiments without needing ancillary qubits.
Authors: Emma Hughes, William Munizzi, Prineha Narang
This work introduces a lightweight simulation framework for evaluating asynchronous entanglement distribution protocols under realistic error models. We focus on two contemporary protocols: sequential, where entanglement is established one node at a time, and parallel, where all nodes attempt to generate entanglement simultaneously. We evaluate the performance of each protocol using two key metrics: the fidelity of distributed entangled states, and the hashing rate, a measure of entanglement efficiency. These metrics are compared between both protocols across a range of network sizes and noise parameters. We demonstrate that the parallel protocol consistently outperforms the sequential, particularly in the hashing rate metric due to reduced runtime, suggesting that parallel protocols are a strong candidate for a realizable quantum Internet. Our framework offers an accessible and scalable tool for evaluating entanglement distribution strategies, by reducing the simulation of complex quantum processes to simple memory time calculations.
Authors: Arkadiusz Kobus, Rafał Demkowicz-Dobrzański
We show an explicit $N$-qubit protocol involving one-axis-twisted spin squeezed states, that allows for simultaneous phase and dephasing strength estimation with precision that asymptotically matches fundamental quantum metrological bounds. The relevance of the protocol goes beyond this particular model, since any uncorrelated noise quantum metrological model, that allows for at most constant asymptotic quantum enhancement, can be reduced to this problem via an appropriately tailored quantum error-correction procedure.
Authors: Clara Wassner, Jack Davis, Sacha Cerf, Ulysse Chabaud, Francesco Arzani
Full reconstruction of quantum states from measurement samples is often a prohibitively complex task, both in terms of the experimental setup and the scaling of the sample size with the system. This motivates the relatively easier task of certifying application-specific quantities using measurements that are not tomographically complete, i.e. that provide only partial information about the state related to the application of interest. Here, we focus on simplifying the measurements needed to certify non-Gaussianity in bosonic systems, a resource related to quantum advantage in various information processing tasks. We show that the statistics of a single quadrature measurement, corresponding to standard homodyne detection in quantum optics, can witness arbitrary degrees of non-Gaussianity as quantified by stellar rank. Our results are based on a version of Hudson's theorem for wavefunctions, proved in a companion paper [1], revealing that the zeros in a homodyne distribution are signatures of quantum non-Gaussianity and higher stellar ranks. The validity of our witnesses is supported by a technical result showing that sets of states with bounded energy and finite stellar rank are compact. We provide an analysis of sample complexity, noise robustness, and experimental prospects. Our work drastically simplifies the setup required to detect quantum non-Gaussianity in bosonic quantum states. and experimental prospects. Our work drastically simplifies the setup required to detect quantum non-Gaussianity in bosonic quantum states.
Authors: Mathieu Beau
We introduce a general framework for defining context-dependent time distributions in quantum systems using projective measurements. The time-of-flow (TF) distribution, derived from population transfer rates into a measurement subspace, yields a time--energy uncertainty relation of the form $\Delta \mathcal{T} \cdot \Delta H \geq \hbar / (6\sqrt{3}) \cdot \delta\theta$, where $\delta\theta$ quantifies net population transfer. This bound applies to arbitrary projectors under unitary dynamics and reveals that time uncertainty is inherently measurement-dependent. We demonstrate the framework with two applications: a general time-of-arrival (TOA)-energy uncertainty relation and a driven three-level system under detuned coherent driving. The TF framework unifies timing observables across spin, atomic, and matter-wave systems, and offers an experimentally accessible route to probing quantum timing in controlled measurements.
Authors: Hae Lim (1), Johannes E. Fröch (1), Christian M. Pluchar (1), Arka Majumdar (1 and 2), Sara L. Mouradian (1) ((1) Department of Electrical and Computer Engineering, The University of Washington at Seattle, (2) Department of Physics, The University of Washington at Seattle)
A scaled trapped-ion quantum computer will require efficient fluorescence collection across a large area. Here we propose and demonstrate a compact monolithically integrated system featuring a metalens fabricated on the backside of a surface ion trap. A 40$\times$100 $\mu$m aperture enables a simulated point-source collection efficiency of 0.91% and a measured point-source detection efficiency of 0.58%. Increasing the aperture area to 40$\times$600 $\mu$m boosts the simulated collection efficiency to 3.17%$-$comparable to that of a conventional objective with a numerical aperture of 0.35. Further improvements are possible by co-optimizing the electrode and aperture geometry. An undercut of the electrode substrate at the aperture ensures a large distance between the ion and dielectric substrate without compromising collection efficiency. The metalens directly collimates the collected fluorescence, eliminating the need for a high numerical aperture objective. An array of such readout zones will offer a compact, scalable solution for high-fidelity parallel readout in next-generation trapped-ion quantum processors.
Authors: Qilin Li, Atharva Vidwans, Yazhen Wang, Micheline B. Soley
We establish a unified statistical framework that underscores the crucial role statistical inference plays in Quantum Amplitude Estimation (QAE), a task essential to fields ranging from chemistry to finance and machine learning. We use this framework to harness Bayesian statistics for improved measurement efficiency with rigorous interval estimates at all iterations of Iterative Quantum Amplitude Estimation. We demonstrate the resulting method, Bayesian Iterative Quantum Amplitude Estimation (BIQAE), accurately and efficiently estimates both quantum amplitudes and molecular ground-state energies to high accuracy, and show in analytic and numerical sample complexity analyses that BIQAE outperforms all other QAE approaches considered. Both rigorous mathematical proofs and numerical simulations conclusively indicate Bayesian statistics is the source of this advantage, a finding that invites further inquiry into the power of statistics to expedite the search for quantum utility.
Authors: Karol Horodecki, Marek Winczewski, Leonard Sikorski, Paweł Mazurek, Mikołaj Czechlewski, Raja Yehia
Inspired by environmental sciences, we develop a framework to quantify the energy needed to generate quantum entanglement via noisy quantum channels, focusing on the hardware-independent, i.e. fundamental cost. Within this framework, we define a measure of the minimal fundamental energy consumption rate per distributed entanglement (expressed in Joule per ebit). We then derive a lower bound on the energy cost of distributing a maximally entangled state via a quantum channel, which yields a quantitative estimate of energy investment per entangled bit for future quantum networks. We thereby show that irreversibility in entanglement theory implies a non-zero energy cost in standard entanglement distribution protocols. We further establish an upper bound on the fundamental energy consumption rate of entanglement distribution by determining the minimal energy required to implement quantum operations via classical control. To this end, we formulate the axioms for an energy cost measure and introduce a Hamiltonian model for classically-controlled quantum operations. The fundamental cost is then defined as the infimum energy over all such Hamiltonian protocols, with or without specific hardware constraints. The study of the energy cost of a quantum operation is general enough to be naturally applicable to quantum computing and is of independent interest. Finally, we evaluate the energy demands of three entanglement distillation protocols for photonic polarization qubits, finding that, due to entanglement irreversibility, their required energy exceeds the fundamental lower bound by many orders of magnitude. The introduced paradigm can be applied to other quantum resources, with appropriate changes depending on their nature.
Authors: Sebastian Kish, Josef Pieprzyk, Seyit Camtepe
Quantum Key Distribution (QKD) is a technology that ensures secure communication by leveraging the principles of quantum mechanics, such as the no-cloning theorem and quantum uncertainty. This chapter provides an overview of this quantum technology's maturity and trends. It highlights significant advancements in single-photon sources and detection technologies that have brought QKD closer to widespread adoption, including real-world deployments by industry leaders. While addressing challenges such as cost, integration, standardization, and the need for quantum repeaters, the chapter emphasizes the growing importance of QKD in securing mission-critical communications against future quantum threats. Through its unique ability to achieve information-theoretic security, QKD is poised to play a vital role in quantum-safe cryptographic algorithms and protocols.
Authors: Morteza Moradi, Maryam Afsary, Piotr Mironowicz, Enky Oudot, Magdalena Stobińska
Device-independent quantum key distribution (DI QKD) offers unparalleled cryptographic security by eliminating trust requirements for quantum devices, yet has remained impractical for long-distance implementation due to fundamental rate limitations. Here, we propose the first experimentally viable schemes for long-distance DI QKD using two fully photonic approaches with heralded entanglement distribution using spontaneous parametric down-conversion (SPDC) sources. Both schemes achieve key rate scaling with the square root of channel transmissivity $\eta_t$, matching the twin-field protocol advantage rather than the prohibitive linear decay of conventional QKD. We demonstrate positive key rates at detector efficiencies as low as 83\%, bringing DI QKD within reach of the current superconducting detector technology. Our security analysis employs the Entropy Accumulation Theorem to establish rigorous finite-size bounds, while numerical optimization yields custom Bell certificates that surpass standard approaches by 2--3 times at maximum transmission distances. This work represents a critical milestone toward device-independent security in quantum communication networks, providing experimentalists with practical implementation pathways while maintaining the strongest possible security guarantees against quantum adversaries.
Authors: Tongyuan Bao, Qi Luo, Ailun Yi, Yingjie Li, Haibo Hu, Xin Ou, Yu Zhou, Qinghai Song
Silicon carbide (SiC) has attracted significant attention as a promising quantum material due to its ability to host long-lived, optically addressable color centers with solid-state photonic interfaces. The CMOS compatibility of 4H-SiCOI (silicon-carbide-on-insulator) makes it an ideal platform for integrated quantum photonic devices and circuits. However, the deterministic integration of single spin defects into high-performance photonic cavities on this platform has remained a key challenge. In this work, we demonstrate the deterministic and scalable coupling of both ensemble (PL4) and single PL6 spin defects into monolithic bullseye cavities on the 4H-SiCOI platform. By tuning the cavity resonance, we achieve a 40-fold enhancement of the zero-phonon line (ZPL) intensity from ensemble PL4 defects, corresponding to a Purcell factor of approximately 5.0. For deterministically coupled single PL6 defects, we observe a threefold increase in the saturated photon count rate, confirm single-photon emission, and demonstrate coherent control of the spin state through optically detected magnetic resonance (ODMR), resonant excitation, and Rabi oscillations. These advancements establish a viable pathway for developing scalable, high-performance SiC-based quantum photonic circuits.
Authors: Javid Ahmad Malik, Muzaffar Qadir Lone, Prince A Ganai
This paper presents strategies for enhancing the robustness of bidirectional quantum teleportation (BQT) through environment-assisted and weak measurement techniques. BQT is a crucial component of distributed quantum networks, allowing for the bilateral transfer of quantum information between two nodes. While perfect teleportation necessitates maximally entangled states, these are vulnerable to degradation due to inherent decoherence. We propose a BQT scheme that enables the bilateral transfer of arbitrary qubits between nodes via amplitude damping channels (ADC), aiming to optimize fidelity using weak measurements in the final step of the process. Environment-assisted measurements (EAM) are used to establish a four-qubit channel composed of two Bell states. We explore two situations: (I) where only the recovery qubits pass through amplitude damping channels and (II) where the entire four-qubit channel is subjected to ADC. Our findings demonstrate a balance between average fidelity and success probability when the weak measurement strength ($q_w$) is constrained by the decay rate ($p$), specifically $q_w \in {[0,p]}$. Perfect BQT is achieved when $q_w = p$, indicating complete suppression of ADC effects. On the other hand, a decline in both average fidelity and success probability is noted when the weak measurement strength surpasses the ADC strength, marking the prohibited domain as $q_w \in {(p,1]}$. Additionally, our secured BQT protocol consistently outperforms the unprotected scheme in both scenarios, highlighting the effectiveness of the proposed protection strategies.
Authors: Pan Zheng, Yanyan Cai, Bin Xu, Shengcheng Wen, Libo Zhang, Zhongchu Ni, Jiasheng Mai, Yanjie Zeng, Lin Lin, Ling Hu, Xiaowei Deng, Song Liu, Jing Shu, Yuan Xu, Dapeng Yu
Quantum metrology enables sensitive dark matter detection, particularly using nonclassical states, such as Schrödinger cat states featuring sub-Planck interference structures in microwave cavities. Here, we report the first experimental application of four-component Schrödinger cat states within a high-quality superconducting microwave cavity to detect dark photons, a potential dark matter candidate. We demonstrate an 8.1-fold enhancement in the signal photon rate and constrain the dark photon kinetic mixing angle to an unprecedented $\epsilon < 7.32 \times 10^{-16}$ near 6.44~GHz (26.6~$\mu$eV). By employing a parametric sideband drive to actively tune the cavity frequency, we achieve dark photon searches and background subtraction across multiple frequency bins, yielding a sensitivity at the $10^{-16}$ level within a 100~kHz bandwidth. Our Schrödinger's cat-assisted detection (SCaD) scheme demonstrates a substantial improvement over previous results, promising potential implications in quantum-enhanced searches for new physics.
Authors: Yue-Zhi Zhang, Leong-Chuan Kwek, Wei Nie
Topological manipulation of light provides a versatile toolbox for photonic technologies. Here, we show that a topological atom array can induce photon localization in a waveguide via symmetry-protected light-matter interaction. Long-lived photon-atom entanglement reveals the existence of a novel topological quasi-bound state in the continuum (quasi-BIC). This hybrid light-matter quasi-BIC is formed at a critical coupling condition via collectively induced absorption, which is produced by quantum interference between edge and bulk states. We uncover the time-reversed relation between topological quasi-BIC and light amplification. Interestingly, one can realize a directional ultranarrow amplifier by means of critical coupling. Our work demonstrates an unconventional quasi-BIC at a topological quantum optics interface with potential applications in quantum devices.
Authors: Panagiotis Dorlis, Nick E. Mavromatos, Sarben Sarkar, Sotirios-Neilos Vlachos
It is shown that both, standard general relativity (GR) and Chern-Simons (CS) gravity, the latter containing chiral gravitational anomaly terms, seed the production of pairs of entangled gravitons in a multi-mode squeezed state. This involves the interaction of gravitons with the axionic cloud surrounding a superradiant Kerr (rotating) black hole background. The order of magnitude of the squeezing effect, specifically the number of graviton excitations in the squeezed vacuum, is estimated in the non-relativistic limit, relevant for the superradiance process. It is found analytically that the squeezing from the GR process of annihilation of two axions into two gravitons, dominates, by many orders of magnitude, that coming from the axion decay into two gravitons, induced by the higher-derivative CS term. It is also shown that significant squeezing effects are produced in the case of long-lived axionic clouds, whose lifetimes are much longer than the timescale for which superradiance is effective.
Authors: Morteza Bajand, Babak Vakili
In this paper, we investigate the Hamiltonian formulation of a spherically symmetric spacetime that corresponds to the interior of a Schwarzschild black hole. The resulting phase space involves two independent dynamical variables along with their conjugate momenta. We quantize the associated minisuperspace using the affine quantization method, which is particularly suited for systems with positive-definite configuration variables. We then explore whether the quantum effects encoded in this wave function can lead to the avoidance of classical singularities.
Authors: Chris Gustin, Juanjuan Ren, Sebastian Franke, Stephen Hughes
Phenomenological approaches to photon loss have long been the workhorse of cavity-QED, but prove inadequate in the presence of sufficiently broadband light-matter interactions. We present a rigorous and ab initio derivation of a quantum master equation for a quantized optical cavity mode coupled to a dipole, using a quasinormal mode (QNM) quantization procedure for plasmonic and dielectric open-system cavity-QED, which is valid in broadband light-matter interaction regimes, including ultrastrong coupling (USC). The theory supports general three-dimensional resonators with arbitrary dispersion and loss, and thus can be applied to a wide range of open cavities. Our ab initio and gauge-invariant approach fully recovers the recent result of Phys. Rev. Lett. 134, 123601 (2025) for the spectral density of a quantized cavity with a single dipole, exhibits a dissipative classical-quantum correspondence for bosonic Hopfield model systems, and reveals important departures from previous heuristic assumptions about system-bath coupling. We identify a new criterion for what we term the "broadband" dissipative regime of cavity-QED, where phenomenological models require corrections in accordance with the intrinsic and spatially-dependent complex phase of the QNM, and also shed light on fundamental limits to single-mode models in extreme coupling regimes. Using plasmonic and dielectric cavity examples, we show validity ranges of our QNM master equation and spectral USC calculations, and discuss prospects for near-term experimental observation of broadband dissipative effects.
Authors: Henok Weldeyesus, Pedro M.T. Vianez, Omid Sharifi Sedeh, Wooi Kiat Tan, Yiqing Jin, María Moreno, Christian P. Scheller, Jonathan P. Griffiths, Ian Farrer, David A. Ritchie, Dominik M. Zumbühl, Christopher J.B. Ford, Oleksandr Tsyplyatyev
Luttinger liquids occupy a special place in physics as the most understood case of essentially quantum many-body systems. The experimental mission of measuring its main prediction, power laws in observable quantities, has already produced a body of exponents in different semiconductor and metallic structures. Here, we combine tunneling spectroscopy with density-dependent transport measurements in the same quantum wires over more than two orders of magnitude in temperature to very low electron temperatures down to $\sim$40 mK. This reveals that, when the second 1D subband becomes populated, the temperature dependence splits into two ranges with different exponents in the power-law dependence of the conductance, both dominated by the finite-size effect of the end-tunneling process. This result demonstrates the importance of measuring the Luttinger parameters as well as the number of modes independently through spectroscopy in addition to the transport exponent in the characterization of Luttinger liquids. This opens a new pathway to unambiguous interpretation of the exponents observed in quantum wires.
Authors: Takanori Ishii, Daichi Takeda
We develop, in the AdS/CFT correspondence, a method to compute correlation functions when the CFT is governed by the Lindblad equation for open quantum systems, via the AdS theory. Using a simple example in AdS$_3$/CFT$_2$, we demonstrate that the predictions of the AdS theory based on our method match the direct computations in the dual CFT. We also briefly discuss the relaxation problem and the holographic entropy in this example.
Authors: Yann Kiefer, Zijie Zhu, Lars Fischer, Samuel Jele, Marius Gächter, Giacomo Bisson, Konrad Viebahn, Tilman Esslinger
Quantum computing represents a central challenge in modern science. Neutral atoms in optical lattices have emerged as a leading computing platform, with collisional gates offering a stable mechanism for quantum logic. However, previous experiments have treated ultracold collisions as a dynamically fine-tuned process, which obscures the underlying quantum- geometry and statistics crucial for realising intrinsically robust operations. Here, we propose and experimentally demonstrate a purely geometric two-qubit swap gate by transiently populating qubit doublon states of fermionic atoms in a dynamical optical lattice. The presence of these doublon states, together with fermionic exchange anti-symmetry, enables a two-particle quantum holonomy -- a geometric evolution where dynamical phases are absent. This yields a gate mechanism that is intrinsically protected against fluctuations and inhomogeneities of the confining potentials. The resilience of the gate is further reinforced by time-reversal and chiral symmetries of the Hamiltonian. We experimentally validate this exceptional protection, achieving a loss-corrected amplitude fidelity of $99.91(7)\%$ measured across the entire system consisting of more than $17'000$ atom pairs. When combined with recently developed topological pumping methods for atom transport, our results pave the way for large-scale, highly connected quantum processors. This work introduces a new paradigm for quantum logic, transforming fundamental symmetries and quantum statistics into a powerful resource for fault-tolerant computation.
Authors: Wei Xia, Yijia Zhou, Xingze Qiu, Xiaopeng Li
Understanding the complexity of quantum many-body systems has been attracting much attention recently for its fundamental importance in characterizing complex quantum phases beyond the scope of quantum entanglement. Here, we investigate Krylov complexity in quantum percolation models (QPM) and establish unconventional phase transitions emergent from the interplay of exponential scaling of the Krylov complexity and the number of spanning clusters in QPM. We develop a general scaling theory for Krylov complexity phase transitions (KCPT) on QPM, and obtain exact results for the critical probabilities and exponents. For non-interacting systems across diverse lattices (1D/2D/3D regular, Bethe, and quasicrystals), our scaling theory reveals that the KCPT coincides with the classical percolation transition. In contrast, for interacting systems, we find the KCPT develops a generic separation from the percolation transition due to the highly complex quantum many-body effects, which is analogous to the Griffiths effect in the critical disorder phase transition. To test our theoretical predictions, we provide a concrete protocol for measuring the Krylov complexity, which is accessible to present experiments.
Authors: Ian Pannemarsh
In recent decades the field of quantum computation has seen remarkable development. While much progress has been made toward the realization of a fully digital, scalable, and fault tolerant quantum computer, there are still many essential challenges to overcome. In the interim, direct emulation of quantum systems of interest can fill an important gap not only for exploring fundamental questions about many-body physics and the quantum to classical transition, but also for potentially providing alternative methods to verify results from quantum simulations. In this work we will demonstrate a method utilizing closed loop control of the collective magnetic moment of an ensemble of cold neutral atoms via non-destructive measurements to emulate various spin system Hamiltonians. By modifying the feedback control law appropriately we are able to generate nonlinear dynamical behavior in the ensemble, allowing us to explore the physics of collective spin systems at mesoscopic scales. Moreover, controlling the number of atoms in the collective spin can potentially allow us to investigate these dynamics in the transition from fully quantum to the classical limit. In particular, we emulate two models: the Lipkin-Meshkov-Glick (LMG) Hamiltonian, and a closely related model, the Kicked Top. In the former case, we show that our system undergoes a symmetry-breaking phase transition in the expected parameter regime. In the latter, we explore two interesting aspects: the formation of chaos, and a dynamically driven time crystal phase. We will then discuss the advantages and limits of this approach.
Authors: Pranet Sharma, Yizhuo Tan, Konstantinos-Nikolaos Papadopoulos, Jakub Szefer
This work explores and evaluates noise and crosstalk in neutral atom quantum computers. Neutral atom quantum computers are a promising platform for analog Hamiltonian simulations, which rely on a sequence of time-dependent Hamiltonians to model the dynamics of the larger system and are particularly useful for problems in optimization, physics, and molecular dynamics. However, the viability of running multiple simulations in a co-located or multi-tenant environment is limited by noise and crosstalk. This work conducts an analysis of how noise faced by simulations changes over time, and investigates the effects of spatial co-location on simulation fidelity. Findings of this work demonstrate that the close proximity of concurrent simulations can increase crosstalk between them. To mitigate this issue, a Moving Target Defense (MTD) strategy is proposed and evaluated. The results confirm that the MTD is a viable technique for enabling safe and reliable co-location of simulations on neutral atom quantum hardware.
Authors: Sutapa Saha, Ujjwal Sen
Quantum evolutions are often non-unitary and in such cases, they are frequently regarded as lossy. Such lossiness, however, does not necessarily persist throughout the evolution, and there can often be intermediate time-spans during which information ebbs in the environment to re-flood the system -- an event known as information backflow. This phenomenon serves as a well-established and sufficient indicator of non-Markovian behavior of open quantum dynamics. Nevertheless, not all non-Markovian dynamics exhibit such backflow. We find that when interference is allowed between two quantum evolutions that individually generate non-Markovianity and yet do not exhibit information backflow, it becomes possible to retrieve information from the environment. Furthermore, we show that this setup involving coherently-controlled quantum operation trajectories provides enhanced performance and is more robust compared to an alternate coherently-controlled arrangement of the quantum switch.
Authors: Markus Weber
In 1935 EPR used the assumption of local realism to conclude in a Gedankenexperiment with two entangled particles that quantum mechanics is not complete. Based on this idea Bell constructed an inequality whereby experimental tests could distinguish between quantum mechanics and local-realistic theories. Many experiments have since been done that are consistent with quantum mechanics, disproving the concept of local realism. But all these tests suffered from loopholes allowing a local-realistic explanation of the experimental observations. In this context, of special interest is entanglement between different quantum objects like atoms and photons, because it allows one to entangle distant atoms by the interference of photons. The resulting space-like separation together with the almost perfect detection efficiency of the atoms will allow a first event-ready Bell test closing detection and locality loopholes. The primary goal of the present thesis was the experimental realization of entanglement between a single localized atom and a single spontaneously emitted photon. In the experiment a single optically trapped R87 atom is excited to a state which has two selected decay channels. In the following spontaneous decay a photon is emitted coherently with equal probability into both decay channels. This accounts for perfect correlations between the polarization state of the emitted photon and the Zeeman state of the atom after spontaneous decay. Because these decay channels are spectrally and in all other degrees of freedom indistinguishable, the spin state of the atom is entangled with the polarization state of the photon. To verify entanglement, appropriate correlation measurements in complementary bases of the photon polarization and the internal quantum state of the atom were performed. It is shown, that the generated atom-photon state yields an entanglement fidelity of 0.82.
Authors: Imre Varga
The complexity of $n$-qubit and many body systems is investigated. In case of an $n$-qubit system the disturbance due to depolarization and dephasing is identified based on a certain complexity quantity defined as the difference of the Shannon-entropy and the Rényi entropy of order two. In case of the effect of the depolarization the quantum system is replaced by a fully separable, i.e. classical state with probability $p$ while it remains unchanged with probability $1-p$. Whereas dephasing is modelled by destructing the appropriate off-diagonal elements of the density matrix also with probability $p$. For both cases the state with maximal complexity marks the border between the most quantum and most classical limits. Furthermore we also show that many body systems modelled using deformed random matrix ensembles, deformed two-body random interaction ensembles and also the system of one-dimensional Heisenberg-model of spins subject to a random, local magnetic field exhibiting many body localization transition, the states with maximal complexity mark the cross-over or the transition point between integrability and full quantum chaos. Finally we address the question of identifying the cross-over in the thermalization properties within large sets of quantum chaotic states using the survival probability of an excitation of a many body system. All these results show that the complexity parameter defined on a combination of the von Neumann entropy and the Rényi entropy of 2nd order is a meaningful and informative parameter to detect whenever a system is in a cross-over state between the otherwise trivial extremal cases of integrability or localization and quantum chaos or ergodic behavior.
Authors: Harsh Kashyap, Denis A. Kopylov, Polina R. Sharapova
The cubic phase state constitutes a nonlinear resource that is essential for universal quantum computing protocols. However, constructing such non-classical states faces many challenges. In this work, we present a protocol for generating a cubic phase state with high fidelity. The protocol is based on an interferometer scheme assisted by a detection operation. To find the proper set of parameters that results in both high fidelity and high detection probability, we provide a numerical multiparameter optimization. We investigate a broad range of target states and study how parameter imperfections influence fidelity.
Authors: Tomer Goldfriend, Or Samimi Golan, Amir Naveh
We implement a quantum linear solver for the one-dimensional Vlasov-Ampere equation, following the model presented in Novikau et. al. [I, Novikau, I. Y. Dodin, and E. this http URL. J. Plasma Phys, 90, 805900401]. We design the relevant block encoding operator with Qmod high-level language, and obtain optimized quantum programs using Classiq synthesis tools. Compared to a rigid baseline implementation, our approach yields a clear reduction in quantum resource requirements.
Authors: Alok Shukla, Prakash Vedula
Quantum Phase Estimation (QPE) is a cornerstone algorithm in quantum computing, with applications ranging from integer factorization to quantum chemistry simulations. However, the resource demands of standard QPE, which require a large number of coherent qubits and deep circuits, pose significant challenges for current Noisy Intermediate Scale Quantum (NISQ) devices. In this work, we introduce the Adaptive Windowed Quantum Phase Estimation (AWQPE) algorithm, a novel method designed to address the limitations of standard QPE. AWQPE utilizes small, independent blocks of $m > 1$ control qubits to estimate multiple phase bits simultaneously within a "window,'' thereby significantly reducing the number of iterations required to achieve a desired precision. These independent blocks are amenable to parallelization and, when combined with a robust least-significant-bit (LSB) to most-significant-bit (MSB) ambiguity resolution mechanism, enhance the algorithm's accuracy while mitigating the risk of error propagation. Our numerical simulations demonstrate AWQPE's accuracy and robustness, showcasing a distinct balance between resource efficiency and computational speed. This makes AWQPE particularly well-suited for near-term quantum platforms.
Authors: Rishabh Batra, Zhili Chen, Rahul Jain, YaoNan Zhang
Pseudorandom quantum states (PRS) and pseudorandom function-like quantum state (PRFS) generators are quantum analogues of pseudorandom generators and pseudorandom functions. It is known that PRS (and PRFS) can exist even if BQP = QMA (relative to a quantum oracle) or if P = NP (relative to a classical oracle), which does not allow for the existence of one-way functions (relative to these oracles). Hence, these are potentially weaker objects than quantum-secure one-way functions, which can be used to do quantum cryptography. A desirable property of PRS and PRFS constructions is scalability, which ensures that the security parameter $\lambda$ (which determines indistinguishability from their Haar-random counterparts) is much larger than $n$ (the number of qubits of the output states). This may be important in some applications where PRS and PRFS primitives are used. We present an isometric procedure to prepare quantum states that can be arbitrarily random (i.e., the trace distance from the Haar-random state can be arbitrarily small for the true random case, or the distinguishing advantage can be arbitrarily small for the pseudorandom case). Our procedure provides a new method for scalable PRS that introduces no entanglement or correlations with the environment. This naturally gives the first construction for scalable and (quantum-accessible) adaptive PRFS assuming quantum-secure one-way functions. Our PRFS construction implies various primitives, including long-input PRFS, short-input PRFS, short-output PRFS, non-adaptive PRFS, and classical-accessible adaptive PRFS. This new construction may be helpful in some simplification of the microcrypt zoo.
Authors: José Garre-Rubio, András Molnár, Germán Sierra
We introduce a framework to define coalgebra and bialgebra structures on two-dimensional (2D) square lattices, extending the algebraic theory of Hopf algebras and quantum groups beyond the one-dimensional (1D) setting. Our construction is based on defining 2D coproducts through horizontal and vertical maps that satisfy compatibility and associativity conditions, enabling the consistent growth of vector spaces over lattice sites. We present several examples of 2D bialgebras, including group-like and Lie algebra-inspired constructions and a quasi-1D coproduct instance that is applicable to Taft-Hopf algebras and to quantum groups. The approach is further applied to the quantum group $U_q[su(2)]$, for which we construct 2D generalizations of its generators, analyze $q$-deformed singlet states, and derive a 2D R-matrix satisfying an intertwining relation in the semiclassical limit. Additionally, we show how tensor network states, particularly PEPS, naturally induce 2D coalgebra structures when supplemented with appropriate boundary conditions. Our results establish a local and algebraically consistent method to embed quantum group symmetries into higher-dimensional lattice systems, potentially connecting to the emerging theory of fusion 2-categories and categorical symmetries in quantum many-body physics.
Authors: Filippo Brozzi, Gloria Turati, Maurizio Ferrari Dacrema, Filippo Caruso, Paolo Cremonesi
In the context of Variational Quantum Algorithms (VQAs), selecting an appropriate ansatz is crucial for efficient problem-solving. Hamiltonian expressibility has been introduced as a metric to quantify a circuit's ability to uniformly explore the energy landscape associated with a Hamiltonian ground state search problem. However, its influence on solution quality remains largely unexplored. In this work, we estimate the Hamiltonian expressibility of a well-defined set of circuits applied to various Hamiltonians using a Monte Carlo-based approach. We analyze how ansatz depth influences expressibility and identify the most and least expressive circuits across different problem types. We then train each ansatz using the Variational Quantum Eigensolver (VQE) and analyze the correlation between solution quality and this http URL results indicate that, under ideal or low-noise conditions and particularly for small-scale problems, ansätze with high Hamiltonian expressibility yield better performance for problems with non-diagonal Hamiltonians and superposition-state solutions. Conversely, circuits with low expressibility are more effective for problems whose solutions are basis states, including those defined by diagonal Hamiltonians. Under noisy conditions, low-expressibility circuits remain preferable for basis-state problems, while intermediate expressibility yields better results for some problems involving superposition-state solutions.
Authors: Nicola Mariella, Enrique Rico, Adam Byrne, Sergiy Zhuk
Krylov quantum diagonalization methods have emerged as a promising use case for quantum computers. However, many existing implementations rely on controlled operations, which pose challenges to near-term quantum hardware. We introduce a novel protocol, which we call Krylov Time Reversal (KTR), which avoids these bottlenecks by exploiting the time-reversal symmetry in Hamiltonian evolution. Using symmetric time dynamics, we show that it is possible to recover real-valued Krylov matrix elements, which significantly reduces the circuit depth and enhances compatibility with shallow quantum architectures. We validate our method through numerical simulations on paradigmatic Hamiltonians exhibiting time-reversal symmetry, including the transverse-field Ising model and a lattice gauge theory, demonstrating accurate spectral estimation and favorable circuit constructions.
Authors: Keita Omiya, Yuya O Nakagawa
We introduce a novel non-equilibrium phase -- the quantum many-body scar (QMBS) phase -- that emerges in non-Hermitian many-body dynamics when scarred wavefunctions are selectively stabilized via non-Hermitian driving. Projective measurements, or non-Hermitian counterparts, preferentially reinforce QMBS, counteracting the entropy growth that drives thermalization. As a result, atypical, high-energy scarred wavefunctions that are negligible in the long-time dynamics of closed systems become non-equilibrium steady states. We establish the existence of the QMBS phase and its sharp, first-order phase transition from an ergodic thermal phase, through both analytical arguments and numerical simulations of three representative models: a random quantum circuit model, the $SU(q)$ spin model, and the paradigmatic spin-1 XY model.
Authors: P. A. S. de Alcântara, Gabriel Audi, Leandro Morais
Advances in quantum computing over the last two decades have required sophisticated mathematical frameworks to deepen the understanding of quantum algorithms. In this review, we introduce the theory of Lie groups and their algebras to analyze two fundamental problems in quantum computing as done in some recent works. Firstly, we describe the geometric formulation of quantum computational complexity, given by the length of the shortest path on the $SU(2^n)$ manifold with respect to a right-invariant Finsler metric. Secondly, we deal with the barren plateau phenomenon in Variational Quantum Algorithms (VQAs), where we use the Dynamical Lie Algebra (DLA) to identify algebraic sources of untrainability
Authors: Walter Zedda, Ilaria Gianani, Vincenzo Berardi, Marco Barbieri
Light detection and ranging is a key technology for a number of applications, from relatively simple distance ranging to environmental monitoring. When dealing with low photon numbers an important issue is the improvement of the signal- to-noise-ratio, which is severely affected by external sources whose emission is captured by the detection apparatus. In this paper, we present an extension of the technique developed in [Phys. Rev. Lett. 123, 203601] to the effects caused by the propagation of light through a turbulent media, as well as the detection through photon counting devices bearing imperfections in terms of efficiency and number resolution. Our results indicate that even less performing technology can result in a useful detection scheme.
Authors: Eliya Blumenthal, Nir Gutman, Ido Kaminer, Shay Hacohen-Gourgy
Advanced bosonic quantum computing architectures demand nonlocal Gaussian operations such as two-mode squeezing to unlock universal control, enable entanglement generation, and implement logical operations across distributed modes. This work presents a novel method for generating conditional squeezing using a Rabi-driven qubit dispersively coupled to one or two harmonic oscillators. A proof that this enables universal control over bosonic modes is provided, expanding the toolkit for continuous-variable quantum information processing. Using modulated Jaynes-Cummings interactions in circuit QED, the simulation predicts intra-cavity squeezing of 13dB (single-mode), 4dB (superimposed single-mode), and 12dB (two-mode), with the latter two yet to be demonstrated experimentally. These results establish a new paradigm for qubit-conditioned control of photonic states, with applications to quantum sensing and continuous-variable computation on readily available systems.
Authors: Samir Belaloui, Nacer Eddine Belaloui, Achour Benslama
We investigate the electronic structure of methanimine (CH2NH) and water (H2O) molecules in an effort to locate conical intersections (CIs) using variational quantum algorithms. Our approach implements and compares a range of hybrid quantum-classical methods, including the Variational Quantum Eigensolver (VQE), Variational Quantum Deflation (VQD), VQE with Automatically-Adjusted Constraints (VQE-AC), and we explore molecular configurations of interest using a State-Average (SA) approach. Exact Diagonalization is employed as the classical benchmark to evaluate the accuracy of the quantum algorithms. We perform simulations across a range of molecular geometries, basis sets, and active spaces to compare each algorithm's performance and accuracy, and to enhance the detectability of CIs. This work confirms the quantum variational algorithms' capability of describing conical intersections in both molecules, as long as appropriate active spaces and geometries of the molecule are chosen. We also compare the accuracy and reliability of VQE-based methods for computing excited states with classical benchmark methods, and we demonstrate good agreement within desired regions.
Authors: Marco Barbieri
The purpose of quantum technologies is to explore how quantum effects can improve on existing solutions for the treatment of information. Quantum photonics sensing holds great promises for reaching a more efficient trade-off between invasivity and quality of the measurement, when compared with the potential of classical means. This tutorial is dedicated to presenting how this advantage is brought about by nonclassical light, examining the basic principles of parameter estimation and reviewing the state of the art.
Authors: Shuaifan Cao, Xiaopeng Li
Developing hardware-efficient implementations of quantum algorithms is crucial in the NISQ era to achieve practical quantum advantage. Here, we construct a generic quantum solver for NP problems based on Grover's search algorithm, specifically tailored for Rydberg-atom quantum computing platforms. We design the quantum oracles in the search algorithm using parallelizable single-qubit and multi-qubit entangling gates in the Rydberg atom system, yielding a unified framework for solving a broad class of NP problems with provable quadratic quantum speedup. We analyze the experimental resource requirements considering the unique qubit connectivity of the dynamically reconfigurable qubits in the optical tweezer array. The required qubit number scales linearly with the problem size, representing a significant improvement over existing Rydberg-based quantum annealing approaches that incur quadratic overhead. These results provide a concrete roadmap for future experimental efforts towards demonstrating quantum advantage in NP problem solving using Rydberg atomic systems. Our construction indicates that atomic qubits offer favorable circuit depth scaling compared to quantum processors with fixed local connectivity.
Authors: Di Fang, Xiaoxu Wu, Avy Soffer
Efficient simulation of many-body quantum systems is central to advances in physics, chemistry, and quantum computing, with a key question being whether the simulation cost scales polynomially with the system size. In this work, we analyze many-body quantum systems with Coulomb interactions, which are fundamental to electronic and molecular systems. We prove that Trotterization for such unbounded Hamiltonians achieves a $1/4$-order convergence rate, with explicit polynomial dependence on the number of particles. The result holds for all initial wavefunctions in the domain of the Hamiltonian, and the $1/4$-order convergence rate is optimal, as previous studies have shown that it can be saturated by a specific initial eigenstate. The main challenges arise from the many-body structure and the singular nature of the Coulomb potential. Our proof strategy differs from prior state-of-the-art Trotter analyses, addressing both difficulties in a unified framework.
Authors: Mikhail Bednov, Waqas Pervez, Ingo Barke, Dieter Bauer
We investigate plasmon-assisted photoelectron emission using a one-dimensional time-dependent density-functional theory (TDDFT) model. Photoelectron spectra are computed with the time-dependent surface-flux (t-SURFF) method. In addition to the expected above-threshold ionization (ATI) comb, we observe peaks that arise from long-lived plasmon oscillations and the associated electron emission occurring after the laser pulse. We further analyze the positions of these peaks and their scaling behavior with the laser intensity.
Authors: Bing-Bing Liu, Shi-Lei Su
For a truly $\mathcal{PT}$-symmetric quantum system, the conventional non-Hermitian Hamiltonian is $H = H_{\rm drive} -i\gamma|1\rangle\langle1| + i\gamma|0\rangle\langle0|$, where $\Omega$ and $\gamma$ are real parameters. The three terms respectively represent coherent coupling, loss (on state $|1\rangle$), and gain (on state $|0\rangle$). However, realizing the gain term $+i\gamma|0\rangle\langle0|$ has remained an outstanding challenge for quantum system, especially on ground state -- no theoretical or experimental schemes have definitively demonstrated its achievement. While systems omitting this gain term can exhibit a passively $\mathcal{PT}$-symmetric energy spectrum (featuring a parallel imaginary shift) and display related phenomena, they fail to capture the full physical behavior and unique properties inherent to truly $\mathcal{PT}$-symmetric systems. In this manuscript, we propose a method to achieve effective gain on the ground state $|0\rangle$ ($+i\gamma|0\rangle\langle0|$) after averaging all trajectories, by integrating the Sørensen-Reiter effective operator method with the Wiseman-Milburn master equation for continuous measurement and instantaneous feedback control after averaging the evolution over all trajectories. This approach provides a possible pathway to efficiently construct truly $\mathcal{PT}$-symmetric quantum devices, offering a powerful platform for engineering quantum resources vital for quantum information technology applications.
Authors: S. Macedonio, L. Lepori, A. Chiesa, S. Chicco, L. Bersani, M. Rubin-Osanz, L. B. Woodcock, A. Mavromagoulos, G. Allodi, E. Garlatti, S. Piligkos, A. Smerzi, S. Carretta
We show that Yb(trensal) molecular nanomagnet, embedding an electronic spin qubit coupled to a nuclear spin qudit, provides an ideal platform to probe entanglement in a qubit-qudit this http URL is demonstrated by developing an optimized pulse sequence to show violation of generalized Bell inequalities and by performing realistic numerical simulations including experimentally measured decoherence. We find that the inequalities are safely violated in a wide range of parameters, proving the robustness of entanglement in the investigated system. Furthermore, we propose a scheme to study qudit-qudit entanglement on a molecular spin trimer, in which two spins 3/2 are linked via an interposed switch to turn on and off their mutual interaction.
Authors: Jiu-Ming Li, Jun-Yan Liu, Yuan-Yuan Liu, Xiao-Ming Xiu, Shao-Ming Fei
We propose a physical system consisting of two optical cavities and a two-level system (TLS), which can be viewed as a double single-sided cavity system. The two cavities are crossed each other in a mutually perpendicular way and are both single-sided. The TLS is coupled to the two cavities. The universal input-output relation of the system, the reflection and transmission coefficients are derived by exploiting the probability amplitude method. Then by using the nitrogen-vacancy center instead of the TLS, we generate the controlled-phase gate and the controlled-controlled-phase gate on the photon qubits, with simple protocols that can be accomplished in both weak and strong coupling regimes. The protocols are shown to give rise to high fidelities and gate efficiencies.
Authors: Triyas Sapui, Keshav Das Agarwal, Tanoy Kanti Konar, Leela Ganesh Chandra Lakkaraju, Aditi Sen De
We demonstrate that the quenched average genuine multipartite entanglement (GME) can approach its maximum value in the ergodic phase of a disordered quantum spin model. In contrast, GME vanishes in the many-body localized (MBL) phase, both in equilibrium and in the long-time dynamical steady state, indicating a lack of useful entanglement in the localized regime. To establish this, we analyze the disordered Heisenberg spin chain subjected to a random magnetic field and incorporating two- and three-body Dzyaloshinskii-Moriya (DM) interactions. We exhibit that the behavior of GME, in both static eigenstates and in dynamically evolved states from an initial Neel configuration, serves as a reliable indicator of the critical disorder strength required for the ergodic-to-MBL transition. The identified transition point aligns well with standard indicators such as the gap ratio and correlation length. Moreover, we find that the presence of DM interactions, particularly the three-body one, significantly stabilizes the thermal phase and delays the onset of localization. This shift in the transition point is consistently reflected in both static and dynamical analyses, reinforcing GME as a robust probe for MBL transitions.
Authors: Shubhodeep Gangopadhyay, Vinayak Jagadish, R. Srikanth
We analyze a system of three or more qubits collectively interacting with a zero-temperature bosonic bath characterized by a Lorentzian spectral density. Our study focuses on the emergence of decoherence-free subspaces and the genuine-entanglement dynamics. Specifically, we study the three qubit system in detail, where the genuine entanglement is quantified through the convex roof extension of negativity. By examining the transition between Markovian and non-Markovian regimes, we reveal how the entanglement in the system evolves under the influence of the environment. Notably, we observe transitions between genuinely multi-qubit entangled and bi-separable states, including a revival of entanglement even in the Markovian regime. These findings provide insights into the robustness of quantum correlations and the conditions under which decoherence-protected states can be sustained.
Authors: Luca Spagnoli, Chiara Lissoni, Alessandro Roggero
The study of real time dynamics of nuclear systems is of great importance to provide theoretical predictions of cross sections relevant for both terrestrial experiments as well as applications in astrophysics. First principles simulations of these dynamical processes is however hindered by an exponential cost in classical resources and the possibility of performing scalable simulations using quantum computers is currently an active field of research. In this work we provide the first complete characterization of the resource requirements for studying nuclear dynamics with the full Leading Order (LO) pionless EFT Hamiltonian in first quantization employing simulation strategies using both product formulas as well as Quantum Signal Processing. In particular, we show that time evolution of such an Hamiltonian can be performed with polynomial resources in the number of particles, and logarithmic resources in the number of single-particle basis states. This result provides an exponential improvement compared with previous work on the same Hamiltonian model in second quantization. We find that interesting simulations for low energy nuclear scattering could be achievable with tens of millions of T gates and few hundred logical qubits suggesting that the study of simple nuclear reactions could be amenable for early fault tolerant quantum platforms.
Authors: Mwezi Koni, Fazilah Nothlawala, Vagharshak Hakobyan, Isaac Nape, Etienne Brasselet, Andrew Forbes
We propose a spin-orbit strategy for generating dual-wavelength quantum skyrmions, realized either as entangled photon pairs at distinct wavelengths or as heralded single-photon states at a given wavelength -- regimes neither previously conceptualized nor demonstrated. By coupling a two-photon entangled state to an electrically tunable liquid crystal topological defect, we engineer both nonlocal and local skyrmionic topologies in a reconfigurable platform. These results open new directions for topological quantum state engineering and the topological richness of liquid crystals.
Authors: A. Alocco, A. Celotto, E. Palumbo, B. Galvano, P. Livreri, L. Fasolo, L. Callegaro, E. Enrico
Cluster states are a fundamental resource for continuous-variable quantum computing, enabling measurement-based protocols that can scale beyond the limitations of qubit-based architectures. Here, we demonstrate on-demand generation of multimode entangled microwave cluster states using a programmable Josephson Traveling-Wave Parametric Amplifier (JTWPA) operated in the three-wave mixing regime. By injecting a tailored, non-equidistant set of pump tones via an arbitrary waveform generator, we engineer frequency-specific nonlinear couplings between multiple frequency modes. The entanglement structure is verified via frequency-resolved heterodyne detection of quadrature nullifiers, confirming the target graph topology of the cluster state. Our approach allows reconfigurability through the pumps spectrum and supports scalability by leveraging the wide bandwidth and spatial homogeneity of the JTWPA. This platform opens new avenues for scalable measurement-based quantum information processing in the microwave domain, compatible with superconducting circuit architectures.
Authors: Majid Haghparast, Ronja Heikkinen, Samuel Ovaskainen, Julian Fuchs, Jussi P P Jokinen, Tommi Mikkonen
This paper presents a tool that simplifies quantum software development by unifying circuit design, code generation, and execution within a single cross-platform environment that supports iterative development. Implemented as open source, the DEQSE Quantum IDE Extension has been developed to provide quantum functionalities within the Visual Studio Code environment, including project creator, code runner, code converter, and embedded quantum circuit simulator. Furthermore, the system provides capabilities that facilitate iterative development and support learning, distinguishing it from other available Visual Studio Code Extensions for quantum computing.
Authors: Daniel D. Stancil, Bojko N. Bakalov, Gregory T. Byrd
A quantum computing circuit is presented that approximates a single spin wave quantum on a linear chain of spin 1/2 particles described by a Heisenberg Hamiltonian. The circuit is a product state where each qubit represents a spin. The spin wave motion is represented by opening the cone angle using Y rotations and then adding progressive Z rotations along the chain to represent wave propagation. We show analytically that this product state yields the correct dispersion relation in the limit of an unbounded chain. This surprising observation is confirmed using both a simulator and various quantum processors. The quantum circuit calculation leads to insight into the connection between classical and quantum descriptions of spin waves, and may also be useful for characterizing the error in quantum processors.
Authors: Yi-Ting Lee, Bibek Pokharel, Jeffrey Cohn, Andre Schleife, Arnab Banerjee
Understanding transport phenomena in quantum spin systems has long intrigued physicists due to their potential applications in spintronic devices and spin qubits. Here, using a superconducting-qubit-based transmon device, we show that pre-fault-tolerant digital quantum simulation is reliable for studying transport phenomena via spin-current autocorrelation function (ACF). While quantum simulations of the spin-spin ACF have been used to probe spin transport, methods based on the spin-current ACF have yet to be demonstrated due to their high gate cost, despite offering more direct information relevant to the transport properties. Overcoming the resource constraints set by indirect measurement schemes like the Hadamard test, we showcase a direct measurement scheme that utilizes non-unitary operations, in particular mid-circuit measurements, to investigate spin transport for the 40-site 1D XXZ Heisenberg model in the near-ballistic, superdiffusive, and diffusive regimes. We successfully reproduce the expected power-law behavior in the superdiffusive regime and vanishing of the Drude weight in the diffusive regime.
Authors: Damià Gomila
We show that the Schrödinger equation describes the ensemble mean dynamics of solitons in a Galilean invariant field theory where we interpret solitons as particles. On a zero background, solitons move classically, following Newton`s second law, however, on a non-zero amplitude chaotic background, their momentum and position fluctuate fulfilling an exact uncertainty relation, which give rise to the emergence of quantum phenomena. The Schrodinger equation for the ensemble of solitons is obtained from this exact uncertainty relation, and the amplitude of the background fluctuations is what corresponds to the value of $\hbar$. We confirm our analytical results running simulations of solitons moving against a potential barrier and comparing the ensemble probabilities with the predictions of the time dependent Schrödinger equation, providing a deterministic version of the quantum tunneling effect. We conclude with a discussion of how our theory does not present statistical independence between measurement and experiment outcome.
Authors: Lodovico Scarpa, Nishan Ranabhat, Amit Te'eni, Abdulla Alhajri, Vlatko Vedral, Fabio Anza, Luis Pedro García-Pintos
A quantum thermodynamic system can conserve non-commuting observables, but the consequences of this phenomenon on relaxation are still not fully understood. We investigate this problem by leveraging an observable-dependent approach to equilibration and thermalization in isolated quantum systems. We extend such approach to scenarios with non-commuting charges, and show that it can accurately estimate the equilibrium distribution of coarse observables without access to the energy eigenvalues and eigenvectors. Our predictions do not require weak coupling and are not restricted to local observables, thus providing an advantage over the non-Abelian thermal state. Within this approach, weak values and quasiprobability distributions emerge naturally and play a crucial role in characterizing the equilibrium distributions of observables. We show and numerically confirm that, due to charges' non-commutativity, these weak values can be anomalous even at equilibrium, which has been proven to be a proxy for non-classicality. Our work thus uncovers a novel connection between the relaxation of observables under non-commuting charges, weak values, and Kirkwood-Dirac quasiprobability distributions.
Authors: Lennart Bittel, Lorenzo Leone
Magic-state resource theory is a fundamental framework with far-reaching applications in quantum error correction and the classical simulation of quantum systems. Recent advances have significantly deepened our understanding of magic as a resource across diverse domains, including many-body physics, nuclear and particle physics, and quantum chemistry. Central to this progress is the stabilizer Rényi entropy, a computable and experimentally accessible magic monotone. Despite its widespread adoption, a rigorous operational interpretation of the stabilizer entropy has remained an open problem. In this work, we provide such an interpretation in the context of quantum property testing. By showing that the stabilizer entropy is the most robust measurable magic monotone, we demonstrate that the Clifford orbit of a quantum state becomes exponentially indistinguishable from Haar-random states, at a rate governed by the stabilizer entropy $M(\psi)$ and the number of available copies. This implies that the Clifford orbit forms an approximate state $k$-design, with an approximation error $\Theta(\exp(-M(\psi))$. Conversely, we establish that the optimal probability of distinguishing a given quantum state from the set of stabilizer states is also governed by its stabilizer entropy. These results reveal that the stabilizer entropy quantitatively characterizes the transition from stabilizer states to universal quantum states, thereby offering a comprehensive operational perspective of the stabilizer entropy as a quantum resource.
Authors: Nico Kirchner, Roderich Moessner, Frank Pollmann, Adam Gammon-Smith
Two-dimensional many-body quantum systems can exhibit topological order and support collective excitations with anyonic statistics different from the usual fermionic or bosonic ones. With the emergence of these exotic point-like particles, it is natural to ask what phases can arise in interacting many-anyon systems. To study this topic, we consider the particular case of Fibonacci anyons subject to an anyonic tight-binding model with nearest-neighbor repulsion on a two-leg ladder. Focusing on the case of half-filling, for low interaction strengths an ''anyonic'' metal is found, whereas for strong repulsion, the anyons form an insulating charge-density wave. Within the latter regime, we introduce an effective one-dimensional model up to sixth order in perturbation theory arising from anyonic superexchange processes. We numerically identify four distinct phases of the effective model, which we characterize using matrix product state methods. These include both the ferro- and antiferromagnetic golden chain, a $\mathbb{Z}_2$ phase, and an incommensurate phase.
Authors: Wojciech J. Jankowski, Robert-Jan Slager, Giandomenico Palumbo
We show that momentum-space tensor monopoles corresponding to nontrivial vector bundle generalizations, known as bundle gerbes, can be realized in bands of three-dimensional topological matter with nontrivial Hopf invariants. We provide a universal construction of tensor Berry connections in these topological phases, demonstrating how obstructions therein lead to $\mathbb{Z}$-quantized bulk magnetoelectric and nonlinear optical phenomena. We then pinpoint that these quantum effects are supported by intraband and interband torsion leading to nontrivial Dixmier-Douady classes in most known Hopf phases and in more general topological insulators realizing gerbe invariants falling beyond the tenfold classification of topological phases of matter. We furthermore provide an interacting generalization upon introducing many-body gerbe invariants by employing twisted boundary conditions. This opens an avenue to study gerbe invariants realized through higher-dimensional charge fractionalizations that can be electromagnetically probed.
Authors: Stefano Baiguera, Nicolas Chagnet, Shira Chapman, Osher Shoval
Quantum complexity of conformal field theory (CFT) states has recently gained significant attention, both as a diagnostic tool in condensed matter systems and in connection with holographic observables probing black hole interiors. Previous studies have primarily focused on cases where all generators of the conformal group contribute equally to the cost of building a circuit. In this work, we present a general framework for studying the complexity of circuits in generic Lie groups, where penalty factors assign relative weights to different generators. Our approach constructs a metric on the coset space of quantum states, induced from a (pseudo-)Riemannian norm on the space of unitary circuits. The geodesics of this metric are interpreted as optimal circuits. The method builds on the formalism of (pseudo-)Riemannian submersions and connects naturally to other prescriptions in the literature, including cost function minimization along stabilizer directions and constructions based on coadjoint orbits. As a concrete application, we compute state complexity for states in one- and two-dimensional CFTs. For specific choices of penalty factors, our prescription yields a positive-definite metric with a viable interpretation as complexity; in other cases, the resulting metric is indefinite. In the viable regime, we derive analytic results when a specific penalty factor is turned off, develop perturbative expansions for small values of the penalty factors, and provide numerical results in the general case. We comment on the relation of our measure of complexity to holography.
Authors: Yicheng Tang, Pradip Kattel, Natan Andrei
We construct a dual symmetry-protected topological (SPT) Hamiltonian for the $U(1)$ symmetric anisotropic spin-$\frac{1}{2}$ Heisenberg chain-a model that has traditionally been used to study spontaneous symmetry breaking (SSB) in both ferromagnetic and antiferromagnetic phases, with an intervening extended Luttinger liquid phase. By performing a non-local unitary transformation, we explicitly construct a local fermionic Hamiltonian that exhibits two nontrivial topological phases separated by an extended Luttinger liquid regime. We demonstrate the topological nature of these phases by analyzing the entanglement structure, deriving a non-local string order parameter, and constructing an exact zero mode operator that connects states in different fermionic parity sectors.
Authors: Yuepeng Guan, Jan M. Pawlowski, Masatoshi Yamada
We study topological properties of a quantum mechanical system with $U(1)$-symmetry within the functional renormalisation group (fRG) approach. These properties include the vacuum energy structure and the topological susceptibility. Our approach works with a complexification of the flow equation, and specifically we embed the original symmetry into the complex plane, $U(1)\rightarrow \mathbb{C}$. We compute the effective potential of a given topological sector by restricting ourselves to field configurations with a given generalised non-trivial Chern-Simons numbers. The full potential is directly constructed from these sector potentials. Our results compare well with the benchmark results obtained from solving the corresponding Schrödinger equation.
Authors: M. Mootz, C. Vaswani, C. Huang, K. J. Lee, A. Khatri, P. Mandal, J. H. Kang, L. Luo, I. E. Perakis, C. B. Eom, J. Wang
Overcoming the decoherence bottleneck remains a central challenge for advancing coherent superconducting quantum device and information technologies. Solitons -- non-dispersive wave packets stabilized by the collective synchronization of quantum excitations -- offer a robust pathway to mitigating dephasing, yet their realization in superconductors has remained experimentally elusive. Here, we report the observation of a driven soliton state in epitaxial thin films of an iron-based superconductor (Co-doped BaFe$_2$As$_2$), induced by intense, multi-cycle terahertz (THz) periodic driving. The dynamical transition to this soliton state is marked by the emergence of Floquet-like spectral sidebands that exhibit a strongly nonlinear dependence on THz laser field strength and a resonant enhancement with temperature. Quantum kinetic simulations corroborate these observations, allowing us to underpin the emergence of synchronized Anderson pseudo-spin oscillations -- analogous to Dicke superradiance -- mediated by persistent order parameter oscillations. In this coherently driven state, the observed sidebands result from difference-frequency mixing between the THz drive and persistent soliton dynamics. These findings establish a robust framework for coherently driving and controlling superconducting soliton time-crystal-like phases using low dissipation, time-periodic THz fields, enabling prospects for THz-speed quantum gate operations, long-lived quantum memory, and robust quantum sensing based on enhanced macroscopic pseudo-spin coherence.
Authors: Nathanan Tantivasadakarn, Xinyu Liu, Xie Chen
Generalized symmetry extends the usual notion of symmetry to ones that are of higher-form, acting on subsystems, non-invertible, etc. The concept was originally defined in the field theory context using the idea of topological defects. On the lattice, an immediate consequence is that a symmetry twist is moved across the system by a sequential quantum circuit. In this paper, we ask how to obtain the full, potentially non-invertible symmetry action from the unitary sequential circuit and how the connection to sequential circuit constrains the properties of the generalized symmetries. We find that for symmetries that contain the trivial symmetry operator as a fusion outcome, which we call annihilable symmetries, the sequential circuit fully determines the symmetry action and puts various constraints on their fusion. In contrast, for unannihilable symmetries, like that whose corresponding twist is the Cheshire string, a further 1D sequential circuit is needed for the full description. Matrix product operator and tensor network operator representations play an important role in our discussion.
Authors: Aviv Taller, Thomas Vidick
We generalize Håstad's long-code test for projection games and show that it remains complete and sound against entangled provers. Combined with a result of Dong et al. \cite{Dong25}, which establishes that $\MIP^*=\RE$ with constant-length answers, we derive that $\LIN^*_{1-\epsilon,s}=\RE$, for some $1/2< s<1$ and for every sufficiently small $\epsilon>0$, where LIN refers to linearity (over $\mathbb{F}_2$) of the verifier predicate. Achieving the same result with $\epsilon=0$ would imply the existence of a non-hyperlinear group.
Authors: Masaya Kunimi, Takafumi Tomita
We investigate the magnetic-field dependence of the interaction between two Rydberg atoms, $|nS_{1/2}, m_J\rangle$ and $|(n+1)S_{1/2}, m_J\rangle$. In this setting, the effective spin-1/2 Hamiltonian takes the form of an {\it XXZ} model. We show that the anisotropy parameter of the {\it XXZ} model can be tuned by applying a magnetic field, and in particular, that it changes drastically near the Förster resonance points. Based on this result, we propose experimental realizations of spin-1/2 and spin-1 Heisenberg-type quantum spin models in Rydberg atom quantum simulators, without relying on Floquet engineering. Our results provide guidance for future experiments of Rydberg atom quantum simulators and offer insight into quantum many-body phenomena emerging in the Heisenberg model.
Authors: Wenbin Li, Ivan Villani, Ylea Vlamidis, Matteo Carrega, Letizia Ferbel, Leonardo Sabattini, Antonio Rossi, Wen Si, Stefano Veronesi, Camilla Coletti, Sergio Pezzini, Masahiro Haze, Yukio Hasegawa, Stefan Heun
Owing to its superconducting properties, Niobium (Nb) is an excellent candidate material for superconducting electronics and applications in quantum technology. Here we perform scanning tunneling microscopy and spectroscopy experiments on Nb films covered by a thin gold (Au) film. We investigate the minigap structure of the proximitized region and provide evidence for a highly transparent interface between Nb and Au, beneficial for device applications. Imaging of Abrikosov vortices in presence of a perpendicular magnetic field is reported. The data show vortex pinning by the granular structure of the polycrystalline Au film. Our results show robust and homogeneous superconducting properties of thin Nb film in the presence of a gold capping layer. The Au film not only protects the Nb from surface oxidation but also preserves its excellent superconducting properties.
Authors: Masoumeh Davoudiniya, Jonas Fransson, Biplab Sanyal
Lattice vibrations critically shape charge and spin transport by governing carrier scattering, spin-charge interactions and spectral redistribution in nanostructures. In this study, we investigate how electron-phonon coupling (EPC) and structural configurations intertwine in magnetic and nonmagnetic $\beta_{12}$-borophene nanoribbons (BNRs). Using a tight-binding framework with site-dependent hopping parameters extracted from ab initio calculations and incorporating phonons within the Holstein model, we compute phonon-renormalized Green's functions and transport currents via the Landauer-Büttiker formalism. We find that spin-dependent EPC enhances spin-dependent current in magnetic zigzag (ZZ) nanoribbons, driven by phonon-induced inelastic scattering and spin-selective band renormalization. Additionally, we observe an enhancement of charge transport current in the nonmagnetic configurations of $\beta_{12}$-BNRs. Structural variations further induce anisotropic EPC effects, significantly reshaping charge and spin transport. These insights establish EPC as a powerful design lever for optimizing borophene-based logic devices through tailored edge engineering.
Authors: Cristian A. Galvis-Florez, Ahmad Farooq, Simo Särkkä
Probabilistic machine learning models are distinguished by their ability to integrate prior knowledge of noise statistics, smoothness parameters, and training data uncertainty. A common approach involves modeling data with Gaussian processes; however, their computational complexity quickly becomes intractable as the training dataset grows. To address this limitation, we introduce a quantum-assisted algorithm for sparse Gaussian process regression based on the random Fourier feature kernel approximation. We start by encoding the data matrix into a quantum state using a multi-controlled unitary operation, which encodes the classical representation of the random Fourier features matrix used for kernel approximation. We then employ a quantum principal component analysis along with a quantum phase estimation technique to extract the spectral decomposition of the kernel matrix. We apply a conditional rotation operator to the ancillary qubit based on the eigenvalue. We then use Hadamard and swap tests to compute the mean and variance of the posterior Gaussian distribution. We achieve a polynomial-order computational speedup relative to the classical method.
Authors: Wei Jia, Yuping Tian, Huanhuan Yang, Xiangru Kong, Zhi-Hao Huang, Wei-Jiang Gong, Jun-Hong An
Exploring topological matters with exotic quantum states can update the understanding of topological phases and broaden the classification of topological materials. Here, we report a class of unconventional hybrid-order topological insulators (HyOTIs), which simultaneously host various different higher-order topological states in a single $d$-dimensional ($d$D) system. Such topological states exhibit a unique bulk-boundary correspondence that is different from first-order topological states, higher-order topological states, and the coexistence of both. Remarkably, we develop a generic surface theory to precisely capture them and firstly discover a $3$D unconventional HyOTI protected by inversion symmetry, which renders both second-order (helical) and third-order (corner) topological states in one band gap and exhibits a novel bulk-edge-corner correspondence. By adjusting the parameters of the system, we also observe the nontrivial phase transitions between the inversion-symmetric HyOTI and other conventional phases. We further propose a circuit-based experimental scheme to detect these interesting results. Particularly, we demonstrate that a modified tight-binding model of bismuth can support the unconventional HyOTI, suggesting a possible route for its material realization. This work shall significantly advance the research of hybrid topological states in both theory and experiment.
Authors: Tao-Ran Hu, Su Chen, Katsuyoshi Sone, Feng-Kun Guo, Tetsuo Hyodo, Ian Low
Recently entanglement suppression was proposed to be one potential origin of emergent symmetries. In this work, we extend this theoretical framework to accommodate particles with arbitrary spins and/or arbitrary group representations. As case studies, we discuss recent efforts to test the entanglement-suppression conjecture in two hadron systems that exhibit possible emergent symmetries. The first concerns interactions involving spin-3/2 baryons, where entanglement suppression gives rise to symmetries such as $\text{SU}(40)$ spin-flavor symmetry. The second system involves low-energy scattering of heavy mesons, where entanglement suppression leads to an enhancement of the inherent heavy-quark spin symmetry to a light-quark spin symmetry, predicting additional siblings for the prominent exotic double-charm meson $T_{cc}(3875)^+$. These predictions should be confronted against experimental data and lattice results to further test the minimal-entanglement conjecture.
Authors: Filippo Utro, Meltem Tolunay, Kahn Rhrissorrakrai, Tanvi P. Gujarati, Jie Shi, Sara Capponi, Mirko Amico, Nate Earnest-Noble, Laxmi Parida
Chimeric antigen receptor (CAR) T-cells are T-cells engineered to recognize and kill specific tumor cells. Through their extracellular domains, CAR T-cells bind tumor cell antigens which triggers CAR T activation and proliferation. These processes are regulated by co-stimulatory domains present in the intracellular region of the CAR T-cell. Through integrating novel signaling components into the co-stimulatory domains, it is possible to modify CAR T-cell phenotype. Identifying and experimentally testing new CAR constructs based on libraries of co-stimulatory domains is nontrivial given the vast combinatorial space defined by such libraries. This leads to a highly data constrained, poorly explored combinatorial problem, where the experiments undersample all possible combinations. We propose a quantum approach using a Projected Quantum Kernel (PQK) to address this challenge. PQK operates by embedding classical data into a high dimensional Hilbert space and employs a kernel method to measure sample similarity. Using 61 qubits on a gate-based quantum computer, we demonstrate the largest PQK application to date and an enhancement in the classification performance over purely classical machine learning methods for CAR T cytotoxicity prediction. Importantly, we show improved learning for specific signaling domains and domain positions, particularly where there was lower information highlighting the potential for quantum computing in data-constrained problems.
Authors: Jules Sueiro, Gian Marcello Andolina, Marco Schirò
We use Floquet theory and the High-Frequency expansion to derive an effective Hamiltonian for electrons coupled to an off resonant cavity mode, either in its vacuum or driven by classical light. For vacuum fields, we show that long-range hopping and cavity-mediated interactions arise as a direct consequence of quantum fluctuations. As an application, this method is applied to the Su-Schrieffer-Heeger (SSH) model. At high light-matter coupling, our results reveal significant deviations from mean-field predictions, with our framework capturing light-matter entanglement through the Floquet micromotion. Furthermore, the cavity-mediated interactions appearing at first order are shown to be crucial to the description of the system at sufficiently strong light-matter coupling for a fixed cavity frequency. Finally, a drive resonant with the cavity is added with the SSH chain displaying dynamical behavior dependent on the cavity parameters.
Authors: Adit Vishnu, Abhay Shastry, Dhruva Kashyap, Chiranjib Bhattacharyya
Density operators, quantum generalizations of probability distributions, are gaining prominence in machine learning due to their foundational role in quantum computing. Generative modeling based on density operator models (\textbf{DOMs}) is an emerging field, but existing training algorithms -- such as those for the Quantum Boltzmann Machine -- do not scale to real-world data, such as the MNIST dataset. The Expectation-Maximization algorithm has played a fundamental role in enabling scalable training of probabilistic latent variable models on real-world datasets. \textit{In this paper, we develop an Expectation-Maximization framework to learn latent variable models defined through \textbf{DOMs} on classical hardware, with resources comparable to those used for probabilistic models, while scaling to real-world data.} However, designing such an algorithm is nontrivial due to the absence of a well-defined quantum analogue to conditional probability, which complicates the Expectation step. To overcome this, we reformulate the Expectation step as a quantum information projection (QIP) problem and show that the Petz Recovery Map provides a solution under sufficient conditions. Using this formulation, we introduce the Density Operator Expectation Maximization (DO-EM) algorithm -- an iterative Minorant-Maximization procedure that optimizes a quantum evidence lower bound. We show that the \textbf{DO-EM} algorithm ensures non-decreasing log-likelihood across iterations for a broad class of models. Finally, we present Quantum Interleaved Deep Boltzmann Machines (\textbf{QiDBMs}), a \textbf{DOM} that can be trained with the same resources as a DBM. When trained with \textbf{DO-EM} under Contrastive Divergence, a \textbf{QiDBM} outperforms larger classical DBMs in image generation on the MNIST dataset, achieving a 40--60\% reduction in the Fréchet Inception Distance.
Authors: Andris Ambainis, Joao F. Doriguello, Debbie Lim
We propose novel classical and quantum online algorithms for learning finite-horizon and infinite-horizon average-reward Markov Decision Processes (MDPs). Our algorithms are based on a hybrid exploration-generative reinforcement learning (RL) model wherein the agent can, from time to time, freely interact with the environment in a generative sampling fashion, i.e., by having access to a "simulator". By employing known classical and new quantum algorithms for approximating optimal policies under a generative model within our learning algorithms, we show that it is possible to avoid several paradigms from RL like "optimism in the face of uncertainty" and "posterior sampling" and instead compute and use optimal policies directly, which yields better regret bounds compared to previous works. For finite-horizon MDPs, our quantum algorithms obtain regret bounds which only depend logarithmically on the number of time steps $T$, thus breaking the $O(\sqrt{T})$ classical barrier. This matches the time dependence of the prior quantum works of Ganguly et al. (arXiv'23) and Zhong et al. (ICML'24), but with improved dependence on other parameters like state space size $S$ and action space size $A$. For infinite-horizon MDPs, our classical and quantum bounds still maintain the $O(\sqrt{T})$ dependence but with better $S$ and $A$ factors. Nonetheless, we propose a novel measure of regret for infinite-horizon MDPs with respect to which our quantum algorithms have $\operatorname{poly}\log{T}$ regret, exponentially better compared to classical algorithms. Finally, we generalise all of our results to compact state spaces.
Authors: Osama A. Alsaiari, Adrien Bouhon, Robert-Jan Slager, F. Nur Ünal
While the landscape of free-fermion phases has drastically been expanded in the last decades, recently novel multi-gap topological phases were proposed where groups of bands can acquire new invariants such as Euler class. As in conventional single-gap topologies, obstruction plays an inherent role that so far has only been incidentally addressed. We here systematically investigate the nuances of the relation between the non-Bravais lattice configurations and the Brillouin zone boundary conditions (BZBCs) for any number of dimensions. Clarifying the nomenclature, we provide a general periodictization recipe to obtain a gauge with an almost Brillouin-zone-periodic Bloch Hamiltonian both generally and upon imposing a reality condition on Hamiltonians for Euler class. Focusing on three-band $\mathcal{C}_2$ symmetric Euler systems in two dimensions as a guiding example, we present a procedure to enumerate the possible lattice configurations, and thus the unique BZBCs possibilities. We establish a comprehensive classification for the identified BZBC patterns according to the parity constraints they impose on the Euler invariant, highlighting how it extends to more bands and higher dimensions. Moreover, by building upon previous work utilizing Hopf maps, we illustrate physical consequences of non-trivial BZBCs in the quench dynamics of non-Bravais lattice Euler systems, reflecting the parity of the Euler invariant. We numerically confirm our results and corresponding observable signatures, and discuss possible experimental implementations. Our work presents a general framework to study the role of non-trivial boundary conditions and obstructions on multi-gap topology that can be employed for arbitrary number bands or in higher dimensions.
Authors: Jacob A. Barandes
This paper argues that every quantum system can be understood as a sufficiently general kind of stochastic process unfolding in an old-fashioned configuration space according to ordinary notions of probability. This argument is based on an exact correspondence between the class of `indivisible' stochastic processes and quantum theory. This new stochastic-quantum correspondence demotes the wave function from a primary ontological ingredient to a secondary mathematical tool, and yields a deflationary account of exotic quantum phenomena, such as interference, decoherence, entanglement, noncommutative observables, and wave-function collapse. At a more practical level, the stochastic-quantum correspondence leads to a novel reconstruction of quantum theory, alongside the Hilbert-space, path-integral, and quasiprobability representations, and also provides a framework for using Hilbert-space methods to formulate highly generic, non-Markovian types of stochastic dynamics, with potential applications throughout the sciences.
Authors: Edoardo Alessandroni, Sergi Ramos-Calderer, Ingo Roth, Emiliano Traversi, Leandro Aolita
A major obstacle for quantum optimizers is the reformulation of constraints as a quadratic unconstrained binary optimization (QUBO). Current QUBO translators exaggerate the weight $M$ of the penalty terms. Classically known as the "Big-$M$" problem, the issue becomes even more daunting for quantum solvers, since it affects the physical energy scale. We take a systematic, encompassing look at the quantum big-$M$ problem, revealing NP-hardness in finding the optimal $M$ and establishing bounds on the Hamiltonian spectral gap $\Delta$, inversely related to the expected run-time of quantum solvers. We propose a practical translation algorithm, based on SDP relaxation, that outperforms previous methods in numerical benchmarks. Our algorithm gives values of $\Delta$ orders of magnitude greater, e.g. for portfolio optimization instances. Solving such instances with an adiabatic algorithm on 6-qubits of an IonQ device, we observe significant advantages in time to solution and average solution quality. Our findings are relevant to quantum and quantum-inspired solvers alike.
Authors: Jiahao Cao, Xinwei Li, Tianwei Mao, Wenxin Xu, Li You
Quantum metrology employs entanglement to enhance measurement precision. The focus and progress so far have primarily centered on estimating a single parameter. In diverse application scenarios, the estimation of more than one single parameter is often required. Joint estimation of multiple parameters can benefit from additional advantages for further enhanced precision. Here we report quantum-enhanced measurement of simultaneous spin rotations around two orthogonal axes, making use of spin-nematic squeezing in an atomic Bose-Einstein condensate. Aided by the $F=2$ atomic ground hyperfine manifold coupled to the nematic-squeezed $F=1$ states as an auxiliary field through a sequence of microwave (MW) pulses, simultaneous measurement of multiple spin-1 observables is demonstrated, reaching an enhancement of 3.3 to 6.3 decibels (dB) beyond the classical limit over a wide range of rotation angles. Our work realizes the first enhanced multi-parameter estimation using entangled massive particles as a probe. The techniques developed and the protocols implemented also highlight the application of two-mode squeezed vacuum states in quantum-enhanced sensing of noncommuting spin rotations simultaneously.
Authors: Sudipta Mondal, Samir Kumar Hazra, Aditi Sen De
Achieving perfect control over the parameters defining a quantum gate is, in general, a very challenging task, and at the same time, environmental interactions can introduce disturbances to the initial states as well. Here we address the problem of how the imperfections in unitaries and noise present in the input states affect the entanglement-generating power of a given quantum gate -- we refer to it as imperfect (noisy) entangling power. We observe that, when the parameters of a given unitary are chosen randomly from a Gaussian distribution centered around the desired mean, the quenched average entangling power -- averaged across multiple random samplings -- exhibits intriguing behavior like it may increase or show nonmonotonic behavior with the increase of disorder strength for certain classes of diagonal unitary operators. For arbitrary unitary operators, the quenched average power tends to stabilize, showing almost constant behavior with variation in the parameters instead of oscillating. Our observations also reveal that, in the presence of a local noise model, the input states that maximize the entangling power of a given unitary operator differ considerably from the noiseless scenario. Additionally, we report that the rankings among unitary operators according to their entangling power in the noiseless case change depending on the noise model and noise strength.
Authors: Bibek Bhandari, Irwin Huang, Ahmed Hajr, Kagan Yanik, Bingcheng Qing, Ke Wang, David I Santiago, Justin Dressel, Irfan Siddiqi, Andrew N Jordan
We theoretically explore an alternative circuit for Kerr-cat qubits based on symmetrically threaded Superconducting Quantum Interference Devices (SQUID). The Symmetrically Threaded SQUIDs (STS) architecture employs a simplified flux-pumped design that suppresses two-photon dissipation, a dominant loss mechanism in high-Kerr regimes, by engineering the drive Hamiltonian's flux operator to generate only even-order harmonics. By fulfilling two critical criteria for practical Kerr-cat qubit operation, the STS emerges as an ideal platform: (1) a static Hamiltonian with diluted Kerr nonlinearity (achieved via the STS's middle branch) and (2) a drive Hamiltonian restricted to even harmonics, which ensures robust two-photon driving with reduced dissipation. For weak Kerr nonlinearity, we find that the coherent state lifetime ($T_\alpha$) is similar between STS and SNAIL circuits. However, STS Kerr-cat qubits exhibit enhanced resistance to higher-order photon dissipation, enabling significantly extended $T_\alpha$ even with stronger Kerr nonlinearities ($\sim$10 MHz). In contrast to SNAIL, STS Kerr-cat qubits display a $T_\alpha$ dip under weak two-photon driving for high Kerr coefficient. We demonstrate that this dip can be suppressed by applying drive-dependent detuning, enabling Kerr-cat qubit operation with only eight Josephson junctions (of energies 80 GHz); fewer junctions suffice for higher junction energies. We further validate the robustness of the STS design by studying the impact of strong flux driving and asymmetric Josephson junctions on $T_\alpha$. With the proposed design and considering a cat size of 10 photons, we predict $T_\alpha$ of the order of tens of milliseconds, even in the presence of multi-photon heating and dephasing effects.
Authors: Ge Bai, Francesco Buscemi, Valerio Scarani
Bayes' rule, which is routinely used to update beliefs based on new evidence, can be derived from a principle of minimum change. This principle states that updated beliefs must be consistent with new data, while deviating minimally from the prior belief. Here, we introduce a quantum analog of the minimum change principle and use it to derive a quantum Bayes' rule by minimizing the change between two quantum input-output processes, not just their marginals. This is analogous to the classical case, where Bayes' rule is obtained by minimizing several distances between the joint input-output distributions. When the change maximizes the fidelity, the quantum minimum change principle has a unique solution, and the resulting quantum Bayes' rule recovers the Petz transpose map in many cases.
Authors: Pei Wang
We propose random non-Hermitian Hamiltonians to model the generic stochastic nonlinear dynamics of a quantum state in Hilbert space. Our approach features an underlying linearity in the dynamical equations, ensuring the applicability of techniques used for solving linear systems. Additionally, it offers the advantage of easily incorporating statistical symmetry, a generalization of explicit symmetry to stochastic processes. To demonstrate the utility of our approach, we apply it to describe real-time dynamics, starting from an initial symmetry-preserving state and evolving into a randomly distributed, symmetry-breaking final state. Our model serves as a quantum framework for the transition process, from disordered states to ordered ones, where symmetry is spontaneously broken.
Authors: Hanyu Xue
In this paper, we develop a mathematical framework that generalizes the concept of anyon statistics, based on string operators and many-body Hilbert space, to invertible topological excitations of any dimensions. This framework is only build on several fundamental facts in quantum mechanics, while providing a novel, rigorous, and self-contained theory of statistics. Our definition of statistics can be computed directly using a computer, and the result aligns with existing physical theories.
Authors: Dharmaraj Ramachandran, Aditya Dubey, Subrahmanyam S. G. Mantha, Radhika Vathsan
We introduce an entanglement measure, the Modified Bloch Norm (MBN), for finite-dimensional bipartite mixed states, based on the improved Bloch matrix criteria. MBN is demonstrated to be effective in analyzing the dynamics of bound entanglement--a valuable resource for quantum protocols where free entanglement may not be available. Through examples, we illustrate the applications of MBN in accurately estimating the Entanglement Sudden Death (ESD) time and detecting behavior such as the freezing of bound entanglement. Additionally, we show that the error rate for entanglement measured using state estimation from a limited number of measurement copies is significantly lower when using MBN compared to negativity. This demonstrates the robustness of MBN under practical constraints.
Authors: Vinayak Jagadish, R. Srikanth
As is well known, unital Pauli maps can be eternally non-CP-divisible. In contrast, here we show that in the case of non-unital maps, eternal non-Markovianity in the non-unital part is ruled out. In the unital case, the eternal non-Markovianity can be obtained by a convex combination of two dephasing semigroups, but not all three of them. We study these results and ramifications arising from them.
Authors: Pei Wang
We propose a relativistic model of spontaneous wave-function collapse, based on a random nonHermitian action where the fermion density operator is coupled to a universal colored noise. Upon quantization, the wave function obeys a nonlinear stochastic differential equation that respects statistical Lorentz symmetry. The localization mechanism is driven by the colored noise, derived from the d'Alembert equation using generalized stochastic calculus in 1+3-dimensional spacetime. We analytically determine the noise-induced localization length, which decreases as the size of the observable universe increases.
Authors: Ivan Novikau, Ilon Joseph
We propose an efficient block-encoding technique for the implementation of the Linear Combination of Hamiltonian Simulations (LCHS) for modeling dissipative initial-value problems. This algorithm approximates a target nonunitary operator as a weighted sum of Hamiltonian evolutions, thereby emulating a dissipative problem by mixing various time scales. We introduce an efficient encoding of the LCHS into a quantum circuit based on a simple coordinate transformation that turns the dependence on the summation index into a trigonometric function. This significantly simplifies block-encoding of a dissipative problem and allows one to perform an exponential number of Hamiltonian simulations by a single Quantum Signal Processing (QSP) circuit. The resulting LCHS circuit has high success probability and scales logarithmically with the number of terms in the LCHS sum and linearly with time. We verify the quantum circuit and its scaling by simulating it on a digital emulator of fault-tolerant quantum computers and, as a test problem, solve the advection-diffusion equation. The proposed algorithm can be used for modeling a wide class of nonunitary initial-value problems including the Liouville equation and linear embeddings of nonlinear systems.
Authors: Jean F. Gomez, Hermann L. Albrecht
Given the current interest in quantum information tasks with higher-dimensional systems, we present three possible extensions of the bit-flip channel to qutrit systems based on different interpretations of the channel. We compared our results with the commonly used cyclic one-parameter trit flip channels and demonstrated that they are particular cases of our more general formulation. Also, we extended these channels to higher-dimensional qudit systems, therefore defining different dit flip channels. Finally, we studied their impact on the Negativity, as an entanglement measure, of qubit-qutrit and 2-qutrit Werner states. In doing so, we demonstrated the inequivalence of these versions, as they affect the states' entanglement in very distinct ways.
Authors: Ankit Kumar, Luis Cort, Marcin Łobejko, Alejandro Jenkins, Michał Horodecki
We investigate the macroscopic dynamics of Josephson charge pumps in the light of Alicki et al.'s theoretical description of the Josephson junction as an open quantum system described by a Markovian master equation. Once the electrostatic interaction between the terminals is taken into account via nonlinear capacitive terms in the Hamiltonian, we find that the resulting description of pumping is physically reasonable and in good qualitative agreement with experimental observations. We comment on how this approach relates to other theoretical treatments of quantum pumps based on time-dependent potentials or scattering amplitudes. We also highlight the significance of our results in the broader context of the dynamics of charge pumping by active systems.
Authors: Igor E. Protsenko, Alexander V. Uskov
The quantum nonlinear Maxwell-Bloch equations for a single-mode laser with a two-level active medium are solved in the LED regime without adiabatic elimination of the medium polarization, when the population fluctuation spectrum is much narrower than the radiation and polarization spectra. It is shown that population fluctuations significantly increase the output power and collective Rabi splitting of a superradiant LED.
Authors: Youngrong Lim, Changhun Oh
We present efficient classical algorithms to approximate expectation values and probability amplitudes in linear optical circuits. Specifically, our classical algorithm efficiently approximates the expectation values of observables in linear optical circuits for arbitrary product input states within an additive error under a mild condition. This result suggests that certain applications of linear optical circuits relying on expectation value estimation, such as photonic variational algorithms, may face challenges in achieving quantum advantage. In addition, the (marginal) output probabilities of boson sampling with arbitrary product input states can be efficiently approximated using our algorithm, implying that boson sampling can be efficiently simulated if its output probability distribution is polynomially sparse. Moreover, our method generalizes Gurvits's algorithm, originally designed to approximate the permanent, to also approximate the hafnian of complex symmetric matrices with an additive error. The algorithm also solves a molecular vibronic spectra problem for arbitrary product input states as precisely as boson samplers. Finally, our method extends to near-Clifford circuits, enabling the classical approximation of their expectation values of any observables and (marginal) output probabilities.
Authors: Khalil Al Salahat, Mohamad El Moussawi, Ali J. Ghandour
Synthetic Aperture Radar (SAR) plays a vital role in remote sensing due to its ability to capture high-resolution images regardless of weather conditions or daylight. However, to transform the raw SAR signals into interpretable imagery, advanced data processing techniques are essential. A widely used technique for this purpose is the Range Doppler Algorithm (RDA), which takes advantage of Fast Fourier Transform (FFT) to convert signals into the frequency domain for further processing. However, the computational cost of this approach becomes significant when dealing with large datasets. This paper presents a Quantum Range Doppler Algorithm (QRDA) that utilizes the Quantum Fourier Transform (QFT) to accelerate processing compared to the classical FFT. Furthermore, it introduces a quantum implementation of the Range Cell Migration Correction (RCMC) in the Fourier domain, a critical step in the RDA pipeline that realigns the received echoes so that the energy from a target is concentrated in a single range bin across all azimuth positions. The performance of the quantum RCMC is evaluated and compared against its classical counterpart, demonstrating the potential of quantum computing in advanced SAR imaging.
Authors: Jayanth Jayakumar, Marco Barbieri, Magdalena Stobińska
The Cramér-Rao bound captures completely the performance of single-parameter quantum sensors. On the other hand, its extension to multiple parameters demands more caution. Different aspects need to be captured at once, including, critically, compatibility. In this article we consider compatibility in quantum interferometry for an important class of probe states, measured by double homodyne or photon counters, standard benchmarks for these applications. We include the presence of loss and phase diffusion in the estimation of a phase. Our results illustrate how different weighting of the precision on individual parameters affects their compatibility, adding to the list of considerations for quantum multiparameter estimation.
Authors: Xinxian Chen, Ignacio Franco
We introduce an efficient method TTN-HEOM for exactly calculating the open quantum dynamics for driven quantum systems interacting with highly structured bosonic baths by combining the tree tensor network (TTN) decomposition scheme to the bexcitonic generalization of the numerically-exact hierarchical equations of motion (HEOM). The method yields a series of quantum master equations for all core tensors in the TTN that efficiently and accurately capture the open quantum dynamics for non-Markovian environments to all orders in the system-bath interaction. These master equations are constructed based on the time-dependent Dirac--Frenkel variational principle which isolates the optimal dynamics for the core tensors given the TTN ansatz. The dynamics converges to the HEOM when increasing the rank of the core tensors, a limit in which the TTN ansatz becomes exact. We introduce TENSO, Tensor Equations for Non-Markovian Structured Open systems, as a general-purpose Python code to propagate the TTN-HEOM dynamics. We implement three general propagators for the coupled master equations: Two fixed-rank methods that require a constant memory footprint during the dynamics, and one adaptive-rank method with variable memory footprint controlled by the target level of computational error. We exemplify the utility of these methods by simulating a two-level system coupled to a structured bath containing one Drude--Lorentz component and eight Brownian oscillators, which is beyond what can presently be computed using the standard HEOM. Our results show that the TTN-HEOM is capable to simulate both dephasing and relaxation dynamics of driven quantum system interacting with structured baths, even those of chemical complexity, with affordable computational cost.
Authors: Andrea Barzaghi, Maëlle Bénéfice, Francesco Ceccarelli, Giacomo Corrielli, Valerio Galli, Marco Gardina, Vittorio Grimaldi, Jakub Kaczorowski, Francesco Malaspina, Roberto Osellame, Ciro Pentangelo, Andrea Rocchetto, Alessandro Rudi
We report the fabrication of the first 24-mode universal photonic processor (UPP) realized through femtosecond laser writing (FLW), marking the most complex UPP demonstrated to date. Optimized for quantum dot emission at 925 nm, the device exhibits total insertion losses averaging only 4.35 dB, enabling its direct application in advanced multi-photon quantum experiments. Leveraging the versatility of FLW, we introduce suspended waveguides and precisely engineered 2D and 3D microstructures, significantly enhancing thermal isolation and minimizing power dissipation. As a result, our processor operates efficiently at less than 10 W, requiring only a simple thermo-electric cooler for stable thermal management. The device exhibits exceptional performance after calibration, implementing Haar-random unitary transformations with an amplitude fidelity of 99.7 %. This work establishes FLW-based integrated photonics as a scalable and robust platform for advancing quantum computing, communication, and sensing technologies.
Authors: Arash Azizi
We investigate the structure and uniqueness of squeezed vacuum states defined by annihilation conditions of the form $(a - \alpha a^\dagger)|\psi\rangle = 0$ and their multimode generalizations, with applications to the Thermofield Double (TFD) state in quantum field theory. For $N=1$ and $N=2$, we demonstrate that these conditions uniquely define the single- and two-mode squeezed vacua, generated by unitary squeezing operators. A key result is the unitary reformulation of the TFD state, expressed as a product of two-mode squeezing operators, ensuring invertibility and resolving the non-unitary paradox in the Minkowski--Rindler vacuum correspondence. Extending to cyclic annihilation conditions $(a_i - \alpha_i a_{i+1}^\dagger)|\psi\rangle = 0$ with $a_{N+1} \equiv a_1$, we find that non-trivial squeezed states exist only for $N=2$. For $N > 2$, we establish a no-go theorem, proving no normalizable, non-trivial solutions exist, revealing a fundamental limit on cyclic multi-mode entanglement. These results highlight the bipartite nature of TFD-like entanglement and constrain multipartite generalizations in multi-region quantum field theories.
Authors: Elizaveta I. Malevannaya, Viktor I. Polozov, Anton I. Ivanov, Aleksei R. Matanin, Nikita S. Smirnov, Vladimir V. Echeistov, Dmitry O. Moskalev, Dmitry A. Mikhalin, Denis E. Shirokov, Yuri V. Panfilov, Ilya A. Ryzhikov, Aleksander V. Andriyash, Ilya A. Rodionov
In this review, we provide a practical guide on protection of superconducting quantum circuits from broadband electromagnetic and infrared-radiation noise by using cryogenic shielding and filtering of microwave lines. Recently, superconducting multi-qubit processors demonstrated quantum supremacy and quantum error correction below the surface code threshold. However, the decoherence-induced loss of quantum information still remains a challenge for more than 100 qubit quantum computing. Here, we review the key aspects of superconducting quantum circuits protection from stray electromagnetic fields and infrared radiation, namely, multilayer shielding design, materials, filtering of the fridge lines and attenuation, cryogenic setup configurations, and methods for shielding efficiency evaluation developed over the last 10 years. In summary, we make recommendations for creation of an efficient and compact shielding system as well as microwave filtering for a large-scale superconducting quantum systems.
Authors: Aleksei R. Matanin, Nikita S. Smirnov, Anton I. Ivanov, Victor I. Polozov, Daria A. Moskaleva, Elizaveta I. Malevannaya, Margarita V. Androshuk, Yulia A. Agafonova, Denis E. Shirokov, Aleksander V. Andriyash, Ilya A. Rodionov
Microwave quantum memory represents a critical component for quantum radars and resource-efficient approaches to quantum error correction. Superconducting microwave resonators provide highly efficient storage, long coherence times, on-demand reading and even in memory pulse engineering, but it is still challenging to overcome design and materials induced loss channels for on-chip realization. In this work, we present a novel architecture of integrated superconducting quantum memory with a dynamically controlled RF-SQUID coupling element in pulse regime, thus ensuring high efficiency storage and cycling storage time. It demonstrates a memory cycle time of 1.51 $\mu s$ and 57.5% storage fidelity with preservation of the stored pulse shape during the retrieval at single-photon level excitations. We establish that while the proposed active coupler realization introduces no measurable fidelity degradation, the primary limitation arises from impedance matching and materials imperfections. The proposed architecture highlights a disruptive potential for on-chip qubit and memory integration for scalable quantum error correction, while identifying specific avenues for near-unity storage fidelity.
Authors: Sacha Greenfield, Archana Kamal, Justin Dressel, Eli Levenson-Falk
The quantum Zeno effect is a striking feature of quantum mechanics with foundational implications and practical applications in quantum control, error suppression, and error correction. The effect has branched off into a variety of different interpretations, making it easy to miss the unifying features of the underlying effect. In particular, the quantum Zeno effect has been studied in the context of both selective and nonselective measurements; for both pulsed and continuous interactions; for suppression and enhancement of decay (Zeno / anti-Zeno effects); and even in the absence of measurement entirely. This concise review presents a unified picture of these effects by examining how they all arise in the context of a driven qubit subjected to measurements or dissipation. Zeno and anti-Zeno effects are revealed as regimes of a unified effect that appears whenever a measurement-like process competes with a non-commuting evolution. The current landscape of Zeno and anti-Zeno effects is reviewed through this unifying lens, with a focus on experimental applications and implementations. The quantum Zeno effect is found to be both ubiquitous and essential for the future of near-term quantum computing.
Authors: Akhil Francis, Abhi D. Rajagopala, Norm M. Tubman, Katherine Klymko, Kasra Nowrouzi
Improving quantum algorithms run-time performance involves several strategies such as reducing the quantum gate counts, decreasing the number of measurements, advancement in QPU technology for faster gate operations, or optimizing the classical processing. This work focuses on the latter, specifically reducing classical processing and compilation time via hardware-assisted parameterized circuit execution (PCE) for computing dynamical properties of quantum systems. PCE was previously validated for QCVV protocols, which leverages structural circuit equivalencies. We demonstrate the applicability of this approach to computing dynamical properties of quantum many-body systems using structurally equivalent time evolution circuits, specifically calculating correlation functions of spin models using constant-depth circuits generated via Cartan decomposition. Implementing this for spin-spin correlation functions in Transverse field XY (up to 6-sites) and Heisenberg spin models (up to 3-sites), we observed a run-time reduction of up to 50\% compared to standard compilation methods. This highlights the adaptability of time-evolution circuit with hardware-assisted PCE to potentially mitigate the classical bottlenecks in near-term quantum algorithms.
Authors: Asghar Ullah, M. Tahir Naseem, Özgür E. Müstecaplıoğlu
We study temperature estimation using quantum probes, including single-mode initial states and two-mode states generated via stimulated parametric down-conversion in a nonlinear crystal at finite temperature. We explore both transient and equilibrium regimes and compare the performance of Gaussian and non-Gaussian probe states for temperature estimation. In the non-equilibrium regime, we show that single-mode non-Gaussian probe states - such as Fock, odd cat, and Gottesman-Kitaev-Preskill states - can significantly enhance the speed of estimation, particularly at short interaction times. In the two-mode setting, entangled states such as the two-mode squeezed vacuum, NOON state, and entangled cat state can enable access to temperature information at earlier times. In the equilibrium regime, we analyze temperature estimation using two-mode squeezed thermal states, which outperform single-mode strategies. We evaluate practical measurement strategies and find that energy-based observables yield optimal precision, population difference observables provide near-optimal precision, while quadrature-based measurements are suboptimal. The precision gain arises from squeezing, which suppresses fluctuations in the population difference.
Authors: Wonjun Lee, Minki Hhan, Gil Young Cho, Hyukjoon Kwon
Random quantum states have various applications in quantum information science. In this work, we discover a new ensemble of quantum states that serve as an $\epsilon$-approximate state $t$-design while possessing extremely low entanglement, magic, and coherence. These resources can reach their theoretical lower bounds, $\Omega(\log (t/\epsilon))$, which are also proven in this work. This implies that for fixed $t$ and $\epsilon$, entanglement, magic, and coherence do not scale with the system size, i.e., $O(1)$ with respect to the total number of qubits $n$. Moreover, we explicitly construct an ancilla-free shallow quantum circuit for generating such states by transforming $k$-qubit approximate state designs into $n$-qubit ones without increasing the support size. The depth of such a quantum circuit, $O(t [\log t]^3 \log n \log(1/\epsilon))$, is the most efficient among existing algorithms without ancilla qubits. A class of quantum circuits proposed in our work offers reduced cost for classical simulation of random quantum states, leading to potential applications in quantum information processing. As a concrete example, we propose classical shadow tomography using an estimator with superpositions between only two states, which improves the runtime of a state certification task by requiring only $O(1)$ measurements and queries.
Authors: Souvik Agasti
We investigate the connection between entanglement and non-locality between continuous-variable bipartite Gaussian states. The investigation initiates with formulating non-locality by using the phase-space Wigner representation of Bell's function. Furthermore, our analysis shows entanglement to be necessary for nonlocality, but not sufficient for it; however, nonlocality is sufficient to ensure entanglement.
Authors: Bora Baran, Timothy Heightman
Accurate and resource-efficient estimation of quantum Hamiltonians is crucial for developing practical quantum technologies, yet current methods typically demand entanglement resources or dynamical control. Here, we demonstrate a method that surpasses the standard quantum limit without requiring entanglement resources, coherent measurements, or dynamical control. Our method relies on trajectory-based Hamiltonian learning, in which we apply local, randomized pre-processing to probe states and apply the maximum-likelihood estimation to optimally scheduled Pauli measurements. Analytically, we establish the emergence of a transient Heisenberg-limited regime for short-time probes for our procedure. Furthermore, we outline how to estimate all Hamiltonian parameters in parallel using ensembles of probe states, removing the need for parameter isolation and structural priors. Finally, we supplement our findings with a numerical study, learning multiple disordered, anisotropic Heisenberg models for a 1D chain of spin-1/2 particles, featuring local transverse fields with both nearest- and next-nearest-neighbour interactions, as well as a gapless XXZ Hamiltonian. Our numerics show that our method needs only one shot per Pauli measurement, making it well-suited for experimental scenarios. The code for our method is available online and open-source.
Authors: Anumita Mukhopadhyay, Praggnyamita Ghosh, Shibdas Roy
Although many quantum channels satisfy Completely Positive Trace Preserving (CPTP) condition, there are valid quantum channels that can be non-completely positive (NCP). In a search of the conditions of noisy evolution to be a useful resource for quantum computing, we study the relation of complete positivity (CP) with unitality, where we find that a map must be non-unital in order to be NCP, but not vice-versa. As memory effects can provide advantages in the dynamics of noisy quantum systems, we investigate the relative CP condition and the CP-divisibility condition of the system and environment subsystems of a joint system-environment quantum state evolving noiselessly. We show that the system and environment channels must be both CP (NCP) or CP-divisible (CP-indivisible) for the evolution in the joint system-environment space to be unitary. We illustrate our results with examples of Bell state created from $|00\rangle$, GHZ state created from $|000\rangle$, W state created from $|100\rangle$, and the partial transpose (PT) operation acting on the Bell state.
Authors: Michelle L. Wu
We present a quantum algorithm for solving perfect mazes by casting the pathfinding task as a structured search problem. Building on Grover's amplitude amplification, the algorithm encodes all candidate paths in superposition and evaluates their proximity to the goal using a reversible fitness operator based on quantum arithmetic. A Grover-compatible oracle marks high-fitness states, and an adaptive cutoff strategy refines the search iteratively. We provide formal definitions, unitary constructions, and convergence guarantees, along with a resource analysis showing efficient scaling with maze size and path length. The framework serves as a foundation for quantum-hybrid pathfinding and planning. The full algorithmic pipeline is specified from encoding to amplification, including oracle design and fitness evaluation. The approach is readily extensible to other search domains, including navigation over tree-like or acyclic graphs.
Authors: Konstantinos Sfairopoulos, Luke Causer, Jamie F. Mair, Juan P. Garrahan
We study the triangular plaquette model (TPM, also known as the Newman-Moore model) in the presence of a transverse magnetic field on a lattice with periodic boundaries in both spatial dimensions. We consider specifically the approach to the ground state phase transition of this quantum TPM (QTPM, or quantum Newman-Moore model) as a function of the system size and type of boundary conditions. Using cellular automata methods, we obtain a full characterization of the minimum energy configurations of the TPM for arbitrary tori sizes. For the QTPM, we use these cycle patterns to obtain the symmetries of the model, which we argue determine its quantum phase transition: we find it to be a first-order phase transition, with the addition of spontaneous symmetry breaking for system sizes which have degenerate classical ground states. For sizes accessible to numerics, we also find that this classification is consistent with exact diagonalization, Matrix Product States and Quantum Monte Carlo simulations.
Authors: Konstantinos Sfairopoulos, Luke Causer, Jamie F. Mair, Juan P. Garrahan
We show how the trajectories of $d$-dimensional cellular automata (CA) can be used to determine the ground states of $(d+1)$-dimensional classical spin models, and we characterise their quantum phase transition, when in the presence of a transverse magnetic field. For each of the 256 one-dimensional elementary CA we explicitly construct the simplest local two-dimensional classical spin model associated to the given CA, and we also describe this method for $d>1$ through selected examples. We illustrate our general observations with detailed studies of: (i) the $d=1$ CA Rule 150 and its $d=2$ four-body plaquette spin model, (ii) the $d=2$ CA whose associated model is the $d=3$ square-pyramid plaquette model, and (iii) two counter-propagating $d=1$ Rule 60 CA that correspond to the two-dimensional Baxter-Wu spin model. For the quantum spin models, we show that the connection to CAs implies a sensitivity on the approach to the thermodynamic limit via finite size scaling for their quantum phase transitions.
Authors: Rukmani Bai, Soumik Bandyopadhyay
Interaction plays key role in the mixing properties of a multi-component system. The miscibility-immiscibility transition (MIT) in a weakly interacting mixture of Bose gases is predominantly determined by the strengths of the intra and inter-component two-body contact interactions. On the other hand, in the strongly interacting regime interaction induced processes become relevant. Despite previous studies on bosonic mixtures in optical lattices, the effects of the interaction induced processes on the MIT remains unexplored. In this work, we investigate the MIT in the strongly interacting phases of two-component bosonic mixture trapped in a homogeneous two-dimensional square optical lattice. Particularly we examine the MIT condition when both the components are in superfluid (SF), one-body staggered superfluid (OSSF), or supersolid (SS) phases. Our study uncovers that MIT condition is significantly shaped by the interplay of competing non-local intra- and inter-component density-induced tunneling effects, as well as off-site interactions. Notably, we demonstrate that the MIT condition for the staggered superfluid phase exhibits an inequality that is inverted compared to the conventional MIT condition associated with superfluid or supersolid phases driven by local contact interactions. In addition, we present the phase diagram of the Bose-Hubbard Model incorporating non-local processes, derived using a site-decoupling mean-field approach with the Gutzwiller ansatz. Our study contributes to the better understanding of miscibility properties of multi-component systems in the strongly interacting regime.
Authors: Sandip Maiti, Debasish Banerjee, Bipasha Chakraborty, Emilie Huffman
The simulation of various properties of quantum field theories is rapidly becoming a testing ground for demonstrating the prowess of quantum algorithms. Some examples include the preparation of ground states, as well as the investigation of various simple wave packets relevant for scattering phenomena. In this paper, we study the ability of quantum algorithms to prepare ground states in a matter-free non-Abelian $SO(3)$ lattice gauge theory in $2+1$D in a phase where the global charge conjugation symmetry is spontaneously broken. This is challenging for two reasons: the necessity of dealing with a large Hilbert space for gauge theories compared to that of quantum spin models, and the closing of the gap between the two ground states, which becomes exponentially small as a function of the volume. To deal with the large Hilbert space of gauge fields, we demonstrate how the exact imposition of the non-Abelian Gauss Law in the rishon representation of the quantum link operator significantly reduces the degrees of freedom. Further, to resolve the gap, we introduce symmetry-guided ansätze in the Gauss-Law-resolved basis for trial states as the starting point for the quantum algorithms to prepare the two lowest energy states. In addition to simulation results for a range of two-dimensional system sizes, we also provide experimental results from the trapped-ion-based quantum hardware, IonQ, when working on systems with four quantum links. The experimental/simulation results derived from our theoretical developments indicate the role of metrics--such as the energy and the infidelity--to assess the obtained results.
Authors: Marek Gazdzicki, Daniel Kikola, Ivan Pidhurskyi, Leonardo Tinti
Teleportation, introduced in science fiction literature, is an instantaneous change of the position of a macroscopic object. Two teleportation-like phenomena have been predicted by quantum mechanics: quantum teleportation and, more recently, quantum particle teleportation. Here, we introduce the third teleportation-like phenomenon - apparent teleportation. It seems to be a natural consequence of the Standard Model's indistinguishable elementary particles and antiparticles. We illustrate the idea within a 1+1D toy model of particle-antiparticle creation and space-time evolution obeying transport locality. Furthermore, we propose a novel method to observe apparent teleportation driven by strong interactions through measurements of correlations between the momenta of charm and anticharm hadrons in nuclear collisions. Observing the apparent teleportation would uncover the basic transport properties of indistinguishable particles.
Authors: Bowen Yang
We provide a complete classification of Clifford quantum cellular automata (QCAs) on arbitrary metric spaces and any qudits (of prime or composite dimensions) in terms of algebraic L-theory. Building on the delooping formalism of Pedersen and Weibel, we reinterpret Clifford QCAs as symmetric formations in a filtered additive category constructed from the geometry of the underlying space. This perspective allows us to identify the group of stabilized Clifford QCAs, modulo circuits and separated automorphisms, with the Witt group of the corresponding Pedersen--Weibel category. Notably, because the Pedersen--Weibel category depends only on the large-scale (coarse) structure of the metric space, so too does the classification of Clifford QCAs. For Euclidean lattices, the classification reproduces and expands upon known results, while for more general spaces -- including open cones over finite simplicial complexes -- we relate nontrivial QCAs to generalized homology theories with coefficients in the L-theory spectrum. We also outline extensions to QCAs with symmetry and mixed qudit dimensions, and discuss how these fit naturally into the L-theoretic framework.
Authors: Taiki Haga
We introduce the time glass, a non-periodic analogue of the discrete time crystal that arises in periodically driven dissipative quantum many-body systems. This phase is defined by two key features: (i) spatial long-range order arising from the spontaneous breaking of an internal symmetry, and (ii) temporally chaotic oscillations of the order parameter, whose lifetime diverges with system size. In other words, a time glass is a state of matter in which all components evolve in a synchronized yet chaotic manner. To characterize the time glass phase, we focus on the spectral gap of the one-cycle (Floquet) Liouvillian, which determines the decay rate of the slowest relaxation mode. Numerical studies of periodically driven dissipative Ising models show that, in the time glass phase, the Liouvillian gap remains finite in the thermodynamic limit, in contrast to time crystals where the gap closes exponentially with system size. We further demonstrate that the Liouvillian gap converges to the decay rate of the order-parameter autocorrelation derived from the classical (mean-field) dynamics in the thermodynamic limit. This result establishes a direct correspondence between microscopic spectral features and emergent macroscopic dynamics in driven dissipative quantum systems. At first glance, the existence of a nonzero Liouvillian gap appears incompatible with the presence of indefinitely persistent chaotic oscillations. We resolve this apparent paradox by showing that the quantum Rényi divergence between a localized coherent initial state and the highly delocalized steady state grows unboundedly with system size. This divergence allows long-lived transients to persist even in the presence of a finite Liouvillian gap.
Authors: Adith Sai Aramthottil, Ali Emami Kopaei, Piotr Sierant, Lev Vidmar, Jakub Zakrzewski
Ergodic isolated quantum many-body systems satisfy the eigenstate thermalization hypothesis (ETH), i.e., the expectation values of local observables in the system's eigenstates approach the predictions of the microcanonical ensemble. However, the ETH does not specify what happens to expectation values of local observables within an energy window when the average over disorder realizations is taken. As a result, the expectation values of local observables can be distributed over a relatively wide interval and may exhibit nontrivial structure, as shown in [Phys. Rev. B \textbf{104}, 214201 (2021)] for a quasiperiodic disordered system for site-resolved magnetization. We argue that the non-Gaussian form of this distribution may \textit{falsely} suggest non-ergodicity and a breakdown of ETH. By considering various types of disorder, we find that the functional forms of the distributions of matrix elements of the site-resolved magnetization operator mirror the distribution of the onsite disorder. We argue that this distribution is a direct consequence of the local observable having a finite overlap with moments of the Hamiltonian. We then demonstrate how to adjust the energy window when analyzing expectation values of local observables in disordered quantum many-body systems to correctly assess the system's adherence to ETH, and provide a link between the distribution of expectation values in eigenstates and the outcomes of quench experiments.
Authors: Pietro Maria Forcella, Cesare Tresca, Antonio Sanna, Gianni Profeta
Mercury chalcogenides are a class of materials that exhibit diverse structural phases under pressure, leading to a range of exotic physical properties, including topological phases and chiral phonons. In particular, the phase diagram of mercury sulfide (HgS) remains difficult to characterize, with significant uncertainty surrounding the transition pressure between phases. Based on recent experimental results, we employ Density Functional Theory and Superconducting Density Functional Theory to investigate the pressure-induced structural phase transition in HgS and its interplay with the emergence of superconductivity as the crystal transitions from the cinnabar phase (space group P3$_1$21) to the rock salt phase (space group Fm$\bar{3}$m). Remarkably, the rocksalt phase hosts a multigap superconducting state driven by distinct Fermi surface sheets, with two dominant gaps; the unusually high critical temperature of $\sim$11 K emerges naturally within this multiband scenario, highlighting the role of interband coupling beyond isotropic models. These results place HgS among the few systems where multiband superconducting gap structures emerge under pressure.
Authors: Arash Azizi
We demonstrate that the quantum vacuum, as perceived by a uniformly accelerating observer, can be harnessed to perform a quantum Z-gate. A two-level Unruh-DeWitt detector, prepared in a superposition of its ground and excited states, undergoes a second-order interaction with the vacuum, resulting in a two-photon emission. We derive the exact analytical form of the final entangled detector-field state and show that this emission is conditional on a phase flip of the detector's initial state-the defining feature of the gate's operation. This process harvests entanglement from the Minkowski vacuum, producing photon pairs entangled across causally disconnected Rindler wedges. This work reframes acceleration-induced radiation not as thermal noise but as a coherent computational resource, offering new pathways for relativistic quantum information.
Authors: Boris Gurevich, Weihua Xie, Mohsen Yarmohammadi, Michael H. Kolodrubetz
We investigate the coupling of two spatially separated qubits via topologically protected edge states in a two-dimensional Hofstadter lattice. In this hybrid platform, the qubits are coupled to distinct edge sites of the lattice, enabling long-range interactions mediated by topological edge modes. We solve the full system Hamiltonian and analyze the resulting eigenstate structure to uncover the conditions under which coherent qubit interactions emerge. Our analysis reveals that the effective coupling is highly sensitive to the qubit placement, energy detuning, and the topological character of the edge spectrum. We obtain an analytical solution that goes beyond the perturbative regime, capturing the full interplay between the qubits and edge modes. These results provide a foundation for exploring information transport and many-body effects in engineered quantum systems where interactions are mediated by topological edge modes.
Authors: Chris Gustin, Juanjuan Ren, Sebastian Franke, Stephen Hughes
Phenomenological approaches to photon loss have long been the workhorse of cavity-QED, but prove inadequate in the presence of sufficiently broadband light-matter interactions. We present a rigorous and \emph{ab initio} derivation of a quantum master equation for a quantized optical cavity mode coupled to a dipole, using a quasinormal mode (QNM) quantization procedure for plasmonic and dielectric open-system cavity-QED, which is valid in broadband light-matter interaction regimes, including ultrastrong coupling (USC). The theory supports general three-dimensional resonators with arbitrary dispersion and loss, and thus can be applied to a wide range of open cavities. Our \emph{ab initio} and gauge-invariant approach fully recovers the recent result of Phys. Rev. Lett. 134, 123601 (2025) for the spectral density of a quantized cavity with a single dipole, exhibits a dissipative classical-quantum correspondence for bosonic Hopfield model systems, and reveals important departures from previous heuristic assumptions about system-bath coupling. We identify a new criterion for what we term the ``broadband'' dissipative regime of cavity-QED, where phenomenological models require corrections in accordance with the intrinsic and spatially-dependent complex phase of the QNM, and also shed light on \emph{fundamental limits} to single-mode models in extreme coupling regimes. Using plasmonic and dielectric cavity examples, we show validity ranges of our QNM master equation and spectral USC calculations, and discuss prospects for near-term experimental observation of broadband dissipative effects.
Authors: Paul A. Albrecht, Eric W. Fischer, Tillmann Klamroth, Peter Saalfrank
The creation of light-matter hybrid states, polaritons, in a cavity offers new intriguing opportunities to manipulate the electronic structure and electron dynamics of atoms and molecules. Here, we investigate the effect of electronic strong coupling (ESC) between atoms or molecules and field modes of a Fabry-Pérot cavity on High-Harmonic Generation (HHG) spectra within a theoretical model study. We assume that the atom or molecule is driven by an intense classical laser field, giving rise to HHG, while being strongly coupled to quantized cavity modes as described by the Pauli-Fierz Hamiltonian in the framework of molecular quantum electrodynamics (QED). Specifically, as a test case, we first consider a model Hamiltonian of a one-dimensional hydrogen atom coupled to a cavity mode, which can be treated ``numerically exact'' using grid methods. Further, a hydrogen molecule coupled to a cavity mode is considered and treated within a recently suggested QED-TD-CI (Quantum Electrodynamics Time-Dependent Configuration Interaction) method [Weidman $\textit{et al.}$, J. Chem. Phys. $\textbf{160}$, 094111 (2024)]. The resulting HHG spectra show (i) a suppression of the harmonic cutoff in line with excitation of quantum light in the cavity and, in some cases, (ii) enhancement of some harmonics of the coupled light-matter system.
Authors: Arash Azizi
Fundamental principles of quantum mechanics, such as superposition, have long been used to create nonclassical "cat states" from classical-like components. We push this concept into the fully quantum regime by introducing the "Janus State"-a superposition of two squeezed vacuum states. Here, we report the discovery that this state, born from two photon-bunched components, exhibits strong photon antibunching. Our exact analytical theory reveals this emergent nonclassicality is driven by pure quantum interference. We identify a universal lower bound of $g^{(2)}=1/2$ in the asymptotic limit of vanishing squeezing, and also find a local minimum of $g^{(2)} \approx 0.567$ in a more experimentally accessible regime. The existence of this operational sweet spot fundamentally challenges the paradigm that strong antibunching requires non-Gaussian resources or post-selection. The Janus State thus constitutes a new foundational tool for quantum engineering, providing a deterministic route to generating highly nonclassical light.
Authors: Andrea Mari, Stefano Zippilli, David Vitali
We propose an experiment to test the non-classicality of the gravitational interaction. We consider two optomechanical systems that are perfectly isolated, except for a weak gravitational coupling. If a suitable resonance condition is satisfied, an optical signal can be transmitted from one system to the other over a narrow frequency band, a phenomenon that we call gravitationally induced transparency. In this framework, the challenging problem of testing the quantum nature of gravity is mapped to the easier task of determining the non-classicality of the gravitationally-induced optical channel: If the optical channel is not entanglement-breaking, then gravity must have a quantum nature. This approach is applicable without making any assumption on the, currently unknown, correct model of gravity in the quantum regime. In the second part of this work, we model gravity as a quadratic Hamiltonian interaction (e.g. a weak Newtonian force), resulting in a Gaussian thermal attenuator channel between the two systems. Depending on the strength of thermal noise, the system presents a sharp transition from an entanglement-breaking to a non-classical channel capable not only of entanglement preservation but also of asymptotically perfect quantum communication.
Authors: Weijun Cheng, Zhihai Wang, Yu-Xi Liu
Nonlocal interactions between photonic resonator array and giant atoms have attracted extensive attentions. Optimization and control of quantum states via giant atoms have been shown. We here study the dynamical scattering of a single-photon wave packet by a giant atom coupled to a two-dimensional photonic resonator array via multiple spatial points. Using several iterations of time evolutions, we can prepare an expected wave packet with a stable size and use it as the incident state for the scattering process. We show that spatially symmetric or asymmetric target scattering states of single-photon wave packet can be generated by adjusting the coupling strengths between the giant atom and different lattice sites of the resonator array. Furthermore, the dynamical scattering of the wave packets enables us to study the atomic excitation and propagating properties of the scattering states. We find that the atomic excitation has negligibly small probability during the scattering process. Our study may provide a new way to generate an expected photon state via photon scattering by a giant atom in two-dimensional photonic array.
Authors: Nils Leber, Luis Adrián Alanís Rodríguez, Alessandro Ferreri, Andreas Wolfgang Schell, David Edward Bruschi
We analyze the domain of validity of a quantum optical model describing the effects of gravitational redshift on the quantum state of photons that propagate in curved spacetime. We find that the model is consistent for small redshift, but fails for larger redshifts. Surprisingly, the range of the redshift for which the model applies is conditional not only to the gravitational parameters, but also to those that define the photonic modes. We provide an analysis to characterize this aspect and propose a solution. Finally, we discuss implications for theoretical science as well as for the development of quantum technologies.
Authors: João C. P. Porto, Carlos H. S. Vieira, Pedro R. Dieguez, Irismar G. da Paz, Lucas S. Marinho
Gaussian quantum probes have been widely used in quantum metrology and thermometry, where the goal is to estimate the temperature of an environment with which the probe interacts. It was recently shown that introducing initial position-momentum (PM) correlations in such probes can enhance the estimation precision compared to standard, uncorrelated Gaussian states. Motivated by these findings, we investigate whether PM correlations can also be advantageous in a simultaneous estimation setting, specifically, when estimating both the PM correlations themselves and the effective environment temperature that interacts with the probe. Using the Quantum Fisher Information Matrix, we derive new precision bounds for this joint estimation task. Additionally, we demonstrate that such correlations can serve as a resource to improve temperature estimation within this multiparameter context. Finally, we analyze the compatibility between the two parameters, establishing conditions under which the derived bounds can be saturated.
Authors: Brandon B. Le, Dustin Keller
We present an extraction of Compton Form Factors (CFFs) from Deeply Virtual Compton Scattering (DVCS) experiments conducted at Thomas Jefferson National Accelerator Facility, utilizing Quantum Deep Neural Networks (QDNNs). The analysis employs the standard Belitsky, Kirchner, and Müller formalism at twist-two, complemented by a fitting procedure designed to minimize model dependence in a manner analogous to conventional local fits. A pseudodata extraction test of the CFFs is performed using both Classical Deep Neural Networks (CDNNs) and QDNNs, with a detailed comparative analysis. Results indicate that QDNNs can outperform CDNNs in particular cases, offering enhanced predictive accuracy and precision even with limited model complexity. Motivated by this, we develop a metric to quantify the extent of the quantum advantage based on characteristics of DVCS experimental data. These findings underscore the promising role of QDNNs in advancing future investigations into multidimensional parton distributions and hadronic physics.
Authors: Gerd Leuchs, Luis L. Sanchez-Soto
All experimental evidence {indicates} that the vacuum is not void, but filled with something truly quantum. This is reflected by terms such as {zero-point} fluctuations, and Dirac's sea of virtual particle-antiparticle pairs, and last but not least the vacuum is the medium responsible for Maxwell's displacement current. While quantum electrodynamics (QED) is an exceptionally successful theory, it remains a perturbative framework rather than a fully self-contained one. Inherently, it includes singularities and divergences, which prevent the precise calculation of fundamental quantities such as the fine-structure constant $\alpha$. Any direct attempt to compute $\alpha$ results in divergent values. However, and most remarkable, what can be determined is how $\alpha$ ``runs", meaning how it varies with energy or exchanged momentum. In this article, we review the historical development of these ideas, the current state of knowledge, and ongoing efforts to find ways to move further. This includes a simple model to describe vacuum polarization in the low-energy regime, when considering only small (linear) deviations from the equilibrium {state}, relating {Maxwell's displacement} in the vacuum, to the quantum properties of the vacuum.
Authors: S. Saner, O. Băzăvan, D. J. Webb, G. Araneda, C. J. Ballance, R. Srinivas, D. M. Lucas, A. Bermúdez
Quantum simulations of lattice gauge theories (LGTs) with both dynamical matter and gauge fields provide a promising approach to studying strongly coupled problems beyond classical computational reach. Yet, implementing gauge-invariant encodings and real-time evolution remains experimentally challenging. Here, we demonstrate a resource-efficient encoding of a $\mathbb{Z}_2$ LGT using a hybrid qubit-oscillator trapped-ion quantum device, where qubits represent gauge fields and vibrational modes naturally encode bosonic matter fields. This architecture utilises synthetic dimensions to construct higher-dimensional lattice geometries and combines digital and analogue techniques to prepare initial states, realise gauge-invariant real-time evolution, and measure the relevant observables. We experimentally probe dynamics obeying Gauss's law in a $\mathbb{Z}_2$ link and extend this to a loop geometry, marking the first steps towards higher-dimensional LGTs. In this quasi-2D setup, we observe Aharonov-Bohm interference for the first time with dynamical gauge fields encoding magnetic flux, demonstrating the interplay between charge and flux. Our results chart a promising path for scalable quantum simulations of bosonic gauge theories and outline a roadmap for realising exotic LGTs in higher dimensions.
Authors: Md Qutubuddin, Ilia Tutunnikov, Jianshu Cao
Motivated by the recent advances in optical imaging and tracking of wave-packet propagation in optical cavities, we systematically explore the non-Hermitian polariton dynamics within a decay- tunable multimode cavity model. The complex eigen-spectrum of the model Hamiltonian allows us to predict the incoherent-coherent transition induced by photon losses, which defines an exceptional point at resonance and evolves analytically as the wavevector shifts off-resonantly. The resulting dispersion relation, group velocity, and relaxation rate exhibit striking signatures, such as curve crossing, level repulsion, turnover, bifurcation, and coalescence, as the decay rate crosses the critical transition or the wavevector crosses the resonance. The spectral characterization leads to surprising features in the non-Hermitian wave packet dynamics: (i) maximal population relaxation rate at the critical transition; (ii) reversed propagation in the center-of-mass motion; (iii) ballistic-to-diffusion transition; (iv) contraction in the displacement and width of the polariton wave-packet. These dynamical features have complementary symmetry between the upper-polariton (UP) branch and lower-polariton (LP) branch in the two-dimensional phase diagram spanned by the photon decay rate and wavevector. Thus, the combination of complex spectral characterization and non-Hermitian wave packet propagation establishes the photon decay rate as a powerful control parameter for polariton transport, reveals the underlying symmetry in lossy cavities, and presents a starting point to incorporate other dissipative mechanisms.
Authors: Elvira Bilokon, Valeriia Bilokon, Abhijit Sen, Mohammed Th. Hassan, Andrii Sotnikov, Denys I. Bondar
Entanglement entropy is a fundamental measure of quantum correlations and a key resource underpinning advances in quantum information and many-body physics. We uncover a universal relationship between bipartite entanglement entropy and particle number after the barrier in a one-dimensional Fermi-Hubbard system with an external asymmetric potential. Using Kolmogorov-Arnold Networks - a novel machine learning architecture - we learn this relationship across a broad range of interaction strengths with near-perfect predictive accuracy. Furthermore, we propose a simple analytical binary-entropy-like expression that quantitatively captures the observed correlation for fixed parameters. Our findings open new avenues for characterizing quantum correlations in transport phenomena and provide a powerful framework for predicting entanglement evolution in quantum systems.
Authors: T. C. Adorno, A. J. D. Farias Junior, D. M. Gitman
In this work, we study the electromagnetic energy and energy rate spectra produced by a point particle in the presence of plane wave fields. Our approach is based on a semiclassical formulation, in which the current distribution that generates electromagnetic radiation is treated classically while the radiation field is quantum. Unlike the classical energy spectrum--which exhibits divergences linked to the duration of interaction between the particle and the external field--the semiclassical spectrum is finite because radiation is produced during the quantum transition from an initial state without photons to the final state with photons at time $t$. In our formulation, we find that the maximum energy spectrum emitted by the particle is linearly proportional to time or phase, depending on the external field. This allowed us not only to extract the maximum energy rate spectra emitted by the particle but also to correlate them with energy rates derived in the framework of Classical Electrodynamics and Quantum Electrodynamics.
Authors: Hanzhen Lin, Yu-Kun Lu, Vitaly Fedoseev, Yoo Kyung Lee, Jiahao Lyu, Wolfgang Ketterle
Young's double slit experiment has often been used to illustrate the concept of complementarity in quantum mechanics. If information can in principle be obtained about the path of the photon, then the visibility of the interference fringes is reduced or even destroyed. This Gedanken experiment discussed by Bohr and Einstein can be realized when the slit is replaced by individual atoms sensitive to the transferred recoil momentum of a photon which "passes through the slit". Early pioneering experiments were done with trapped ions and atom pairs created via photo-dissociation. Recently, it became possible to perform interference experiments with single neutral atoms cooled to the absolute ground state of a harmonic oscillator potential. The slits are now single atoms representing a two-level system, and the excitation in the harmonic oscillator potential is the which-way marker. In this note, we analyze and generalize two recent experiments performed with single atoms and emphasize the different ways they record which-way information.
Authors: Yunyan Lee, Ian R. Petersen, Daoyi Dong
We propose a variational framework for solving ground-state problems of open quantum systems governed by quantum stochastic differential equations (QSDEs). This formulation naturally accommodates bosonic operators, as commonly encountered in quantum chemistry and quantum optics. By parameterizing a dissipative quantum optical system, we minimize its steady-state energy to approximate the ground state of a target Hamiltonian. The system converges to a unique steady state regardless of its initial condition, and the design inherently guarantees physical realizability. To enhance robustness against persistent disturbances, we incorporate H-infinity control into the system architecture. Numerical comparisons with the quantum approximate optimization algorithm (QAOA) highlight the method's structural advantages, stability, and physical implementability. This framework is compatible with experimental platforms such as cavity quantum electrodynamics (QED) and photonic crystal circuits.
Authors: Ali Pedram, Özgür E. Müstecaplıoğlu
We study the ultimate precision limits of a spin chain, strongly coupled to a heat bath, for measuring a general parameter and report the results for specific cases of magnetometry and thermometry. Employing a full polaron transform, we derive the effective Hamiltonian and obtain analytical expressions for the quantum Fisher information (QFI) of equilibrium states in both weak coupling (WC) and strong coupling (SC) regimes for a general parameter, explicitly accounting for finite-size (FS) effects. Furthermore, we utilize Hill's nanothermodynamics to calculate an effective QFI expression at SC. Our results reveal a potential advantage of SC for thermometry at low temperatures and demonstrate enhanced magnetometric precision through control of the anisotropy parameter. Crucially, we show that neglecting FS effects leads to considerable errors in ultimate precision bounds for equilibrium thermometry. This work also highlights the inadequacy of phenomenological approaches in describing the metrological capability and thermodynamic behavior of systems at SC. Additionally, we demonstrate the effect of bath on system's phase transition at SC.
Authors: Luke M. Stewart, Gefen Baranes, Joshua Ramette, Josiah Sinclair, Vladan Vuletić
Cluster states are a useful resource in quantum computation, and can be generated by applying entangling gates between next-neighbor qubits. Heralded entangling gates offer the advantage of high post-selected fidelity, and can be used to create cluster states at the expense of large space-time overheads. We propose a low-overhead protocol to generate and merge high-fidelity many-atom entangled states into a 3D cluster state that supports fault-tolerant universal logical operations. Our simulations indicate that a state-of-the-art high-finesse optical cavity is sufficient for constructing a scalable fault-tolerant cluster state with loss and Pauli errors remaining an order of magnitude below their respective thresholds. This protocol reduces the space-time resource requirements for cluster state construction, highlighting the measurement-based method as an alternative approach to achieving large-scale error-corrected quantum processing with neutral atoms.
Authors: Jessica John Britto
Bosons with density-dependent hopping on a one dimensional lattice have been shown to emulate anyonic particles with fractional exchange statistics. Leveraging this, we construct a Josephson junction setup, where an insulating barrier in the form of a Mott-insulator is sandwiched between two superfluid phases. This is obtained by spatially varying either the statistical phase or the strength of the on-site interaction potential on which the ground state of the system depends. Utilizing numerical methods such as exact diagonalization and density renormalization group theory, the ground state properties of this setup are investigated to understand the Josephson effect in a strongly correlated regime. The dynamical properties of this model for different configurations of this model are analyzed to find the configurations that can produce the Josephson effect. Furthermore, it is observed that continuous particle flow over time is achievable in this proposed model solely by creating an initial phase difference without any external biasing.
Authors: Jen-Tsung Hsiang, Bei-Lok Hu
This third paper in this series continues the investigation of atom-field interactions in the presence of a conductor or a dielectric medium, focusing on quantum information related basic issues such as decoherence and entanglement. Here we consider the entanglement between two atoms with internal degrees of freedom modeled by a harmonic oscillator, with varying separations between them and varying distances between them and a conducting surface. These are configurations familiar in the Casimir-Polder effect, but the behavior of atom-surface entanglement is quite different from the well-studied behavior of field-induced forces. For one, while the attractive force between an atom and a conducting surface increases as they come closer, the entanglement between the atom and the quantum field actually decreases as the atom gets closer to the conductor, as shown in \cite{Rong,AFD2}. We show how different factors play out, ranging from the coupling between the atoms and the field to the coupling between the atoms, going beyond the weak coupling restrictions often found necessary in the literature. Gathering our results for the entanglement dependence on each variable concerned, we can provide a spatial topography of quantum entanglement, thus enabling a visualized understanding of the behavior of quantum field-mediated entanglement. In particular we can quantify the definition of a three-dimensional \textit{entanglement domain} between the two atoms, how it varies with their coupling, their separation and their distances from the conducting surface, and for practical applications, how to exercise effective control of the entanglement between two atoms by changing these parameters. Our findings are expected to be useful for studies of atom-field-medium interactions in vacuum and surface physics.
Authors: César Feniou, Christopher Cherfan, Julien Zylberman, Baptiste Claudon, Jean-Philip Piquemal, Emmanuel Giner
First-quantized, real-space formulations of quantum chemistry on quantum computers are appealing: qubit count scales logarithmically with spatial resolution, and Coulomb operators achieve quadratic instead of quartic computational scaling of two-electron interactions. However, existing schemes employ uniform discretizations, so the resolution required to capture electron-nuclear cusps in high-density regions oversamples low-density regions, wasting computational resources. We address this by deploying non-uniform, molecule-adaptive grids that concentrate points where electronic density is high. Using Voronoi partitions of these grids, the molecular Hamiltonian is expressed in a Hermitian form and in a transcorrelated, isospectral form that eliminates Coulomb singularities and yields cusp-free eigenfunctions. Both formulations slot naturally into quantum eigenvalue solvers: Hermitian Quantum Phase Estimation (QPE) and the recent generalised Quantum Eigenvalue Estimation (QEVE) protocol for its non-Hermitian, transcorrelated counterpart. Numerical validation on benchmark systems confirms that this non-heuristic ab initio framework offers a promising path for accurate ground-state chemistry on quantum hardware.
Authors: Domenico Pomarico, Federico Dell'Anna, Riccardo Cioli, Saverio Pascazio, Francesco V. Pepe, Paolo Facchi, Elisa Ercolessi
We present the simulation of the quench dynamics of the Z3 Schwinger model, that describes an approximation of one-dimensional Quantum Electrodynamics, on a digital noisy Rydberg atom platform, aiming at the observation of multiple dynamical quantum phase transitions. In order to reach long-time dynamics, we exploit an enconding dictated by the symmetries, combined with a circuit compression procedure. We focus on a quench that evolves the Dirac vacuum by means of a Hamiltonian depending on a negative mass parameter. This leads to resonant Rabi oscillations between the Dirac vacuum and mesonic states. The population concentration exhibits oscillations with negligible fluctuations of detuned states also with the inclusion of combined noise sources, from which we can clearly detect multiple dynamical phase transitions.
Authors: Ioana Moflic, Alan Robertson, Simon J. Devitt, Alexandru Paler
Utility-scale quantum programs contain operations on the order of $>10^{15}$ which must be prepared and piped from a classical co-processor to the control unit of the quantum device. The latency of this process significantly increases with the size of the program: existing high-level classical representations of quantum programs are typically memory intensive and do not naïvely efficiently scale to the degree required to execute utility-scale programs in real-time. To combat this limitation, we propose the utilization of high-level quantum circuit caches and compressors. The first save on the time associated with repetitive tasks and sub-circuits, and the latter are useful for representing the programs/circuits in memory-efficient formats. We present numerical evidence that caches and compressors can offer five orders of magnitude lower latencies during the automatic transpilation of extremely large quantum circuits.
Authors: Samuel Schöpa, Falk-Erik Wiechmann, Franziska Fennel, Dieter Bauer
We utilize few-level model systems to analyze the polarization and phase properties of below-threshold harmonics (BTH) in aligned molecules. In a two-level system (TLS), we find that the phase of emitted harmonics undergoes a distinct change. For harmonics with photon energies below the transition between the dominant field-dressed states, the phase alternates by $\pi$ between successive odd harmonic orders but remains constant above. Exploiting this behavior, we construct a four-level model composed of two uncoupled TLS subsystems aligned along orthogonal directions. We demonstrate that with selected transition frequencies lower-order harmonics follow the polarization of the linearly polarized driving field while higher-order harmonics exhibit a mirrored polarization. The model predicts that aligned systems with orthogonal transition dipoles may show analogous phase and polarization features in the BTH regime.
Authors: M. Rademacher, A. Pontin, J. M. H. Gosling, P. F. Barker, M. Toroš
Levitated optomechanics, the interaction between light and small levitated objects, is a new macroscopic quantum system that is being used as a testing ground for fundamental physics and for the development of sensors with exquisite sensitivity. The utility of this system, when compared to other quantum optomechanical systems, is its extreme isolation from the environment and, by the relatively few degrees of freedom that a levitated object has. While work in the field has strongly focused on the three translational degrees of freedom of this system, it has become increasingly important to understand the induced rotational motion of levitated objects, particularly in optical trapping fields, but also in magnetic and electric traps. These additional three degrees of freedom, which are intrinsic to levitated systems, offer a new set of optomechanical nonlinear interactions that lead to a rich and yet largely unexplored roto-translational motion. The control and utilization of these interactions promise to extend the utility of levitated optomechanics in both fundamental studies and applications. In this review, we provide an overview of levitated optomechanics, before focusing on the roto-translational motion of optically levitated anisotropic objects. We first present a classical treatment of this induced motion, bridging the gap between classical and quantum formalisms. We describe the different types of roto-translational motion for different particle shapes via their interaction with polarized optical trapping fields. Subsequently, we provide an overview of the theoretical and experimental approaches as well as applications that have established this new field. The review concludes with an outlook of promising experiments and applications, including the creation of non-classical states of roto-translational motion, quantum-limited torque sensing and particle characterization methods.
Authors: Tamila Kuanysheva, Brian Kendrick, Lukasz Cincio, Dmitri Babikov
We explore how the fundamental problems in quantum molecular dynamics can be modelled using classical simulators (emulators) of quantum computers and the actual quantum hardware available to us today. The list of problems we tackle includes propagation of a free wave packet, vibration of a harmonic oscillator, and tunneling through a barrier. Each of these problems starts with the initial wave packet setup. Although Qiskit provides a general method for initializing wavefunctions, in most cases it generates deep quantum circuits. While these circuits perform well on noiseless simulators, they suffer from excessive noise on quantum hardware. To overcome this issue, we designed a shallower quantum circuit for preparing a Gaussian-like initial wave packet, which improves the performance on real hardware. Next, quantum circuits are implemented to apply the kinetic and potential energy operators for the evolution of a wavefunction over time. The results of our modelling on classical emulators of quantum hardware agree perfectly with the results obtained using the traditional (classical) methods. This serves as a benchmark and demonstrates that the quantum algorithms and Qiskit codes we developed are accurate. However, the results obtained on the actual quantum hardware available today, such as IBM's superconducting qubits and IonQ's trapped ions, indicate large discrepancies due to hardware limitations. This work highlights both the potential and challenges of using quantum computers to solve fundamental quantum molecular dynamics problems.
Authors: Byungmin Kang, Patrick A. Lee
We propose cyclotron resonance as an optical probe for emergent fractionalized excitations in $\mathrm{U}(1)$ quantum spin liquids, focusing on kagome antiferromagnets. In contrast to conventional systems, where cyclotron resonance directly couples to charged carriers, spinons in spin liquids are charge-neutral and interact only through an emergent gauge field. We identify two key mechanisms by which an external physical electromagnetic field induces emergent electric and magnetic fields, enabling indirect coupling to spinons. Using these mechanisms, we compute the absorption rate of the cyclotron resonance response for Dirac spinons forming Landau levels. Our analysis shows that, although the absorption per layer is small, the absence of a skin-depth limitation in insulating spin liquids allows for cumulative absorption comparable to graphene in realistic sample sizes for the recently discovered spin-liquid candidate material YCu${}_3$(OH)${}_6$Br${}_2$[Br${}_{1-y}$(OH)${}_y$]. Our findings shows that cyclotron resonance is a viable experimental probe of spinon Landau quantization and emergent gauge fields, providing powerful positive experimental signatures of quantum spin liquids.
Authors: Bindu, Amandeep Singh, Amir Hen, Lukas Drago Cavar, Sebastian Maria Ulrich Schultheis, Shira Yochelis, Yossi Paltiel, Andrew F. May, Angela Wittmann, Mathias Klaui, Dmitry Budker, Hadar Steinberg, Nir Bar-Gill
Recently discovered 2D van der Waals magnetic materials, and specifically Iron-Germanium-Telluride ($\rm Fe_{5}GeTe_{2}$), have attracted significant attention both from a fundamental perspective and for potential applications. Key open questions concern their domain structure and magnetic phase transition temperature as a function of sample thickness and external field, as well as implications for integration into devices such as magnetic memories and logic. Here we address key questions using a nitrogen-vacancy center based quantum magnetic microscope, enabling direct imaging of the magnetization of $\rm Fe_{5}GeTe_{2}$ at sub-micron spatial resolution as a function of temperature, magnetic field, and thickness. We employ spatially resolved measures, including magnetization variance and cross-correlation, and find a significant spread in transition temperature yet with no clear dependence on thickness down to 15 nm. We also identify previously unknown stripe features in the optical as well as magnetic images, which we attribute to modulations of the constituting elements during crystal synthesis and subsequent oxidation. Our results suggest that the magnetic anisotropy in this material does not play a crucial role in their magnetic properties, leading to a magnetic phase transition of $\rm Fe_{5}GeTe_{2}$ which is largely thickness-independent down to 15 nm. Our findings could be significant in designing future spintronic devices, magnetic memories and logic with 2D van der Waals magnetic materials.
Authors: Ahmik Virani, Devraj, Anirudh Suresh, Lei Zhang, M V Panduranga Rao
Quantum computing in the Noisy Intermediate-Scale Quantum (NISQ) era presents significant challenges in differentiating quantum software bugs from hardware noise. Traditional debugging techniques from classical software engineering cannot directly resolve this issue due to the inherently stochastic nature of quantum computation mixed with noises from NISQ computers. To address this gap, we propose a statistical approach leveraging probabilistic metrics to differentiate between quantum software bugs and hardware noise. We evaluate our methodology empirically using well-known quantum algorithms, including Grover's algorithm, Deutsch-Jozsa algorithm, and Simon's algorithm. Experimental results demonstrate the efficacy and practical applicability of our approach, providing quantum software developers with a reliable analytical tool to identify and classify unexpected behavior in quantum programs.
Authors: V. E. Kuzmichev, V. V. Kuzmichev (Bogolyubov Institute for Theoretical Physics)
In this note, it is shown that a nonvanishing spatial curvature can generate primordial matter in the post-inflation era. This matter does not depend on the curvature parameter and is described by a stiff equation of state. It can have the properties of spin matter, consisting of particles with a spin quantum number (spin) s = 1/2. Such a matter can be plausibly identified with quark matter at the early stage of the evolution of the universe.
Authors: Zahra Jalali-Mola, Niklas Käming, Luca Asteria, Utso Bhattacharya, Ravindra W. Chhajlany, Klaus Sengstock, Maciej Lewenstein, Tobias Grass, Christof Weitenberg
Fluctuations are fundamental in physics and important for understanding and characterizing phase transitions. In this spirit, the phase transition to the Bose-Einstein condensate (BEC) is of specific importance. Whereas fluctuations of the condensate particle number in atomic BECs have been studied in continuous systems, experimental and theoretical studies for lattice systems were so far missing. Here, we explore the condensate particle number fluctuations in an optical lattice BEC across the phase transition in a combined experimental and theoretical study. We present both experimental data using ultracold $^{87}$Rb atoms and numerical simulations based on a hybrid approach combining the Bogoliubov quasiparticle framework with a master equation analysis for modeling the system. We find strongly anomalous fluctuations, where the variance of the condensate number $\delta N_{\rm BEC}^2$ scales with the total atom number as $N^{1+\gamma}$ with an exponent around $\gamma_{\rm theo}=0.74$ and $\gamma_{\rm exp}=0.62$, which we attribute to the 2D/3D crossover geometry and the interactions. Our study highlights the importance of the trap geometry on the character of fluctuations and on fundamental quantum mechanical properties.
Authors: D. Ojeda-Guillén, R. D. Mota, M. Salazar-Ramírez, V. D. Granados
We extend the $(1+1)$-dimensional Dirac-Moshinsky oscillator by changing the standard derivative by the Dunkl derivative. We demonstrate in a general way that for the Dirac-Dunkl oscillator be parity invariant, one of the spinor component must be even, and the other spinor component must be odd, and vice versa. We decouple the differential equations for each of the spinor component and introduce an appropriate $su(1,1)$ algebraic realization for the cases when one of these functions is even and the other function is odd. The eigenfunctions and the energy spectrum are obtained by using the $su(1,1)$ irreducible representation theory. Finally, by setting the Dunkl parameter to vanish, we show that our results reduce to those of the standard Dirac-Moshinsky oscillator.
Authors: R.D. Mota, D. Ojeda-Guillén
We introduce a generalization of the Dunkl-derivative with two parameters to study the Schrödinger equation in Cartesian and polar coordinates in two dimensions. The eigenfunctions and the energy spectrum for the harmonic oscillator and the Coulomb problem are derived in an analytical way and it is shown that our results are properly reduced to those previously reported for the Dunkl derivative with a single parameter.
Authors: Tomislav Piskor, Florian G. Eich, Michael Marthaler, Frank K. Wilhelm, Jan-Michael Reiner
We propose and analyze a method for improving quantum chemical energy calculations on a quantum computer impaired by decoherence and shot noise. The error mitigation approach relies on the fact that the one- and two-particle reduced density matrices (1- and 2-RDM) of a chemical system need to obey so-called N-representability constraints. We post-process the result of an RDM measurement by projecting it into the subspace where certain N-representability conditions are fulfilled. Furthermore, we utilize that such constraints also hold in the hole and particle-hole sector and perform projections in these sectors as well. We expand earlier work by conducting a careful analysis of the method's performance in the context of quantum computing. Specifically, we consider typical decoherence channels (dephasing, damping, and depolarizing noise) as well as shot noise due to a finite number of projective measurements. We provide analytical considerations and examine numerically three example systems, \ch{H2}, \ch{LiH}, and \ch{BeH2}. From these investigations, we derive our own practical yet effective method to best employ the various projection options. Our results show the approach to significantly lower energy errors and measurement variances of (simulated) quantum computations.
Authors: Yu-Ao Chen, Chengkai Zhu, Keming He, Mingrui Jing, Xin Wang
Quantum Markov chains generalize classical Markov chains for random variables to the quantum realm and exhibit unique inherent properties, making them an important feature in quantum information theory. In this work, we propose the concept of virtual quantum Markov chains (VQMCs), focusing on scenarios where subsystems retain classical information about global systems from measurement statistics. As a generalization of quantum Markov chains, VQMCs characterize states where arbitrary global shadow information can be recovered from subsystems through local quantum operations and measurements. We present an algebraic characterization for virtual quantum Markov chains and show that the virtual quantum recovery is fully determined by the block matrices of a quantum state on its subsystems. Notably, we find a distinction between two classes of tripartite entanglement by showing that the W state is a VQMC while the GHZ state is not. Furthermore, we introduce the virtual non-Markovianity to quantify the non-Markovianity of a given quantum state, which also assesses the optimal sampling overhead for virtually recovering this state. Our findings elucidate distinctions between quantum Markov chains and virtual quantum Markov chains, extending our understanding of quantum recovery to scenarios prioritizing classical information from measurement statistics.
Authors: Ahmed A. Akhtar, Namit Anand, Jeffrey Marshall, Yi-Zhuang You
We introduce ``dual-unitary shadow tomography'' (DUST), a classical shadow tomography protocol based on dual-unitary brick-wall circuits. To quantify the performance of DUST, we study operator spreading and Pauli weight dynamics in one-dimensional qubit systems, evolved by random two-local dual-unitary gates arranged in a brick-wall structure, ending with a measurement layer. We do this by deriving general constraints on the Pauli weight transfer matrix and specializing to the case of dual-unitarity. Remarkably, we find that operator spreading in these circuits have a rich structure resembling that of relativistic quantum field theories, with massless chiral excitations that can decay or fuse into each other, which we call left- or right-movers. We develop a mean-field description of the Pauli weight in terms of $\rho(x,t)$, which represents the probability of having nontrivial support at site $x$ and depth $t$ starting from a fixed weight distribution. We develop an equation of state for $\rho(x,t)$ and simulate it numerically using Monte Carlo simulations. For the task of predicting operators with (nearly) full support, we show that DUST outperforms brick-wall Clifford shadows of equal depth. This advantage is further pronounced for small system sizes and our results are generally robust to finite-size effects.
Authors: Ryan Smith, Zlatko Papić, Andrew Hallam
Non-stabilizer states are a fundamental resource for universal quantum computation. However,despite broad significance in quantum computing, the emergence of "many-body" non-stabilizerness in interacting quantum systems remains poorly understood due to its analytical intractability. Here we show that Rydberg atom arrays provide a natural reservoir of non-stabilizerness that extends beyond single qubits and arises from quantum correlations engendered by the Rydberg blockade. We demonstrate that this non-stabilizerness can be experimentally accessed in two complementary ways, either by performing quench dynamics or via adiabatic ground state preparation. Using the analytical framework based on matrix product states, we explain the origin of Rydberg nonstabilizerness via a quantum circuit decomposition of the wave function.
Authors: Le Chang, Saitej Yavvari, Rance Cleaveland, Samik Basu, Runzhou Tao, Liyi Li
The next generation of distributed quantum processors combines single-location quantum computing and quantum networking techniques to permit large entangled qubit groups to be established through remote processors, and quantum algorithms can be executed distributively. We present DisQ, as the first formal model of distributed quantum processors, and permit the analysis of distributed quantum programs in the new computation environment. The core of DisQ is a distributed quantum programming language that combines the concepts of Chemical Abstract Machine (CHAM) and Markov Decision Processes (MDP) with the objective of providing clearly distinguishing quantum concurrent and distributed behaviors. Based on the DisQ language, we develop a simulation relation, based on classical simulation infrastructure, to check the equivalence of a quantum algorithm and its distributed versions so that users can develop the distributed version of a sequential quantum program via a simulation check.
Authors: Zekun He, A. F. Kemper, J. K. Freericks
Adiabatic state preparation provides an analytical solution for generating the ground state of a target Hamiltonian, starting from an easily prepared ground state of the initial Hamiltonian. While effective for time-dependent Hamiltonians with an energy gap to the first coupled excited state, the process becomes exceedingly slow as the gap narrows. Rather than strictly following the adiabatic theorem, a more robust approach allows controlled diabatic excitations during the evolution and numerically optimizes the path to eliminate these excitations by the end. In this work, this is achieved via modulated time evolution, using a time dependent oscillating field lambda(t) to modulate the Hamiltonian, in conjunction with a transverse field B(t) whose optimized shape closely resembles a local adiabatic ramp. Beyond modulated time evolution, the quantum approximate optimization algorithm (QAOA), which also employs a transverse field defined as beta(t) over gamma(t), exhibits a shape similar to the local adiabatic ramp. This resemblance offers a more intuitive and physically motivated way to understand the QAOA algorithm through the lens of time evolution.
Authors: Zhu Sun, Gregory Boyd, Zhenyu Cai, Hamza Jnane, Balint Koczor, Richard Meister, Romy Minko, Benjamin Pring, Simon C. Benjamin, Nikitas Stamatopoulos
We explore the important task of applying a phase $\exp(i\,f(x))$ to a computational basis state $\left| x \right>$. The closely related task of rotating a target qubit by an angle depending on $f(x)$ is also studied. Such operations are key in many quantum subroutines, and frequently $f(x)$ can be well-approximated by a piecewise function; examples range from the application of diagonal Hamiltonian terms (such as the Coulomb interaction) in grid-based many-body simulation, to derivative pricing algorithms. Here we exploit a parallelisation of the piecewise approach so that all constituent elementary rotations are performed simultaneously, that is, we achieve a total rotation depth of one. Moreover, we explore the use of recursive catalyst `towers' to implement these elementary rotations efficiently. We find that strategies prioritising execution speed can achieve circuit depth as low as $O(\log{n}{+}\log{S})$ for a register of $n$ qubits and a piecewise approximation of $S$ sections (presuming prior preparation of enabling resource states), albeit total qubit count then scales with $S$. In the limit of multiple repetitions of the oracle, we find that catalyst tower approaches have an $O(S\cdot n)$ T-count.
Authors: Lauritz van Luijk, Alexander Stottmeister, Henrik Wilming
Embezzlement of entanglement refers to the task of extracting entanglement from an entanglement resource via local operations and without communication while perturbing the resource arbitrarily little. Recently, the existence of embezzling states of bipartite systems of type III von Neumann algebras was shown. However, both the multipartite case and the precise relation between embezzling states and the notion of embezzling families, as originally defined by van Dam and Hayden, was left open. Here, we show that finite-dimensional approximations of multipartite embezzling states form multipartite embezzling families. In contrast, not every embezzling family converges to an embezzling state. We identify an additional consistency condition that ensures that an embezzling family converges to an embezzling state. This criterion distinguishes the embezzling family of van Dam and Hayden from the one by Leung, Toner, and Watrous. The latter generalizes to the multipartite setting. By taking a limit, we obtain a multipartite system of commuting type III$_1$ factors on which every state is an embezzling state. We discuss our results in the context of quantum field theory and quantum many-body physics. As open problems, we ask whether vacua of relativistic quantum fields in more than two spacetime dimensions are multipartite embezzling states and whether multipartite embezzlement allows for an operator-algebraic characterization.
Authors: Ruoming Peng, Xuntao Wu, Yang Wang, Jixing Zhang, Jianpei Geng, Durga Bhaktavatsala Rao Dasari, Andrew N. Cleland, Jörg Wrachtrup
Solid-state spin systems hold great promise for quantum information processing and the construction of quantum networks. However, the considerable inhomogeneity of spins in solids poses a significant challenge to the scaling of solid-state quantum systems. A practical protocol to individually control and entangle spins remains elusive. To this end, we propose a hybrid spin-phonon architecture based on spin-embedded SiC optomechanical crystal (OMC) cavities, which integrates photonic and phononic channels allowing for interactions between multiple spins. With a Raman-facilitated process, the OMC cavities support coupling between the spin and the zero-point motion of the OMC cavity mode reaching 0.57 MHz, facilitating phonon preparation and spin Rabi swap processes. Based on this, we develop a spin-phonon interface that achieves a two-qubit controlled-Z gate with a simulated fidelity of $96.80\%$ and efficiently generates highly entangled Dicke states with over $99\%$ fidelity, by engineering the strongly coupled spin-phonon dark state which is robust against loss from excited state relaxation as well as spectral inhomogeneity of the defect centers. This provides a hybrid platform for exploring spin entanglement with potential scalability and full connectivity in addition to an optical link, and offers a pathway to investigate quantum acoustics in solid-state systems.
Authors: Inge S. Helland
In this article the weakest possible theorem giving a foundation behind the Hilbert space formalism of quantum theory is stated. The necessary postulates are formulated, and the mathematics is spelled out in detail. It is argued that, from this approach, a general epistemic interpretation of quantum mechanics is natural. Some applications to the Bell experiment and to decision theory are briefly discussed. The article represents the conclusion of a series of articles and books on quantum foundations.
Authors: Peng-Yu Sun, Hang Zhou, Fu-Quan Dou
Machine learning offers a promising methodology to tackle complex challenges in quantum physics. In the realm of quantum batteries (QBs), model construction and performance optimization are central tasks. Here, we propose a cavity-Heisenberg spin chain quantum battery (QB) model with spin-$j (j=1/2,1,3/2)$ and investigate the charging performance under both closed and open quantum cases, considering spin-spin interactions, ambient temperature, and cavity dissipation. It is shown that the charging energy and power of QB are significantly improved with the spin size. By employing a reinforcement learning algorithm to modulate the cavity-battery coupling, we further optimize the QB performance, enabling the stored energy to approach, even exceed its upper bound in the absence of spin-spin interaction. We analyze the optimization mechanism and find an intrinsic relationship between cavity-spin entanglement and charging performance: increased entanglement enhances the charging energy in closed systems, whereas the opposite effect occurs in open systems. Our results provide a possible scheme for design and optimization of QBs.
Authors: Jing-Min Zhu
TThe organization and structure of bipartite mixed-state quantum entanglement (QE) are more complex and less well understood compared to bipartite pure-state QE. Bipartite mixed-state QEs and their measures play a crucial role in both theory and practical applications. Some existing measures involve quantifying the minimum QE and reflect the inherently complex nature of their computation, while others are only applicable to highly limited-dimensional quantum systems. In this context, we propose a method termed Reduction-induced Variation of Partial Von Neumann Entropy to quantify QE in any bipartite states, particularly focusing on bipartite mixed states. Partial Von Neumann Entropy is merely a special case of this method,Its intuitive and clear physical representation, along with easy computation and wide applicability, facilitates exploring its potential applications. Furthermore, we present examples to demonstrate the superiorities of this method in identifying bipartite QE by comparing with other existing bipartite mixed-state QE measures through both their physical implications and mathematical structures.
Authors: Andreas Juul Bay-Smidt, Frederik Ravn Klausen, Christoph Sünderhauf, Róbert Izsák, Gemma C. Solomon, Nick S. Blunt
Quantum simulations of strongly interacting fermionic systems, such as those described by the Hubbard model, are promising candidates for useful early fault-tolerant quantum computing applications. This paper presents Tile Trotterization, a generalization of plaquette Trotterization (PLAQ), which uses a set of tiles to construct Trotter decompositions of arbitrary lattice Hubbard models. The Tile Trotterization scheme also enables the simulation of more complex models, including the extended Hubbard model. We improve previous Hubbard model commutator bounds, further provide tight commutator bounds for periodic extended Hubbard models, and demonstrate the use of tensor network methods for this task. We consider applications of Tile Trotterization to simulate hexagonal lattice Hubbard models and compare the resource requirements of Tile Trotterization for performing quantum phase estimation to a qubitization-based approach, demonstrating that Tile Trotterization scales more efficiently with system size. These advancements significantly broaden the potential applications of early fault-tolerant quantum computers to models of practical interest in materials research and organic chemistry.
Authors: Yi-Hsiang Chen, Charles H. Baldwin
Leakage errors are unwanted transfer of population outside of a defined computational subspace and they occur in almost every platform for quantum computing. While prevalent, leakage is often overlooked when measuring and reporting the fidelity of quantum gates with standard methods. In fact, when leakage is substantial it can cause a large overestimation of fidelity from the typi- cal method used to measure fidelity, randomized benchmarking. We provide several methods for properly estimating fidelity in the presence of leakage errors that are applicable in different error regimes with carefully chosen sequence lengths. Then, we numerically demonstrate the methods for two-qubit randomized benchmarking, which often have the largest errors. Finally, we reanalyze previously shared data from Quantinuum systems with some of the methods provided.
Authors: Bin Sun, Geng-Bin Cao, Xi-Dan Hu, Dan-Bo Zhang
Quantum operator scrambling describes the spreading of local operators into the whole system in the picture of Heisenberg evolution, which is often quantified by the operator size growth. Here we propose a measure of quantum operator scrambling via Holevo information of operators, by taking its capacity to distinguish operator information locally. We show that the operator size is closely related to a special kind of Holevo information of operators. Moreover, we propose a feasible protocol for measuring Holevo information of operators on digital quantum simulators based on random states. \textcolor{black}{For the mixed-field Ising model,} our numerical simulations show that the integrable system can be told apart from the chaotic system by measuring the spatial-temporal patterns of Holevo information. Furthermore, we find that error mitigation is required to restore the time-oscillation behavior of Holevo information for the integrable system, a crucial feature distinct from the chaotic one. Our work provides a new perspective to understand the information scrambling and quantum chaos from aspects of Holevo information of operators.
Authors: Dmitry Melnikov
These notes review a description of quantum mechanics in terms of the topology of spaces, basing on the axioms of Topological Quantum Field Theory and path integral formalism. In this description quantum states and operators are encoded by the topology of spaces that are used as modules to build the quantum mechanical model, while expectation values and probabilities are given by topological invariants of spaces, knots and links. The notes focus on the specific way the topology encodes quantum mechanical features, or, equivalently, on how these features can be controlled through the topology. A topological classification of entanglement is discussed, as well as properties of entanglement entropy and basic quantum protocols. The primary aim is to build a less conventional diagrammatic intuition about quantum mechanics, expanding the paradigm of ``Quantum Picturalism".
Authors: Matthew Brooks, Foster Sabatino, Charles Tahan, Silas Hoffman
While the adiabatic exchange of Majorana zero modes (MZMs) enables a non-universal set of geometrically protected gates, realising an experimental implementation of MZM braiding remains challenging. In an alternative proposal, charge-parity measurement of two neighboring MZMs supports braiding by teleportation. Moreover, owing to the lack of definitive evidence of MZMs in semiconducting systems, there have been several simulations of MZMs on NISQ devices which more naturally lend themselves to braiding. In this work, measurement-based braiding about MZM Y-junctions are simulated by multi-qubit Pauli-parity measurements of a logical qubit. Logical single-qubit geometric $S^{(\dagger)}$-gates and entangling two-qubit gates is shown using two-physical-qubit joint measurements alone, whilst partial phase rotations such as a $T^{(\dagger)}$-gates require at least one three-qubit joint measurement. These relatively small scale circuits offer both novel measurement-based geometric gates as well as a measurement-based demonstration of quantum Hamiltonian simulation.
Authors: Tobias Kehrer, Christoph Bruder
Recently, the synchronization of coupled quantum oscillators has attracted a great deal of interest. Synchronization requires driven constituents, and in such systems, the coupling can be designed to be nonreciprocal. Nonreciprocally coupled oscillators exhibit a rich variety of behavior including active traveling-wave-type states. In this work, we study the interplay of three competing synchronization mechanisms in a setup of two nonreciprocally coupled quantum van der Pol oscillators. One of the oscillators is driven externally which induces phase locking. A dissipative interaction leads to antiphase locking, whereas a coherent interaction nurtures bistable phase locking and active states. We approximate the phase diagram of the quantum case by evaluating the synchronization measure of a perturbation expansion of the steady state. Effective unidirectional interactions lead to synchronization blockades between the undriven oscillator and the external drive as well as between both oscillators. Furthermore, we study the phase diagrams of two and three oscillators in the mean-field limit and find highly nontrivial active states.
Authors: Fengxia Liu, Zhiyong Zheng, Kun Tian, Yi Zhang, Heng Guo, Zhe Hu, Oleksiy Zhedanov, Zixian Gong
This paper introduces a novel lower bound on communication complexity using quantum relative entropy and mutual information, refining previous classical entropy-based results. By leveraging Uhlmann's lemma and quantum Pinsker inequalities, the authors establish tighter bounds for information-theoretic security, demonstrating that quantum protocols inherently outperform classical counterparts in balancing privacy and efficiency. Also explores symmetric Quantum Private Information Retrieval (QPIR) protocols that achieve sub-linear communication complexity while ensuring robustness against specious adversaries: A post-quantum cryptography based protocol that can be authenticated for the specious server; A ring-LWE-based protocol for post-quantum security in a single-server setting, ensuring robustness against quantum attacks; A multi-server protocol optimized for hardware practicality, reducing implementation overhead while maintaining sub-linear efficiency. These protocols address critical gaps in secure database queries, offering exponential communication improvements over classical linear-complexity methods. The work also analyzes security trade-offs under quantum specious adversaries, providing theoretical guarantees for privacy and correctness.
Authors: Matheus Sena, Mael Flament, Shane Andrewski, Ioannis Caltzidis, Niccolò Bigagli, Thomas Rieser, Gabriel Bello Portmann, Rourke Sekelsky, Ralf-Peter Braun, Alexander N. Craddock, Maximilian Schulz, Klaus D. Jöns, Michaela Ritter, Marc Geitz, Oliver Holschke, Mehdi Namazi
The Quantum Internet, a network of quantum-enabled infrastructure, represents the next frontier in telecommunications, promising capabilities that cannot be attained by classical counterparts. A crucial step in realizing such large-scale quantum networks is the integration of entanglement distribution within existing telecommunication infrastructure. Here, we demonstrate a real-world scalable quantum networking testbed deployed within Deutsche Telekom's metropolitan fibers in Berlin. Using commercially available quantum devices and standard add-drop multiplexing hardware, we distributed polarization-entangled photon pairs over dynamically selectable fiber paths ranging from 10~m to 60 km, and showed entanglement distribution over up to approximately 100~km. Quantum signals, transmitted at 1324~nm (O-band), coexist with conventional bidirectional C-band traffic without dedicated fibers or infrastructure changes. Active stabilization of the polarization enables robust long-term performance, achieving entanglement Bell-state fidelity bounds between 85-99% and Clauser-Horne-Shimony-Holt parameter $S$-values between 2.36-2.74 during continuous multiday operation. By achieving a high-fidelity entanglement distribution with less than 1.5% downtime, we confirm the feasibility of hybrid quantum-classical networks under real-world conditions at the metropolitan scale. These results establish deployment benchmarks and provide a practical roadmap for telecom operators to integrate quantum capabilities.
Authors: Pervez Hoodbhoy, M. Haashir Ismail, M. Mufassir
Coupled instantons are introduced by generalizing the double well potential to multiple mutually coupled wells. Physically this corresponds to the simultaneous tunneling of multiple degrees of freedom. A system with four equal minima is examined in detail. It has three instanton types or flavors with distinct actions. For weak coupling and subject to there being a single large (or small) parameter, the interactive system can be handled perturbatively. The zero mode problem arising from time translation symmetry is handled via the Fadeev-Popov procedure. A diagrammatic procedure allows corrections to the fluctuation determinant to be calculated systematically. Independent instanton contributions are summed over by extending the dilute gas approximation to three flavors and energy splittings of the lowest four states is calculated. All tunneling amplitudes are concisely expressed in terms of elementary functions. While the model is possibly useful for a variety of physical systems, an application is made here to the tunneling of a composite particle in one dimension.
Authors: Jiongning Che, Nils Gluth, Simon Köhnes, Thomas Guhr, Barbara Dietz
We report on experimental studies of the distribution of the off-diagonal elements of the scattering matrix of open microwave networks with symplectic symmetry and a chaotic wave dynamics. These consist of two geometrically identical subgraphs with unitary symmetry described by complex conjugate Hamiltonians, that are coupled by a pair of bonds. The results are compared to random-matrix theory predictions obtained on the basis of the Heidelberg approach for the scattering matrix of open quantum-chaotic systems. We demonstrate that deviations from random-matrix theory predictions observed in the distributions may be attributed to the fact that the subgraphs are not fully connected.
Authors: Jake Xuereb
We demonstrate how a single heat exchange between a probe thermal qubit and multi-qubit thermal machine encoding a Boolean function, can determine whether the function is balanced or constant, thus providing a novel thermodynamic solution to the Deutsch-Jozsa problem. We introduce a thermodynamic model of quantum query complexity, showing how qubit thermal machines can act as oracles, queried via heat exchange with a probe. While the Deutsch-Jozsa problem requires an exponential encoding in the number of oracle bits, we also explore the Bernstein-Vazirani problem, which admits a linear thermal oracle and a single thermal query solution. We establish bounds on the number of samples needed to determine the probe temperature encoding the solution for the Deutsch-Jozsa problem, showing that it remains constant with problem size. Additionally, we propose a proof-of-principle experimental implementation to solve the 3-bit Bernstein-Vazirani problem via thermal kickback. This work bridges thermodynamics and complexity theory, suggesting a new test bed for quantum thermodynamic computing.
Authors: Xu-Dan Xie, Zheng-Yuan Xue, Dan-Bo Zhang
In open quantum systems, the Liouvillian gap characterizes the relaxation time toward the steady state. However, accurately computing this quantity is notoriously difficult due to the exponential growth of the Hilbert space and the non-Hermitian nature of the Liouvillian superoperator. In this work, we propose a variational quantum algorithm for efficiently estimating the Liouvillian gap. By utilizing the Choi-Jamiokowski isomorphism, we reformulate the problem as finding the first excitation energy of an effective non-Hermitian Hamiltonian. Our method employs variance minimization with an orthogonality constraint to locate the first excited state and adopts a two-stage optimization scheme to enhance convergence. Moreover, to address scenarios with degenerate steady states, we introduce an iterative energy-offset scanning technique. Numerical simulations on the dissipative XXZ model confirm the accuracy and robustness of our algorithm across a range of system sizes and dissipation strengths. These results demonstrate the promise of variational quantum algorithms for simulating open quantum many-body systems on near-term quantum hardware.
Authors: Xiaoning Feng, Hans Hon Sang Chan, David P. Tew
We establish and analyse the performance and resource requirements of an end-to-end fault-tolerant quantum algorithm for computing the absorption spectrum and population dynamics of photoexcited pyrazine. The quantum circuit construction consists of initial state preparation using uniformly controlled rotations, the time-dependent Hamiltonian propagation based on the grid-based Split Operator Quantum Fourier Transform (SO-QFT) method, and cost-effective measurements including statistical and canonical phase estimation. We use classical emulations to validate the quantum resources required for the task, and propose generalised formulae for the qubit count and gate depth calculation. Simulating the vibronic dynamics of pyrazine in a low-dimensional abstraction requires 17-qubit circuits with a gate depth of $\mathcal{O}(10^4)$, whereas a full-dimensional simulation of pyrazine in 24 modes requires at least 97-qubit circuits with a gate depth of $\mathcal{O}(10^6)$. Our work provides a foundational framework for understanding high-dimensional wavepacket-based quantum simulations of photo-induced dynamics and vibronic spectra, anticipating future applications in the simulation of even larger molecular systems on fault-tolerant quantum computers.
Authors: Fionnuala Curran, Morteza Moradi, Gabriel Senno, Magdalena Stobinska, Antonio Acín
Quantum physics exhibits an intrinsic and private form of randomness with no classical counterpart. Any setup for quantum randomness generation involves measurements acting on quantum states. In this work, we consider the following question: Given a quantum measurement, how much randomness can be generated from it? In real life, measurements are noisy and thus contain an additional, extrinsic form of randomness due to ignorance. This extrinsic randomness is not private since, in an adversarial model, it takes the form of quantum side information held by an eavesdropper who can use it to predict the measurement outcomes. Randomness of measurements is then quantified by the guessing probability of this eavesdropper, when minimized over all possible input states. This optimization is in general hard to compute, but we solve it here for any two-outcome qubit measurement and for projective measurements in arbitrary dimension mixed with white noise. We also construct, for a given measured probability distribution, different realizations with (i) a noisy state and noiseless measurement (ii) a noiseless state and noisy measurement and (iii) a noisy state and measurement, and we show that the latter gives an eavesdropper significantly higher guessing power.
Authors: Abeynaya Gnanasekaran, Amit Surana
We present a novel framework for Linear Combination of Unitaries (LCU)-style decomposition tailored to structured sparse matrices, which frequently arise in the numerical solution of partial differential equations (PDEs). While LCU is a foundational primitive in both variational and fault-tolerant quantum algorithms, conventional approaches based on the Pauli basis can require a number of terms that scales quadratically with matrix size. We introduce the Sigma basis, a compact set of simple, non-unitary operators that can better capture sparsity and structure, enabling decompositions with only polylogarithmic scaling in the number of terms. We develop both numerical and semi-analytical methods for computing Sigma basis decompositions of arbitrary matrices. Given this new basis is comprised of non-unitary operators, we leverage the concept of unitary completion to design efficient quantum circuits for evaluating observables in variational quantum algorithms and for constructing block encodings in fault-tolerant quantum algorithms. We compare our method to related techniques like unitary dilation, and demonstrate its effectiveness through several PDE examples, showing exponential improvements in decomposition size while retaining circuit efficiency.
Authors: Sergei Trofimov, Christos Thessalonikios, Victor Deinhart, Alexander Spyrantis, Lucas Tsunaki, Kseniia Volkova, Katja Höflich, Boris Naydenov
Substitutional nitrogen atoms in a diamond crystal (P1 centers) are, on one hand, a resource for creation of nitrogen-vacancy (NV) centers, that have been widely employed as nanoscale quantum sensors. On the other hand, P1's electron spin is a source of paramagnetic noise that degrades the NV's performance by shortening its coherence time. Accurate quantification of nitrogen concentration is therefore essential for optimizing diamond-based quantum devices. However, bulk characterization methods based on optical absorption or electron paramagnetic resonance often overlook local variations in nitrogen content. In this work, we use a helium ion microscope to fabricate nanoscale NV center ensembles at predefined sites in a diamond crystal containing low concentrations of nitrogen. We then utilize these NV-based probes to measure the local nitrogen concentration on the level of 230 ppb (atomic parts per billion) using the double electron-electron resonance (DEER) technique. Moreover, by comparing the DEER spectra with numerical simulations, we managed to determine the concentration of other unknown paramagnetic defects created during the ion implantation, reaching 15 ppb depending on the implantation dose.
Authors: William Huie, Cianan Conefrey-Shinozaki, Zhubing Jia, Patrick Draper, Jacob P. Covey
Simulating nuclear matter described by quantum chromodynamics using quantum computers is notoriously inefficient because of the assortment of quark degrees of freedom such as matter/antimatter, flavor, color, and spin. Here, we propose to address this resource efficiency challenge by encoding three qubits within individual ytterbium-171 atoms of a neutral atom quantum processor. The three qubits are encoded in three distinct sectors: an electronic "clock" transition, the spin-1/2 nucleus, and the lowest two motional states in one radial direction of the harmonic trapping potential. We develop a family of composite sideband pulses and demonstrate a universal gate set and readout protocol for this three-qubit system. We then apply it to single-flavor quantum chromodynamics in 1+1D axial gauge for which the three qubits directly represent the occupancy of quarks in the three colors. We show that two atoms are sufficient to simulate both vacuum persistence oscillations and string breaking. We consider resource requirements and connections to error detection/correction. Our work is a step towards resource-efficient digital simulation of nuclear matter and opens new opportunities for versatile qubit encoding in neutral atom quantum processors.
Authors: Geneviève Dusson, Louis Garrigue, Benjamin Stamm
Standard perturbation theory of eigenvalue problems consists of obtaining approximations of eigenmodes in the neighborhood of an operator where the corresponding eigenmode is known. Nevertheless, if the corresponding eigenmodes of several nearby operators are known, standard perturbation theory cannot simultaneously use all this knowledge to provide a better approximation. We derive a resolvent formula enabling such an approximation result, and provide numerical examples for which this method is more competitive than standard perturbation theory.
Authors: Mohamed Hibat-Allah, Ejaaz Merali, Giacomo Torlai, Roger G Melko, Juan Carrasquilla
Rydberg atom array experiments have demonstrated the ability to act as powerful quantum simulators, preparing strongly-correlated phases of matter which are challenging to study for conventional computer simulations. A key direction has been the implementation of interactions on frustrated geometries, in an effort to prepare exotic many-body states such as spin liquids and glasses. In this paper, we apply two-dimensional recurrent neural network (RNN) wave functions to study the ground states of Rydberg atom arrays on the kagome lattice. We implement an annealing scheme to find the RNN variational parameters in regions of the phase diagram where exotic phases may occur, corresponding to rough optimization landscapes. For Rydberg atom array Hamiltonians studied previously on the kagome lattice, our RNN ground states show no evidence of exotic spin liquid or emergent glassy behavior. In the latter case, we argue that the presence of a non-zero Edwards-Anderson order parameter is an artifact of the long autocorrelations times experienced with quantum Monte Carlo (QMC) simulations, and we show that autocorrelations can be systematically reduced by increasing numerical effort. This result emphasizes the utility of autoregressive models, such as RNNs, in conjunction with QMC, to explore Rydberg atom array physics on frustrated lattices and beyond.
Authors: Karl P. Horn, Meenu Upadhyay, Baruch Margulis, Daniel M. Reich, Edvardas Narevicius, Markus Meuwly, Christiane P. Koch
High-quality potential energy surfaces (PES) are a prerequisite for quantitative atomistic simulations, with both quantum and classical dynamics approaches. The ultimate test for the validity of a PES are comparisons with judiciously chosen experimental observables. Here we ask whether cold collision measurements are sufficiently informative to validate and distinguish between high-level, state-of-the art PESs for the strongly interacting Ne-H$_2^+$ system. We show that measurement of the final state distributions for a process that involves only several metastable intermediate states is sufficient to identify the PES that captures the long-range interactions properly. Furthermore, we show that a modest increase in the experimental energy resolution will allow for resolving individual Feshbach resonances and enable a quantitative probe of the interactions at short and intermediate range.
Authors: Marco Cattaneo, Marco Baldovin, Dario Lucente, Paolo Muratore-Ginanneschi, Angelo Vulpiani
We establish an analytical criterion for dynamical thermalization within harmonic systems, applicable to both classical and quantum models. Specifically, we prove that thermalization of various observables, such as particle energies in physically relevant random quadratic Hamiltonians, is typical for large systems ($N \gg 1$) with initial conditions drawn from the microcanonical distribution. Moreover, we show that thermalization can also arise from non-typical initial conditions, where only a finite fraction of the normal modes is excited. A different choice of initial conditions, such as all the initial energy localized in a single particle, instead leads to energy equipartition without thermalization. Since the models we consider are integrable, our findings provide a general dynamical basis for an approach to thermalization that bypasses chaos and ergodicity, focusing instead on the physical requirement that thermodynamic observables depend on a large number of normal modes, and build a bridge between the classical and quantum theories of thermalization.
Authors: Si-Han Li, Si-Han Shang, Shu-Min Wu
In a relativistic framework, it is generally accepted that quantum steering of maximally entangled states provide greater advantages in practical applications compared to non-maximally entangled states. In this paper, we investigate quantum steering for four different types of Bell-like states of fermionic modes near the event horizon of a Schwarzschild black hole. In some parameter spaces, the peak of steering asymmetry corresponds to a transition from two-way to one-way steerability for Bell-like states under the influence of the Hawking effect. It is intriguing to find that the fermionic steerability of the maximally entangled states experiences sudden death with the Hawking temperature, while the fermionic steerability of the non-maximally entangled states maintains indefinite persistence at infinite Hawking temperature. In contrast to prior research, this finding suggests that quantum steering of non-maximally entangled states is more advantageous than that of maximally entangled states for processing quantum tasks in the gravitational background. This surprising result overturns the traditional idea of ``the advantage of maximally entangled steering in the relativistic framework" and provides a new perspective for understanding the Hawking effect of the black hole.
Authors: Zhi-Hao Huang, Kou-Han Ma, Bao-Zong Wang, W. Vincent Liu, Xiong-Jun Liu
Spin and orbital are two basic degrees of freedom that play significant roles in exploring exotic quantum phases in optical lattices with synthetic spin-orbit coupling (SOC) and high orbital bands, respectively. Here, we combine these two crucial ingredients for the first time by proposing a completely new orbital optical Raman lattice scheme to explore exotic high-orbital Bose condensates with Raman-induced SOC in a square lattice. We find that both the SOC and p-orbital interactions influence the condensed state of bosons. Their interplay results in two novel high-orbital many-body quantum phases: the uniform angular momentum superfluid phase, which exhibits a global topological chiral orbital current characterized by a uniform Chern number, and the two-dimensional topological spin-orbital supersolid phase, which is characterized by the spin and orbital angular momentum density wave patterns and topological excitations with opposite Chern numbers, respectively protecting the chiral and antichiral edge modes in the neighboring supersolid clusters. Our scheme may open a new avenue for exploring exotic SOC and high-orbital physics in optical lattices, and is expected to advance the experimental realization of novel supersolids in higher dimensions.
Authors: Andreas Deuchert, Phan Thanh Nam, Marcin Napiorkowski
We consider the homogeneous Bose gas in the three-dimensional unit torus, where $N$ particles interact via a two-body potential of the form $N^{-1} v(x)$. The system is studied at inverse temperatures of order $N^{-2/3}$, which corresponds to the temperature scale of the Bose--Einstein condensation phase transition. We show that spontaneous $U(1)$ symmetry breaking occurs if and only if the system exhibits Bose--Einstein condensation in the sense that the one-particle density matrix of the Gibbs state has a macroscopic eigenvalue.
Authors: Kay Brandner, Keiji Saito
We derive a universal thermodynamic uncertainty relation for Fermionic coherent transport, which bounds the total rate of entropy production in terms of the mean and fluctuations of a single particle current. This bound holds for any multi-terminal geometry and arbitrary chemical and thermal biases, as long as no external magnetic fields are applied. It can further be saturated in two-terminal settings with boxcar-shaped transmission functions and reduces to its classical counterpart in linear response. Upon insertion of a numerical factor, our bound also extends to systems with broken time-reversal symmetry. As an application, we derive trade-off relations between the figures of merit of coherent thermoelectric heat engines and refrigerators, which show that such devices can attain ideal efficiency only at vanishing mean power or diverging power fluctuations. To illustrate our results, we work out a model of a coherent conductor consisting of a chain of quantum dots.
Authors: Xudong Huai, Luke Pritchard Cairns, Bridget Delles, Michal J. Winiarski, Maurice Sorolla II, Xinshu Zhang, Youzhe Chen, Stuart Calder, Tatenda Kanyowa, Anshul Kogar, Huibo Cao, Danielle Yahne, Robert Birgeneau, James Analytis, Thao T. Tran
Diamond lattice magnets, formed by a framework of corner-sharing tetrahedra of magnetic cations, offer unique opportunities to realize novel states of matter for potential utility in information technology. However, research has mostly focused on AB2X4 spinels with Td magnetic ions. This hinders the atomically enabled tunability of competing interactions at different energy scales and the ability to harness many-body electronic states in quantum materials, making the discovery of quantum fluctuations and spin dynamics less accessible. We discover a new material CaCo2TeO6 featuring a diamond lattice of two distinct Oh-Co2+ sites. This material displays strong quantum fluctuations, increased competing magnetic exchange interactions, and field-induced tunability of magnetic structures. The results demonstrate how simple, fundamental refinements in ligand fields can profoundly influence the phase space of quantum matter.
Authors: Robbie Cruickshank, Francesco Lorenzi, Arthur La Rooij, Ethan Kerr, Timon Hilker, Stefan Kuhr, Luca Salasnich, Elmar Haller
We report the experimental observation of discrete bright matter-wave solitons with attractive interaction in an optical lattice. Using an accordion lattice with adjustable spacing, we prepare a Bose-Einstein condensate of cesium atoms across a defined number of lattice sites. By quenching the interaction strength and the trapping potential, we generate both single-site and multi-site solitons. Our results reveal the existence and characteristics of these solitons across a range of lattice depths and spacings. We identify stable regions of the solitons, based on interaction strength and lattice properties, and compare these findings with theoretical predictions. The experimental results qualitatively agree with a Gaussian variational model and match quantitatively with numerical simulations of the three-dimensional Gross-Pitaevskii equation, extended with a quintic term to account for the loss of atoms. Our results provide insights into the quench dynamics and collapse mechanisms, paving the way for further studies on transport and dynamical properties of matter-wave solitons in lattices.
Authors: Adam B. Cahaya
The momentum of light in dielectric media has been a century-long controversy that continues to attract significant interest. In a linear dielectric medium with refractive index n, the momentum is predicted to be smaller by a factor of n according to Max Abraham, and larger by the same factor according to Hermann Minkowski. By studying the coupled dynamics of electromagnetic waves and dipoles in a dielectric medium, we show that the change in momentum of the dipole, expressed by the Lorentz force, corresponds to the Abraham momentum. On the other hand, the Minkowski momentum arises as the eigenvalue of the Hamiltonian and determines the direction of refraction in accordance with Snell's law. Our model also predicts a zitterbewegung-like oscillation due to helicity mixing between left- and right-handed wave components, mediated by dipole oscillation. These internal wave dynamics may be observable via wavepacket motion or polarization-sensitive measurements.
Authors: Balázs Rácsai, Péter Jeszenszki, Ádám Margócsy, Edit Mátyus
Relativistic, quantum electrodynamics, as well as non-adiabatic corrections and couplings, are computed for the b $^3\Pi_\mathrm{g}$ and c $^3\Sigma_\mathrm{g}^+$ electronic states of the helium dimer. The underlying Born-Oppenheimer potential energy curves are converged to 1 ppm ($1:10^6$) relative precision using a variational explicitly correlated Gaussian approach. The quantum nuclear motion is computed over the b $^3\Pi_\mathrm{g}$-c $^3\Sigma_\mathrm{g}^+$ (and B $^1\Pi_\mathrm{g}$-C $^1\Sigma_\mathrm{g}^+$) 9-(12-)dimensional electronic-spin subspace coupled by non-adiabatic and relativistic (magnetic) interactions. The electron's anomalous magnetic moment is also included; its effect is expected to be visible in high-resolution experiments. The computed rovibronic energy intervals are in excellent agreement with available high-resolution spectroscopy data, including the rovibronic b $^3\Pi_\mathrm{g}$-state fine structure. Fine-structure splittings are also predicted for the c $^3\Sigma_\mathrm{g}^+$ levels, which have not been fully resolved experimentally, yet.
Authors: MA. Khajeian
Long, human-generated passwords pose significant challenges to both classical and quantum attacks due to their irregular structure and large search space. In this work, we propose an enhanced classical-quantum hybrid attack specifically designed for this scenario. Our approach constructs rainbow tables using dictionary-based password generation augmented with transformation rules that better capture real-world user behavior. These tables are organized into buckets, enabling faster lookup and reduced space complexity. For the search within each bucket, we employ a distributed exact variant of Grover's algorithm. This method provides deterministic success and significantly lower circuit depth, enhancing robustness against noise-particularly depolarizing errors common in near-term quantum devices. Overall, our hybrid framework improves the efficiency and practicality of password recovery for long, human-readable passwords in realistic adversarial settings.
Authors: Michal P. Heller, Fabio Ori, Alexandre Serantes
Recently, several notions of entanglement in time have emerged as a novel frontier in quantum many-body physics, quantum field theory and gravity. We propose a systematic prescription to characterize temporal entanglement in relativistic quantum field theory in a general state for an arbitrary subregion on a flat, constant-time slice in a flat spacetime. Our prescriptions starts with the standard entanglement entropy of a spatial subregion and amounts to transporting the unchanged subregion to boosted time slices all the way across the light cone when it becomes in general a complex characterization of the corresponding temporal subregion. For holographic quantum field theories, our prescription amounts to an analytic continuation of all codimension-two bulk extremal surfaces satisfying the homology constraint and picking the one with the smallest real value of the area as the leading saddle point. We implement this prescription for holographic conformal field theories in thermal states on both a two-dimensional Lorentzian cylinder and three-dimensional Minkowski space, and show that it leads to results with self-consistent physical properties of temporal entanglement.
Authors: Andrei Lunin (1), Mustafa Bal (1), Akshay Murthy (1), Shaojiang Zhu (1), Anna Grassellino (1), Alexander Romanenko (1) ((1) Fermi National Accelerator Laboratory)
The Josephson junction and shunt capacitor form a transmon qubit, which is the cornerstone of modern quantum computing platforms. For reliable quantum computing, it is important how long a qubit can remain in a superposition of quantum states, which is determined by the coherence time (T1). The coherence time of a qubit effectively sets the "lifetime" of usable quantum information, determining how long quantum computations can be performed before errors occur and information is lost. There are several sources of decoherence in transmon qubits, but the predominant one is generally considered to be dielectric losses in the natural oxide layer formed on the surface of the superconductor. In this paper, we present a numerical study of microwave surface losses in planar superconducting antennas of different transmon qubit designs. An asymptotic method for estimating the energy participation ratio in ultrathin films of nanometer scales is proposed, and estimates are given for the limits of achievable minimum RF losses depending on the electrical properties of the surface oxide and the interface of the qubit with the substrate material.
Authors: Flavio Baccari, Pavel Kos, Georgios Styliaris
As quantum devices progress towards a quantum advantage regime, they become harder to benchmark. A particularly relevant challenge is to assess the quality of the whole computation, beyond testing the performance of each single operation. Here we introduce a scheme for this task that combines the target computation with variants of it, which, when averaged, allow for classically solvable correlation functions. Importantly, the variants exactly preserve the circuit architecture and depth, without simplifying the gates into a classically-simulable set. The method is based on replacing each gate by an ensemble of similar gates, which when averaged together form space-time channels [P. Kos and G. Styliaris, Quantum 7, 1020 (2023)]. We introduce explicit constructions for ensembles producing such channels, all applicable to arbitrary brickwork circuits, and provide a general recipe to find new ones through semidefinite programming. The resulting average computation retains important information about the original circuit and is able to detect noise beyond a Clifford benchmarking regime. Moreover, we provide evidence that estimating average-computation expectation values requires running only a limited number of different circuit realizations.
Authors: Léon Carde, Ronan Gautier, Nicolas Didier, Alexandru Petrescu, Joachim Cohen, Alexander McDonald
In this work, we use path integral techniques to predict the switching rate in a single-mode bistable open quantum system. While analytical expressions are well-known to be accessible for systems subject to Gaussian noise obeying classical detailed balance, we generalize this approach to a class of quantum systems, those which satisfy the recently-introduced hidden time-reversal symmetry [1]. In particular, in the context of quantum computing, we deliver precise estimates of bit-flip error rates in cat-qubit architectures, circumventing the need for costly numerical simulations. Our results open new avenues for exploring switching phenomena in multistable single- and many-body open quantum systems.
Authors: Pritam Roy, Subhankar Bera, A. S. Majumdar
We study the security of a quantum key distribution (QKD) protocol under the one-sided device-independent (1sDI) setting, which assumes trust in only one party's measurement device. This approach effectively provides a balance between the experimental viability of device-dependent (DD-QKD) and the minimal trust assumptions of device-independent (DI-QKD). An analytical lower bound on the asymptotic key rate is derived to provide security against collective attacks, in which the eavesdropper's information is limited only by the function of observed violation of a linear quantum steering inequality, specifically the three-setting Cavalcanti--Jones--Wiseman--Reid (CJWR) inequality. We provide a closed-form key rate formula by reducing the security analysis to mixtures of Bell-diagonal states by utilizing symmetries of the steering functional. We show that the protocol tolerates higher quantum bit error rates (QBER) than present DI-QKD protocols by benchmarking its performance under depolarizing noise. Furthermore, we explore the impact of detection inefficiencies and show that, in contrast to DI-QKD, which requires near-perfect detection, secure key generation can be achieved even with lower detection efficiency on the untrusted side. These findings demonstrate the viability of using 1sDI-QKD with current technology and highlight its advantages as a steering-based substitute for secure quantum communication.
Authors: Raul Corrêa, Marcelo F. Santos, Carlos H. Monken, Ado Jorio
The process in which Raman scattering produces correlated Stokes and anti-Stokes radiation is known as Stokes-anti-Stokes (SaS) scattering. It has been shown recently that this process can generate entangled photon pairs, making it a promising tool for quantum optical technologies, but a proper quantum theoretical description was lacking. In this paper, a fully quantum derivation of the electric polarization in a medium with vibrational Raman response, with quantized electromagnetic fields, is developed. Using quantum perturbation theory for Heisenberg operators, we find the solution for the material electric polarization and show that the correlated SaS scattering appears in the first order of perturbation, corresponding to a four-wave mixing phenomenon. We also discuss how to construct the third-order non-linear optical susceptibility for the SaS scattering from the quantum formalism, and show that it coincides with the one derived for classical fields in stimulated Raman.
Authors: Lucas Tsunaki, Anmol Singh, Sergei Trofimov, Boris Naydenov
Color centers defects in solids, and particularly the nitrogen-vacancy (NV) center in diamond, are crucial physical platforms for quantum technology applications. Therefore, a precise and rigorous numerical modeling of the NV system is indispensable for the continued advancement of the field. Within this context, this work develops a Python software for simulating the NV spin dynamics in pulsed sequences under general experimental conditions, i.e. a digital twin. The library accounts for electromagnetic pulses and other environmental inputs, which are used to solve the system's time-evolution dynamics, resulting in a physical output in the form of a quantum mechanical observable given by the fluorescence. The simulations framework is based on a non-perturbative time-dependent Hamiltonian model, solved in the laboratory frame. Whereby eliminating oversimplifications such as the assumption of instantaneous $\delta$-pulses and rotating wave approximations, our simulations reveal subtle dynamics from the realistic pulse constraints that impact quantum systems. Three use-case examples illustrate the use of the software and validate it by comparing the simulation results with well-established experimental works, relevant to the fields of quantum computing (two-qubit conditional logic gates), sensing (noise resilient dynamical decoupling sequences between NV and couples spins) and networks (teleportation of a quantum state between entangled NVs). Overall, this digital twin delivers a robust and detailed numerical modeling of the NV spin dynamics, with simple and accessible usability, for an extensive application range.
Authors: Alejandro Lopez-Bezanilla, Pavel A. Dub, Avadh Saxena
We investigate a hybrid modeling framework in which a quantum annealer is used to simulate magnetic interactions in molecular qubit lattices inspired by experimentally realizable systems. Using phthalocyanine assemblies as a structurally constrained prototype, we model a continuous deformation from a Lieb to a kagome lattice, revealing frustration-driven disorder and magnetic field-induced reordering in the spin structure. The annealer provides access to observables such as the static structure factor and magnetization over a wide parameter space, enabling the characterization of magnetic arrangements beyond the reach of current molecular architectures. This surrogate modeling approach supports a feedback loop between experiment and programmable quantum hardware, offering a pathway to explore and iteratively design tunable magnetic states in synthetic quantum materials. The synthetic design, structural characterization, and quantum simulation framework established here defines a modular and scalable paradigm for probing the limits of engineered quantum matter across chemistry, condensed matter, and quantum information science.
Authors: Peng Zhao, Peng Xu, Zheng-Yuan Xue
Fluxoniums have emerged as a compelling qubit modality for superconducting quantum computing, owing to their strong anharmonicity and high coherence. However, it remains to be seen whether the implementation of high-fidelity entangling gates in a scalable manner will be achieved or not for fluxoniums. Here, we show that fluxonium architectures with tunable plasmon interactions have the capability to implement scalable multi-qubit gates natively while remaining compatible with existing single- and two-qubit gate realizations. Within the architecture, even though qubit states are decoupled, engineered interactions in noncomputational manifolds can facilitate the realization of $(C^{\otimes N})Z$ gates (for $N\geq 2$). This is accomplished by driving qubit-state selective transitions into noncomputational manifolds. Our case studies show that CCZ, CCCZ, and CCCCZ gates with errors of approximately 0.01 (0.001) are achievable, with gate lengths of $50\,(100)\,\text{ns}$, $100\,(250)\,\text{ns}$, and $150\,(300)\,\text{ns}$, respectively. These results underscore the potential of the fluxonium architecture for scalable quantum computation through fast, high-fidelity native multi-qubit gates.
Authors: Laurenz Monzel, Stella Stopkowicz
We introduce a generalization of the quantum electrodynamic coupled cluster (QED-CC)wave function ansatz, to describe the strongly coupled light-matter system in an unpolarized optical Fabry-Pérot cavity. This is achieved by explicitly treating two cavity modes in our calculation with perpendicular polarizations and demonstrate that this ansatz preserves the symmetry of an unpolarized cavity. Furthermore, exploiting point-group symmetry enables the assignment of polaritonic excited states as well as their targeted calculation. Using our implementation, the aromatic species benzene, fluorobenzene and azulene are investigated. We demonstrate that molecules in unpolarized cavities have a complicated excited-state landscapes with a plethora of avoided-crossings. We compare the results for a cavity with a single polarization to those of an unpolarized cavity described by two perpendicular polarization vectors using the excited states of the H$_2$ molecule as an example.
Authors: Andrei Aralov, Émilie Gillet, Viet Nguyen, Andrea Cosentino, Mattia Walschaers, Massimo Frigerio
Multimode multiphoton states are at the center of many photonic quantum technologies, from photonic quantum computing to quantum sensing. In this work, we derive a procedure to generate exactly, and with a controlled number of steps, any such state by using only multiport interferometers, photon number resolving detectors, photon additions, and displacements. We achieve this goal by establishing a connection between photonic quantum state engineering and the algebraic problem of symmetric tensor decomposition. This connection allows us to solve the problem by using corresponding results from algebraic geometry and unveils a mechanism of photon catalysis, where photons are injected and subsequently retrieved in measurements, to generate entanglement that cannot be obtained through Gaussian operations. We also introduce a tensor decomposition, that generalizes our method and allows to construct optimal circuits for particular classes of states. As a benchmark, we numerically evaluate our method and compare its performance with state-of-the art results, confirming 100% fidelity on different classes of states.
Authors: Tongmiao Fan, Yilin Tang, Shaun Lung, Maximilian Weissflog, Jinyong Ma, Saniya Shinde, Sina Saravi, Mudassar Nauman, Wenkai Yang, Hao Qin, Shuyao Qiu, Andrey A. Sukhorukov, Yuerui Lu, Frank Setzpfandt
Quantum photon pairs play a pivotal role in many quantum applications. Metasurfaces, two-dimensional arrays of nanostructures, have been studied intensively to enhance and control pair generation via spontaneous parametric downconversion (SPDC). Van der Waals (VdW) layered materials have emerged as promising candidates for nonlinear materials in quantum light sources, owing to their high nonlinear susceptibility and compatibility with on-chip integration. In this work, we present the first demonstration of SPDC from a metasurface composed of the VdW material 3R-MoS2. The nanoresonators support quasi-bound states in the continuum (qBIC) with a quality factor of up to 120, enhancing light-matter interactions. This design achieves a 20-fold increase in SPDC rate compared to an unstructured film and significantly higher brightness, resulting in enhanced quantum photon-pair generation. This work establishes a new approach for utilizing van der Waals metasurfaces in the generation of quantum photon pairs, opening avenues for advanced quantum applications.
Authors: Ishaan Ganti, Jianshu Cao
Cavity polaritons, quasiparticles formed by coherent light-matter coupling, are at the heart of fundamental concepts of quantum optics. The quintessential signature of this coherent coupling is the Rabi oscillation, which results from the neglect of the counter-rotating-wave (CRW) effect in the weak-coupling regime. The goal of this letter is to predict resonant beatings that envelop the Rabi oscillation on the second or higher excitation manifold. These polariton beatings arise from the CRW term in the Dicke or Pauli-Fierz model and are directly correlated with the asymmetry in polariton eigenenergies. Our findings highlight the relevance of the CRW effect even in the weak-coupling regime, offer novel perspectives about coherent polariton dynamics, and shed new light on experiments of coupled quantum systems.
Authors: Ivan Villani, Matteo Carrega, Alessandro Crippa, Elia Strambini, Francesco Giazotto, Vaidotas Miseikis, Camilla Coletti, Fabio Beltram, Kenji Watanabe, Takashi Taniguchi, Stefan Heun, Sergio Pezzini
The combination of superconductivity and quantum Hall (QH) effect is regarded as a key milestone in advancing topological quantum computation in solid-state systems. Recent quantum interference studies suggest that QH edge states can effectively mediate a supercurrent across high-quality graphene weak links. In this work we report the observation of a supercurrent associated with transitions between adjacent QH plateaus, where transport paths develop within the compressible two-dimensional bulk. We employ a back-gated graphene Josephson junction, comprising high-mobility CVD-grown graphene encapsulated in hexagonal Boron Nitride (hBN) and contacted by Nb leads. Superconducting pockets are detected persisting beyond the QH onset, up to 2.4 T, hence approaching the upper critical field of the Nb contacts. We observe an approximate $\Phi_0=h/2e$ periodicity of the QH-supercurrent as a function of the magnetic field, indicating superconducting interference in a proximitized percolative phase. These results provide a promising experimental platform to investigate the transport regime of percolative supercurrents, leveraging the flexibility of van der Waals devices.
Authors: Paul Aigner, Wolfgang Dür
We investigate how unitary control can improve parameter estimation by designing the effective spectrum of the imprinting Hamiltonian. We show that, for commuting Hamiltonians, the general problem of spectral manipulation via unitary control simplifies to a finite sequence of elementary switching operations. Furthermore, we demonstrate that any desired relative spacing of energy levels can be achieved. We also show that any modified spectrum can be expressed as a convex combination of the original eigenvalues, with the convex weights forming a bi-stochastic matrix. Through several single-parameter estimation examples, we demonstrate that our spectral engineering method substantially enhances estimation accuracy.
Authors: Yahui Chai, Joe Gibbs, Vincent R. Pascuzzi, Zoë Holmes, Stefan Kühn, Francesco Tacchino, Ivano Tavernelli
We develop and demonstrate methods for simulating the scattering of particle wave packets in the interacting Thirring model on digital quantum computers, with hardware implementations on up to 80 qubits. We identify low-entanglement time slices of the scattering dynamics and exploit their efficient representation by tensor networks. Circuit compression based on matrix product state techniques yields on average a reduction by a factor of 3.2 in circuit depth compared to conventional approaches, allowing longer evolution times to be evaluated with higher fidelity on contemporary quantum processors. Utilizing zero-noise extrapolation in combination with Pauli twirling, on quantum hardware we accurately simulate the full scattering dynamics on 40 qubits, and further demonstrate the wave packet state-preparation on 80 qubits.
Authors: Wonjun Lee, Minki Hhan, Gil Young Cho, Hyukjoon Kwon
Random quantum states have various applications in quantum information science, including quantum cryptography, quantum simulation, and benchmarking quantum devices. In this work, we discover a new ensemble of quantum states that serve as an $\epsilon$-approximate state $t$-design while possessing extremely low entanglement, magic, and coherence. We show that those resources such quantum states can reach their theoretical lower bounds, $\Omega\left(\log (t/\epsilon)\right)$, which are also proven in this work. This implies that for fixed $t$ and $\epsilon$, those resources do not scale with the system size, i.e., $O(1)$ with respect to the total number of qubits $n$ in the system. Moreover, we explicitly construct an ancilla-free shallow quantum circuit for generating such states. To this end, we develop an algorithm that transforms $k$-qubit approximate state designs into $n$-qubit ones through a sequence of multi-controlled gates, without increasing the support size. The depth of such a quantum circuit is $O\left(t [\log t]^3 \log n \log(1/\epsilon)\right)$, which is the most efficient among existing algorithms without ancilla qubits. A class of shallow quantum circuits proposed in our work offers reduced cost for classical simulation of random quantum states, leading to potential applications in various quantum information processing tasks. As a concrete example for demonstrating utility of our algorithm, we propose classical shadow tomography using an $O(1)$-entangled estimator, which can achieve shorter runtime compared to conventional schemes.
Authors: Cookey Iyen, Muhammad Sanusi Liman, Benedict O. Ayomanor, Emem-obong Solomon James, Yame Mwanzang Philemon, Babatunde James Falaye
Quantum information processing promises significant advantages over classical methods but remains vulnerable to decoherence induced by environmental interactions and spacetime effects. This work investigates the behavior of Quantum Fisher Information (QFI) as a diagnostic tool for entanglement and parameter estimation in a three-qubit entangled Dirac system subjected to dissipative noisy channels in the curved spacetime of a Schwarzschild black hole. In particular, we examine the influence of the squeezed generalized amplitude damping (SGAD) channel, along with its subchannels -- generalized amplitude damping (GAD) and amplitude damping (AD) -- on the QFI with respect to entanglement weight ($\theta$) and phase ($\phi$) parameters. Our results show that under strong squeezing ($r = 1$), the QFI with respect to $\theta$ becomes completely resistant to variations in the Hawking temperature ($T_H$), while still exhibiting degradation with increasing channel temperature ($T_C$). The QFI decay is significantly slower at $r = 1$ compared to $r = 0$, suggesting that squeezing can function as an error mitigation strategy. For QFI with respect to $\phi$, a transient spike is observed at $T_C = 2$, potentially due to thermal resonance or non-monotonic decoherence, and this behavior is unaffected by $T_H$. Similar patterns are noted in the GAD and AD channels, where $T_C$ consistently dominates as the principal source of decoherence. Overall, the results highlight the intricate interplay between environmental noise, relativistic effects, and quantum error resilience in curved spacetime.
Authors: Changqing Wang, Deyuan Hu, Silvia Zorzetti, Anna Grassellino, Alexander Romanenko, Zheshen Zhang
Pushing the boundaries of measurement precision is central for sensing and metrology, pursued by nonclassical resources such as squeezing, and non-Hermitian degeneracies with distinct spectral response. Their convergence, however, remains challenging. We find extraordinary enhancement of sensitivity by unifying both effects in a general framework for quantum sensing in open systems. At the parametric oscillation threshold and an exceptional point, the sensing precision exhibits a unique quartic scaling with the perturbation strength. The result generalizes to multimode squeezed-state sensors with higher-order exceptional points catered to various quantum sensing platforms.
Authors: Mateus R. L. da Motta, Sandra S. Vianna
We present a detailed theoretical treatment of four-wave mixing (FWM) in a quantized paraxial framework, capturing the multi-spatial-mode nature of the biphoton state generated in the process. By analyzing the biphoton state both in position and momentum representations, we identify the conditions under which these descriptions become equivalent. We also highlight formal and physical similarities between FWM and spontaneous parametric down-conversion (PDC), showing that the transfer of pump structure to the spatial coincidence profile, an important and well-known characteristic of the biphoton state, carries over naturally to FWM. In addition, our treatment captures the transition from position correlations in the near field to momentum correlations in the far field, reflecting the underlying spatial entanglement. The measures of entanglement, including the spiral bandwidth and the Schmidt rank, are discussed. Our work consolidates known and new results on spatial correlations in FWM and provides a theoretical framework that may support future studies in nonlinear and quantum optics with structured light.
Authors: Joshua J. Goings, Kyujin Shin, Seunghyo Noh, Woomin Kyoung, Donghwi Kim, Jihye Baek, Martin Roetteler, Evgeny Epifanovsky, Luning Zhao
We extend correlated sampling from classical auxiliary-field quantum Monte Carlo to the quantum-classical (QC-AFQMC) framework, enabling accurate nuclear force computations crucial for geometry optimization and reaction dynamics. Stochastic electronic structure methods typically encounter prohibitive statistical noise when computing gradients via finite differences. To address this, our approach maximizes correlation between nearby geometries by synchronizing random number streams, aligning orbitals, using deterministic integral decompositions, and employing a consistent set of classical shadow measurements defined at a single reference geometry. Crucially, reusing this single, reference-defined shadow ensemble eliminates the need for additional quantum measurements at displaced geometries. Together, these methodological choices substantially reduce statistical variance in computed forces. We validate the method across hydrogen chains, confirming accuracy throughout varying correlation regimes, and demonstrate significant improvements over single-reference methods in force evaluations for N$_2$ and stretched linear H$_4$, particularly in strongly correlated regions where conventional coupled cluster approaches qualitatively fail. Orbital-optimized trial wave functions further boost accuracy for demanding cases such as stretched CO$_2$, without increasing quantum resource requirements. Finally, we apply our methodology to the MEA-CO$_2$ carbon capture reaction, employing quantum information metrics for active space selection and matchgate shadows for efficient overlap evaluations, establishing QC-AFQMC as a robust framework for exploring complex reaction pathways.
Authors: Chanaprom Cholsuk, Sujin Suwanna, Tobias Vogl
Quantum emitters in hexagonal boron nitride (hBN) have gained significant attention due to a wide range of defects that offer high quantum efficiency and single-photon purity at room temperature. Most theoretical studies on hBN defects simulate monolayers, as this is computationally cheaper than calculating bulk structures. However, most experimental studies are carried out on multilayer to bulk hBN, which creates additional possibilities for discrepancies between theory and experiment. In this work, we present an extended database of hBN defects that includes a comprehensive set of bulk hBN defects along with their excited-state photophysical properties. The database features over 120 neutral defects, systematically evaluated across charge states ranging from -2 to 2 (600 defects in total). For each defect, the most stable charge and spin configurations are identified and used to compute the zero-phonon line, photoluminescence spectrum, absorption spectrum, Huang-Rhys (HR) factor, interactive radiative lifetimes, transition dipole moments, and polarization characteristics. Our analysis reveals that the electron-phonon coupling strength is primarily influenced by the presence of vacancies, which tend to induce stronger lattice distortions and broaden phonon sidebands. Additionally, correlation analysis shows that while most properties are independent, the HR factor strongly correlates with the configuration coordinates. All data are publicly available at this https URL, along with a new application programming interface (API) to facilitate integration with machine learning workflows. This database is therefore designed to bridge the gap between theory and experiment, aid in the reliable identification of quantum emitters, and support the development of machine-learning-driven approaches in quantum materials research.
Authors: Dong An, Chao Yu, Ming-Yang Zheng, Anran Guo, Junsong Wang, Ruizhi Li, Huaping Ma, Xiu-Ping Xie, Xiao-Hui Bao, Qiang Zhang, Jun Zhang, Jian-Wei Pan
Silicon single-photon detectors (Si SPDs) play a crucial role in detecting single photons in the visible spectrum. For various applications, photon detection efficiency (PDE) is the most critical characteristic for effectively collecting photons. Here, we present a Si SPD with a remarkable PDE of up to 84.4% at 785 nm, supporting multiple operation modes. We design and fabricate a thick-junction Si single-photon avalanche diode (SPAD) that enhances the avalanche probability through a backside-illumination structure, while minimizing noise through the design of a doping-compensated avalanche region. To maximize PDE, we implement a readout circuit with a 50 V quenching voltage, enabling operation in free-running, gating, or hybrid modes. The SPAD, along with its readout circuits and affiliated circuits, is integrated into a compact SPD module. In free-running mode, the module achieves a maximum PDE of 84.4%, with a dark count rate of 260 cps, and an afterpulse probability of 2.9% at 268 K. This work provides a practical solution for applications requiring ultra-high-efficiency Si SPD with multiple operation modes.
Authors: Jiannis K. Pachos, Patricio Salgado-Rebolledo, Martine Schut
Exploring potential empirical manifestations of quantum gravity is a challenging pursuit. In this study, we utilise a lattice representation of a (2+1)D massive gravity toy model interacting with Dirac fermions that can support specific spacetime fluctuations. We focus on the evolution of the fermion's spin due to its coupling to spacetime fluctuations. To monitor their dynamics a minimal model is required that comprises two bosonic modes describing spacetime geometry fluctuations coupled with the spin of the fermion. A possible emulation of this system involves encoding spin degrees of freedom in an atom coupled with a bimodal optical cavity that provides the two bosonic modes. We observe diverse spin evolution patterns based on its interaction with fluctuating spacetime geometry. Our proposal introduces a novel approach for modelling the effect of interactions between quantum gravity and matter that can be probed with current technology.
Authors: Pei-Kun Yang
In structure-based virtual screening, it is often necessary to evaluate the binding free energy of protein-ligand complexes by considering not only molecular conformations but also how these structures shift and rotate in space. The number of possible combinations grows rapidly and can become overwhelming. While classical computing has limitations in this context, quantum computing offers a promising alternative due to its inherent parallelism. In this study, we introduce a quantum machine learning approach that encodes molecular information into quantum states and processes them using parameterized quantum gates. The model is implemented and trained using PyTorch, and its performance is evaluated under three settings: ideal simulation, limited-shot sampling, and simulations with quantum noise. With six quantum circuit units, the model achieves an RMSD of 2.37 kcal/mol and a Pearson correlation of 0.650. Even when using 100,000 shots, the predictions remain consistent, indicating that the model is compatible with near-term quantum hardware. Although noise slightly reduces accuracy, the ranking of ligand affinities remains largely unchanged. These findings point to a practical and scalable strategy that balances robustness and predictive power, offering a viable path to accelerate virtual screening through moderately deep quantum circuits.
Authors: William Huie, Cianan Conefrey-Shinozaki, Zhubing Jia, Patrick Draper, Jacob P. Covey
Simulating nuclear matter described by quantum chromodynamics using quantum computers is notoriously inefficient because of the assortment of quark degrees of freedom such as matter/antimatter, flavor, color, and spin. Here, we propose to address this resource efficiency challenge by encoding three qubits within individual ytterbium-171 atoms of a neutral atom quantum processor. The three qubits are encoded in three distinct sectors: an electronic "clock" transition, the spin-1/2 nucleus, and the lowest two motional states in one radial direction of the harmonic trapping potential. We develop a family of composite sideband pulses and demonstrate a universal gate set and readout protocol for this three-qubit system. We then apply it to single-flavor quantum chromodynamics in 1+1D axial gauge for which the three qubits directly represent the occupancy of quarks in the three colors. We show that two atoms are sufficient to simulate both vacuum persistence oscillations and string breaking. We consider resource requirements and connections to error detection/correction. Our work is a step towards resource-efficient digital simulation of nuclear matter and opens new opportunities for versatile qubit encoding in neutral atom quantum processors.
Authors: Jugal Talukdar, Scott E. Smart, Prineha Narang
Quantum states of motion are critical components in the second quantum revolution. We investigate the generation and control of non-Gaussian motional states in a tripartite hybrid quantum system consisting of a collection of qubits coupled to a mechanical resonator, which in turn interacts with an externally driven photonic cavity. This hybrid architecture provides a versatile platform for quantum control by integrating nonlinear interactions and multiple control parameters. Operating in the strong coupling regime, we study the transient dynamics resulting from a time-dependent external drive that has a boxcar profile. Starting from coherent states in both the mechanical and cavity subsystems, we show that this drive protocol, combined with time-independent interaction and frequency configurations, leads to the emergence of highly non-Gaussian quantum states in the intermediary mechanical degree of freedom. These states are characterized by a pronounced negative volume in the Wigner quasi-probability distribution and enhanced quantum Fisher information, indicative of their quantum utility. We systematically analyze the impact of the qubit phase, interaction strengths, and drive parameters on the degree of non-Gaussianity. Our findings underscore the tunability and richness of this hybrid platform, paving the way for advanced quantum state engineering and applications in quantum sensing, metrology, and information processing.
Authors: Gilberto Cunha, Alexandra Ramôa, André Sequeira, Michael de Oliveira, Luís Barbosa
Reinforcement learning (RL) provides a principled framework for decision-making in partially observable environments, which can be modeled as Markov decision processes and compactly represented through dynamic decision Bayesian networks. Recent advances demonstrate that inference on sparse Bayesian networks can be accelerated using quantum rejection sampling combined with amplitude amplification, leading to a computational speedup in estimating acceptance probabilities.\\ Building on this result, we introduce Quantum Bayesian Reinforcement Learning (QBRL), a hybrid quantum-classical look-ahead algorithm for model-based RL in partially observable environments. We present a rigorous, oracle-free time complexity analysis under fault-tolerant assumptions for the quantum device. Unlike standard treatments that assume a black-box oracle, we explicitly specify the inference process, allowing our bounds to more accurately reflect the true computational cost. We show that, for environments whose dynamics form a sparse Bayesian network, horizon-based near-optimal planning can be achieved sub-quadratically faster through quantum-enhanced belief updates. Furthermore, we present numerical experiments benchmarking QBRL against its classical counterpart on simple yet illustrative decision-making tasks. Our results offer a detailed analysis of how the quantum computational advantage translates into decision-making performance, highlighting that the magnitude of the advantage can vary significantly across different deployment settings.
Authors: Leonardo Badurina, Diego Blas, John Ellis, Sebastian A. R. Ellis
We explore how recent advances in the manipulation of single-ion wave packets open new avenues for detecting weak magnetic fields sourced by ultralight dark matter. A trapped ion in a ``Schrödinger cat'' state can be prepared with its spin and motional degrees of freedom entangled and be used as a matter-wave interferometer that is sensitive to the Aharonov-Bohm-like phase shift accumulated by the ion over its trajectory. The result of the spin-motion entanglement is a parametrically-enhanced sensitivity to weak magnetic fields as compared with an un-entangled ion in a trap. Taking into account the relevant boundary conditions, we demonstrate that a single trapped ion can probe unexplored regions of kinetically-mixed dark-photon dark matter parameter space in the $10^{-15}~\text{eV} \lesssim m_{A'} \lesssim 10^{-14}$~eV mass window. We also show how such a table-top quantum device will also serve as a complementary probe of axion-like particle dark matter in the same mass window.
Authors: Michal P. Heller, Fabio Ori, Alexandre Serantes
Recently, several notions of entanglement in time have emerged as a novel frontier in quantum many-body physics, quantum field theory and gravity. We propose a systematic prescription to characterize temporal entanglement in relativistic quantum field theory in a general state for an arbitrary subregion on a flat, constant-time slice in a flat spacetime. Our prescriptions starts with the standard entanglement entropy of a spatial subregion and amounts to transporting the unchanged subregion to boosted time slices all the way across the light cone when it becomes in general a complex characterization of the corresponding temporal subregion. For holographic quantum field theories, our prescription amounts to an analytic continuation of all codimension-two bulk extremal surfaces satisfying the homology constraint and picking the one with the smallest real value of the area as the leading saddle point. We implement this prescription for the holographic conformal field theories in thermal states on both a two-dimensional Lorentzian cylinder and three-dimensional Minkowski space, and show that it leads to results with self-consistent physical properties of temporal entanglement.
Authors: Ruoyu Chen, Jun Li, Qiaofei Pan, Dingguo Zheng, Bin Zhang, Ye Tian, Jianqi Li, Huaixin Yang, Yiming Pan
Efficient coupling between light and bulk plasmons (BPs) remains a central challenge because of their inherent mode mismatch, limited penetration depth, and pronounced resonant energy mismatch between visible-range photons and BPs. In this work, we demonstrate that ultrafast free electrons can coherently mediate an interaction between electromagnetic fields and BPs at the nanoscale. An electron pulse emitted from the photocathode of ultrafast transmission electron microscope, functions as a quantum intermediary that is capable of simultaneously interacting with the laser field by multiphoton processes and BPs by perturbative scattering. Electron energy-loss spectroscopy can capture this indirect interaction, the final electron energy distribution encodes both quantum pathways arising from distinct combinations of multiphoton absorption and emission and BP scattering events. Interference among these pathways gives rise to characteristic spectral modulations, directly revealing the exchange of energy and information between photons and BPs via the electron delivery. Our results show that femtosecond-driven, ultrafast electrons provide a viable route to modulate and even control bulk plasmon excitations in a volume, thereby extending beyond the conventional nanoplasmonics schemes on manipulating surface plasmons by light. This indirect light-BP interaction paves the promising way for exploring fundamental light-matter interaction at ultrafast and nanometer scales.
Authors: Chao Liu, Shuai Zhao, Chenhao Jia, Gengran Hu, Tingting Cui
Due to the merits of high efficiency and strong security against timing and side-channel attacks, ChaCha has been widely applied in real-time communication and data streaming scenarios. However, with the rapid development of AI-assisted cryptanalysis and quantum computing technologies, there are serious challenges to the secure implementation of ChaCha cipher. To further strengthen the security of ChaCha cipher, we propose an improved variant based on quantum random numbers, i.e., Quantum Random Number Enhanced ChaCha (QRE-ChaCha). Specifically, the design XORs the initial constants with quantum random numbers and periodically injects quantum random numbers into selected state words during odd rounds to enhance diffusion. Compared with the original ChaCha, the present variant shows stronger resistance to differential attacks and generates a keystream with statistical randomness, thereby offering increased robustness against both classical and quantum attacks. To evaluate the security and performance of the present ChaCha, our analysis proceeds in three main parts. Firstly, we analyze its theoretical security in terms of quantum randomness and attack testing, and conduct differential cryptanalysis with an automated search method based on the Boolean satisfiability problem (SAT). Secondly, we subject the keystream generated by the cipher to randomness tests using the NIST statistical test suite and the GM/T 0005-2021 randomness testing standard. Finally, we assess its encryption and decryption performance by measuring its encryption speed on files of various sizes. According to the results, the present ChaCha is significantly improved to resist differential attacks while maintaining the high efficiency of the original ChaCha cipher, and its keystream successfully passes statistical randomness tests using the NIST and GM/T 0005-2021 standards, meeting cryptographic application requirements.
Authors: Ryota Kojima, Masahiko Kamoshita, Keita Kanno
Quantum chemistry is a key target for quantum computing, but benchmarking quantum algorithms for large molecular systems remains challenging due to the lack of exactly solvable yet structurally realistic models. In particular, molecular Hamiltonians typically contain $O(N^4)$ Pauli terms, significantly increasing the cost of quantum simulations, while many exactly solvable models, such as the one-dimensional Fermi-Hubbard (1D FH) model, contain only $O(N)$ terms. In this work, we introduce the orbital-rotated Fermi-Hubbard (ORFH) model as a scalable and exactly solvable benchmarking problem for quantum chemistry algorithms. Starting from the 1D FH model, we apply a spin-involved orbital rotation to construct a Hamiltonian that retains the exact ground-state energy but exhibits a Pauli term count scaling as $O(N^4)$, similar to real molecular systems. We analyze the ORFH Hamiltonian from multiple perspectives, including operator norm and electronic correlation. We benchmark variational quantum eigensolver (VQE) optimizers and Pauli term grouping methods, and compare their performance with those for hydrogen chains. Furthermore, we show that the ORFH Hamiltonian increases the computational difficulty for classical methods such as the density matrix renormalization group (DMRG), offering a nontrivial benchmark beyond quantum algorithms. Our results demonstrate that the ORFH model provides a versatile and scalable testbed for benchmarking quantum chemistry algorithms under realistic structural conditions, while maintaining exact solvability even at large system sizes.
Authors: Vahid R. Asadi, Eric Culf, Alex May
A non-local quantum computation (NLQC) replaces an interaction between two quantum systems with a single simultaneous round of communication and shared entanglement. We study two classes of NLQC, $f$-routing and $f$-BB84, which are of relevance to classical information theoretic cryptography and quantum position-verification. We give the first non-trivial lower bounds on entanglement in both settings, but are restricted to lower bounding protocols with perfect correctness. Within this setting, we give a lower bound on the Schmidt rank of any entangled state that completes these tasks for a given function $f(x,y)$ in terms of the rank of a matrix $g(x,y)$ whose entries are zero when $f(x,y)=0$, and strictly positive otherwise. This also leads to a lower bound on the Schmidt rank in terms of the non-deterministic quantum communication complexity of $f(x,y)$. Because of a relationship between $f$-routing and the conditional disclosure of secrets (CDS) primitive studied in information theoretic cryptography, we obtain a new technique for lower bounding the randomness complexity of CDS.
Authors: Selomit Ramírez-Uribe, Andrés E. Rentería-Olivo, Germán Rodrigo
Quantum algorithms are a promising framework for unfolding the causal configurations of multiloop Feynman diagrams, which is equivalent to querying the \textit{directed acyclic graph} (DAG) configurations of undirected graphs in graph theory. In this paper, we present a quantum algorithm for querying in both types of applications, using a systematic and sparing logic in the design of an oracle operator. The construction of the quantum oracle is based exclusively on multicontrolled Toffoli (MCX) gates and quantum NOT (Pauli-$X$) gates. The efficiency of the algorithm is evaluated by comparison with a quantum algorithm based on binary clauses. Furthermore, we analyse the impact of traspilation and introduce an appropriate metric to assess the complexity of the algorithm, the \emph{quantum circuit area}. We explicitly analyse three-, four- and five-eloop topologies, which have not previously been explored due to their higher complexity and the current limitations of quantum simulators.
Authors: Ahmed Zahia, M. Y. Abd-Rabbou, Atta ur Rahman, Cong Feng Qiao
Quantum Information scrambling (QI-scrambling) is a pivotal area of inquiry within the study of quantum many-body systems. This research derives mathematical upper and lower bounds for the scrambling rate by applying the Maligranda inequality. Our results indicate that the upper bounds, lower bounds, and scrambling rates coincide precisely when local operators exhibit to be unitary-Hermitian. Crucially, the convergence or divergence of these upper and lower bounds relative to the scrambling rate is contingent upon the system's initial state. The spin-star model to validate this theoretical framework is investigated, considering thermal and pure initial states. The implantation of the ancilla or external qubit aligns the scrambling rate with the established bounds. The upper and lower bounds may diverge from the scrambling rate based on the system's initial state when both local operators are multi-quit systems. The scrambling rate found grows with the increase of the qubit number in local operators.
Authors: Christopher Gerhard, Todd A. Brun
Many current quantum error-correcting codes that achieve full fault tolerance suffer from having low ratios of logical to physical qubits and significant overhead. This makes them difficult to implement on current noisy intermediate-scale quantum (NISQ) computers and results in the inability to perform quantum algorithms at useful scales with near-term quantum processors. As a result, calculations are generally done without encoding. We propose a middle ground between these two approaches: constructions in the $[[n,n-2,2]]$ quantum error-detecting code that can detect any error from a single faulty gate by measuring the stabilizer generators of the code and additional ancillas at the end of the computation. This achieves weak fault tolerance. As we show, this yields a significant improvement over no error correction for small computations with low enough physical error probabilities and requires much less overhead than codes that achieve full fault tolerance. We give constructions for a set of gates that achieve universal quantum computation in this error-detecting code, while satisfying weak fault tolerance up to analog imprecision on the physical rotation gate.
Authors: Ya-Feng Jiao, Jie Wang, Dong-Yang Wang, Lei Tang, Yan Wang, Yun-Lan Zuo, Wan-Su Bao, Le-Man Kuang, Hui Jing
The generation and manipulation of multipartite entanglement and EPR steering in macroscopic systems not only play a fundamental role in exploring the nature of quantum mechanics, but are also at the core of current developments of various nascent quantum technologies. Here we report a theoretical method using squeezing-phase-controlled quantum noise flows to selectively generate and manipulate quantum entanglement and asymmetric EPR steering in a nonlinear $\chi^{(2)}$ whispering-gallery-mode (WGM) optomechanical resonator. We show that by pumping the $\chi^{(2)}$ nonlinear medium with two-photon optical fields and broadband squeezed lights, a pair of counterpropagating squeezed optical modes could be introduced to the WGM resonator, each coupled with an independent squeezed vacuum reservoir. This configuration could enable squeezing-phase-controlled light-reservoir interaction for each squeezed optical mode, providing a flexible tool for tailoring asymmetric optical noise flows in the counterpropagating modes. Based on this unique feature, it is found that with the injection of asymmetric noise flows, the generation of various types of bipartite and tripartite entanglement become phase-dependent and thus they can be produced in an asymmetric way. More excitingly, it is also found that by further properly adjusting the squeezing parameters, the overall asymmetry of EPR steering can also be stepwise driven from no-way regime, one-way regime to two-way regime. These findings, holding promise for preparing rich types of entangled quantum resources with asymmetric features, may have potential applications in the area of secure quantum information processing such as quantum secure direct communication and one-way quantum computing.
Authors: Lukas Hantzko, Lennart Binkowski, Sabhyata Gupta
Analysis of quantum processes, especially in the context of noise, errors, and decoherence is essential for the improvement of quantum devices. An intuitive representation of those processes modeled by quantum channels are Pauli transfer matrices. They display the action of a linear map in the $n$-qubit Pauli basis in a way, that is more intuitive, since Pauli strings are more tangible objects than the standard basis matrices. We set out to investigate classical algorithms that convert the various representations into Pauli transfer matrices. We propose new algorithms that make explicit use of the tensor product structure of the Pauli basis. They convert a quantum channel in a given representation (Chi or process matrix, Choi matrix, superoperator, or Kraus operators) to the corresponding Pauli transfer matrix. Moreover, the underlying principle can also be used to calculate the Pauli transfer matrix of other linear operations over $n$-qubit matrices such as left-, right-, and sandwich multiplication as well as forming the (anti-)commutator with a given operator. Finally, we investigate the runtime of these algorithms, derive their asymptotic scaling and demonstrate improved performance using instances with up to seven qubits.
Authors: Hao Ai, Yu-xi Liu
To demonstrate supremacy of quantum computing, increasingly large-scale superconducting quantum computing chips are being designed and fabricated. However, the complexity of simulating quantum systems poses a significant challenge to computer-aided design of quantum chips, especially for large-scale chips. Harnessing the scalability of graph neural networks (GNNs), we here propose a parameter designing algorithm for large-scale superconducting quantum circuits. The algorithm depends on the so-called 'three-stair scaling' mechanism, which comprises two neural-network models: an evaluator supervisedly trained on small-scale circuits for applying to medium-scale circuits, and a designer unsupervisedly trained on medium-scale circuits for applying to large-scale ones. We demonstrate our algorithm in mitigating quantum crosstalk errors. Frequencies for both single- and two-qubit gates (corresponding to the parameters of nodes and edges) are considered simultaneously. Numerical results indicate that the well-trained designer achieves notable advantages in efficiency, effectiveness, and scalability. For example, for large-scale superconducting quantum circuits consisting of around 870 qubits, our GNNs-based algorithm achieves 51% of the errors produced by the state-of-the-art algorithm, with a time reduction from 90 min to 27 sec. Overall, a better-performing and more scalable algorithm for designing parameters of superconducting quantum chips is proposed, which initially demonstrates the advantages of applying GNNs in superconducting quantum chips.
Authors: Zichang He, Rudy Raymond, Ruslan Shaydulin, Marco Pistoia
Solving hard optimization problems is one of the most promising application domains for quantum computers due to the ubiquity of such problems in industry and the availability of broadly applicable quantum speedups. However, the ability of near-term quantum computers to tackle industrial-scale optimization problems is limited by their size and the overheads of quantum error correction. Quantum Random Access Optimization (QRAO) has been proposed to reduce the space requirements of quantum optimization. However, to date QRAO has only been implemented using variational algorithms, which suffer from the need to train instance-specific variational parameters, making them difficult to scale. We propose and benchmark a non-variational approach to QRAO based on the Quantum Alternating Operator Ansatz (QAOA) for the MaxCut problem. We show that instance-independent ``fixed" parameters achieve good performance, removing the need for variational parameter optimization. Additionally, we evaluate different design choices, such as various mixers, initial states, and QRAO-specific implementations of the QAOA cost operator, and identify a strategy that performs well in practice. Our results pave the way for the practical execution of QRAO on early fault-tolerant quantum computers.
Authors: Chiara Capecci, Gopal Chandra Santra, Alberto Bottarelli, Emanuele Tirrito, Philipp Hauke
Quantum optimization has emerged as a promising approach for tackling complicated classical optimization problems using quantum devices. However, the extent to which such algorithms harness genuine quantum resources and the role of these resources in their success remain open questions. In this work, we investigate the resource requirements of the Quantum Approximate Optimization Algorithm (QAOA) through the lens of the resource theory of nonstabilizerness. We demonstrate that the nonstabilizerness in QAOA increases with circuit depth before it reaches a maximum, to fall again during the approach to the final solution state -- creating a barrier that limits the algorithm's capability for shallow circuits. We find curves corresponding to different depths to collapse under a simple rescaling, and we reveal a nontrivial relationship between the final nonstabilizerness and the success probability. Finally, we identify a similar nonstabilizerness barrier also in adiabatic quantum annealing. Our results provide deeper insights into how quantum resources influence quantum optimization.
Authors: Qianxu Wang, Sara Magdalena Gómez, Juan S. Salcedo-Gallo, Roy Leibovitz, Jake Freeman, Salil Bedkihal, Mattias Fitzpatrick
Two-level system (TLS) defects in dielectrics are a major source of decoherence in superconducting circuits, yet their atomistic origin, frequency distribution, and dipole moments remain poorly understood. Current probes, which are predominantly based on qubits or resonators, require complex fabrication and only measure defects within a narrow frequency band and limited mode volume, hindering direct insight into TLS defect behaviour in isolated materials and interfaces. Here, we introduce a broadband 3D waveguide spectroscopy technique that enables cryogenic probing of ensembles of TLS defects that we call Broadband Cryogenic Transient Dielectric Spectroscopy (BCTDS). Complementary to the dielectric dipper method, this approach probes a broader spectrum and reveals interference of drive-induced sidebands of the ensembles of TLS defects. The broadband and power-tunable nature of BCTDS makes it especially well-suited to the study of dressed-state physics in driven ensembles of TLS defects, including multi-photon processes and sideband-resolved dynamics. Additionally, BCTDS enables the identification of eigenmode frequencies of the undriven ensembles of TLS defects through characteristic V-shaped features obtained via Fourier analysis of time-domain signals, and shows evidence of memory effects arising from interactions and the broadband nature of our approach. Crucially, our method is modular and can be applied throughout the device fabrication process, informing mitigation strategies and advancing the design of low-loss materials with broad implications for quantum technologies and materials science.
Authors: Ariel Caticha, Hassaan Saleem
The general framework of Entropic Dynamics (ED) is used to construct non-relativistic models of relational quantum mechanics from well known inference principles - probability, entropy and information geometry. Although only partially relational - the absolute structures of simultaneity and Euclidean geometry are still retained - these models provide a useful testing ground for ideas that will prove useful in the context of more realistic relativistic theories. The fact that in ED the positions of particles have definite values, just as in classical mechanics, has allowed us to adapt to the quantum case some intuitions from Barbour and Bertotti's classical framework. Here, however, we propose a new measure of the mismatch between successive states that is adapted to the information metric and the symplectic structures of the quantum phase space. We make explicit that ED is temporally relational and we construct non-relativistic quantum models that are spatially relational with respect to rigid translations and rotations. The ED approach settles the longstanding question of what form should the constraints of a classical theory take after quantization: the quantum constraints that express relationality are to be imposed on expectation values. To highlight the potential impact of these developments, the non-relativistic quantum model is parametrized into a generally covariant form and we show that the ED approach evades the analogue of what in quantum gravity has been called the problem of time.
Authors: Elijah Pelofske, Frank Barrows, Pratik Sathe, Cristiano Nisoli
While quantum annealers have emerged as versatile and controllable platforms for experimenting on correlated spin systems, the important phenomenology of magnetic memory and hysteresis remain unexplored on hardware designed to escape metastable states via quantum tunneling. Here, we present the first general protocol to experiment on magnetic hysteresis on programmable quantum annealers, and implement it on three D-Wave superconducting qubit quantum annealers, using up to thousands of spins, for both ferromagnetic and disordered Ising models, and across different graph topologies. We observe hysteresis loops whose area depends non-monotonically on quantum fluctuations, exhibiting both expected and unexpected features, such as disorder-induced steps and non-monotonicities. Our work establishes quantum annealers as a platform for probing non-equilibrium emergent magnetic phenomena, thereby broadening the role of analog quantum computers into foundational questions in condensed matter physics.
Authors: Klim D. Bondar, Ivan S. Sushchev, Daniil D. Bulavkin, Kirill E. Bugai, Anna S. Sidelnikova, Dmitriy M. Melkonian, Veronika M. Vakhrusheva, Dmitriy A. Dvoretskiy
In this paper, we present a method for security analysis against the Trojan-horse attacks (THA) launched to a practical fiber-based quantum key distribution (QKD) system across a wide spectral range. To achieve this, we utilize optical time-domain reflectometry (OTDR) for the spectral reflectance analysis in the near-infrared range 1100 - 1800 nm with centimeter-level resolution and with a noise floor down to -80 dB. Finally, the total theoretical-security analysis against the THA side channel considering the spectral reflectance and transmittance data over this spectral range is conducted. To the best of our knowledge, our OTDR setup and the corresponding results are the first of their kind in a wide spectral range.
Authors: Yujie Zhang, Jonah Spodek, David Schmid, Carter Reid, Liam J. Morrison, Thomas Jennewein, Kevin J. Resch, Robert W. Spekkens
By combining the assumption of Bell locality with that of generalized noncontextuality, we define different classes of noncontextuality inequalities for correlations in a bipartite Bell scenario. These classes are distinguished by which operational identities are enforced, where certain natural subsets form a hierarchy and provide a new way to understand and classify different forms of quantum correlations. Specifically, we show that violations of inequalities at different levels of this hierarchy can serve as witnesses to determine whether a bipartite quantum state exhibits entanglement, steering, or nonlocality, thereby giving a systematic and unified method for certifying these distinct bipartite quantum resources. To illustrate the power of this approach, we demonstrate the violation of one such inequality in an experiment with polarization-entangled photons. This experimental implementation enables us to certify entanglement in certain two-qubit isotropic states for which certification by Bell or steering inequalities is known to be impossible. Our entanglement certification scheme is the first to combine all of the following desirable features: it requires no prior characterization of the measurements, it delivers a verdict based on quantities that are independent of the tomographic gauge freedom, and it is able to certify {\em any} entangled state without requiring any auxiliary entangled sources.
Authors: Dinesh Kumar Panda, Colin Benjamin
Topological phases, edge states, and flat bands in synthetic quantum systems are a key resource for topological quantum computing and noise-resilient information processing. We introduce a scheme based on step-dependent quantum walks on cyclic graphs, termed cyclic quantum walks (CQWs), to simulate exotic topological phenomena using discrete Fourier transforms and an effective Hamiltonian. Our approach enables the generation of both gapped and gapless topological phases, including Dirac cone-like energy dispersions, topologically nontrivial flat bands, and protected edge states, all without resorting to split-step or split-coin protocols. Odd and even-site cyclic graphs exhibit markedly different spectral characteristics, with rotationally symmetric flat bands emerging exclusively in $4n$-site graphs ($n\in \mathbf{N}$). We analytically establish the conditions for the emergence of topological, gapped flat bands and show that gap closings in rotation space imply the formation of Dirac cones in momentum space. Further, we engineer protected edge states at the interface between distinct topological phases in both odd and even cycle graphs. We numerically demonstrate that the edge states are robust against moderate static and dynamic gate disorder and remain stable against phase-preserving perturbations. This scheme serves as a resource-efficient and versatile platform for engineering topological phases, transitions, edge states, and flat bands in quantum systems, opening new avenues for fault-tolerant quantum technologies.
Authors: Yoshisuke Ban, Kimihiko Kato, Shota Iizuka, Hiroshi Oka, Shigenori Murakami, Koji Ishibashi, Satoshi Moriyama, Takahiro Mori, Keiji Ono
Pauli spin blockade (PSB) is a spin-dependent charge transport process that typically appears in double quantum dot (QD) devices and is employed in fundamental research on single spins in nanostructures to read out semiconductor qubits. The operating temperature of PSB is limited by that of the QDs and remains below 10 K, limiting wide application development. Herein, we confirm that a single deep dopant in the channel of a silicon field effect transistor functions as a room-temperature QD; consequently, transport through two different deep dopants exhibits PSB up to room temperature. The characteristic magnetoconductance provides a means to identify PSB and enables the PSB device to function as a magnetic sensor with a sensitivity below geomagnetic field. Lifting in PSB caused by magnetic resonance (50 K) and Rabi oscillations (10 K) are also observed. Further development of this unique system may lead to room-temperature quantum technologies based on silicon technology.
Authors: Juan Polo, Wayne Jordan Chetcuti, Tobias Haug, Anna Minguzzi, Kevin Wright, Luigi Amico
Persistent currents flowing in spatially closed tracks define one of the most iconic concepts in mesoscopic physics. They have been studied in solid-state platforms such as superfluids, superconductors and metals. Cold atoms trapped in magneto-optical toroidal circuits and driven by suitable artificial gauge fields allow us to study persistent currents with unprecedented control and flexibility of the system's physical conditions. Here, we review persistent currents of ultracold matter. Capitalizing on the remarkable progress in driving different atomic species to quantum degeneracy, persistent currents of single or multicomponent bosons/fermions, and their mixtures can be addressed within the present experimental know-how. This way, fundamental concepts of quantum science and many-body physics, like macroscopic quantum coherence, solitons, vortex dynamics, fermionic pairing and BEC-BCS crossover can be studied from a novel perspective. Finally, we discuss how persistent currents can form the basis of new technological applications like matter-wave gyroscopes and interferometers.
Authors: Juntao Huang, Kun Ding, Jiangping Hu, Zhesen Yang
We introduce the complex frequency fingerprint (CFF), an experimentally accessible method for detecting the complex frequency Green's function (GF). Unlike the real frequency GF, where $\omega$ is real, this complex frequency GF is shown to play a necessary role in both non-Hermitian and quantum many-body systems. For non-Hermitian systems, we will prove that our method detects complex energy spectra, eigenstates, and complex frequency GFs throughout the complex plane, providing necessary identification of the non-Hermitian skin effect. For quantum many-body systems, our method reveals quasiparticle peaks across the complex plane and intuitively illustrates interaction effects. This information is difficult to obtain with real frequency detection. Our method paves the way for exploring exotic phenomena in both non-Hermitian and quantum many-body systems, bridging theory and experiment across diverse physical areas.
Authors: Shivam Sharma, Amartya S. Banerjee
In recent years, materials with topological flat bands have attracted significant attention due to their association with extraordinary transport properties and strongly correlated electrons. Yet, generic principles linking lattice architecture, strain, and band topology remain scarce. Here, using a unified graph-theoretic framework we generate entire families of two-dimensional lattices and, using analytical tight-binding calculations, demonstrate that a single mechanical knob -- uniform in-plane strain -- drives universal transitions between trivial insulating, Dirac semimetal, and quantum spin-Hall phases across all lattices. The framework yields several flat band lattices that were hitherto absent or largely unexplored in the literature -- for example, the checkerboard split-graph and triangular-Kagome lattices -- whose strain-driven topological phase diagrams we establish here for the first time. The design rules implied by our studies provide a blueprint for engineering topological states in a wide variety of 2D materials, photonic crystals, and circuit lattices, and are anticipated to accelerate the discovery of strain-programmable quantum matter.
Authors: Shu-Tong Zhou, Meng Cheng, Tibor Rakovszky, Curt von Keyserlingk, Tyler D. Ellison
We identify a three-dimensional system that exhibits long-range entanglement at sufficiently small but nonzero temperature--it therefore constitutes a quantum topological order at finite temperature. The model of interest is known as the fermionic toric code, a variant of the usual 3D toric code, which admits emergent fermionic point-like excitations. The fermionic toric code, importantly, possesses an anomalous 2-form symmetry, associated with the space-like Wilson loops of the fermionic excitations. We argue that it is this symmetry that imbues low-temperature thermal states with a novel topological order and long-range entanglement. Based on the current classification of three-dimensional topological orders, we expect that the low-temperature thermal states of the fermionic toric code belong to an equilibrium phase of matter that only exists at nonzero temperatures. We conjecture that further examples of topological orders at nonzero temperatures are given by discrete gauge theories with anomalous 2-form symmetries. Our work therefore opens the door to studying quantum topological order at nonzero temperature in physically realistic dimensions.
Authors: Gaia Da Prato, Yong Yu, Ronald Bode, Simon Gröblacher
A tunable magnetic field at low temperatures is essential for numerous applications, including spintronics, magnetic resonance imaging, and condensed matter physics. While commercial superconducting vector magnets are available, they are complex, expensive, and often not adaptable to specific experimental needs. As a result, simple in-house designs are often being used in research environments. However, no comprehensive step-by-step guide for their construction currently exists. In this work, we provide a detailed manual for designing and building a cryogenically compatible three-axis vector magnet. The system is tested at the mixing chamber of a dilution refrigerator at temperatures ranging from 15 mK to 4 K, with no significant increase in base temperature. Safety measures are implemented to mitigate heating from quenching. The coils are successfully driven with DC currents as high as 3 A, generating magnetic fields of up to 2.5 T in the bobbin's bore and 0.4 T at the sample position. Magnetic field measurements using Hall sensors demonstrate good agreement with the predictions of the designed performance.
Authors: Ryan T. Grimm, Alexander J. Staat, Joel D. Eaves
The solutions to many problems in the mathematical, computational, and physical sciences often involve multidimensional integrals. A direct numerical evaluation of the integral incurs a computational cost that is exponential in the number of dimensions, a phenomenon called the curse of dimensionality. The problem is so substantial that one usually employs sampling methods, like Monte Carlo, to avoid integration altogether. Here, we derive and implement a quantum algorithm to compress a multidimensional integrand into a product of matrix-valued functions - a spectral tensor train - changing the computational complexity of integration from exponential to polynomial. The algorithm compresses the integrand by applying a sequence of quantum gates to an unentangled quantum state, where each term corresponds to a body-ordered term in the potential. Because it allows for the systematic elimination of small contributions to the integral through decimation, we call the method integral decimation. The functions in the spectral basis are analytically differentiable and integrable, and in applications to the partition function, integral decimation numerically factorizes an interacting system into a product of noninteracting ones. We illustrate integral decimation by evaluating the absolute free energy and entropy of a chiral XY model as a continuous function of temperature. We also compute the nonequilibrium time-dependent reduced density matrix of a quantum chain with between two and forty levels, each coupled to colored noise. When other methods provide numerical solutions to these models, they quantitatively agree with integral decimation. When conventional methods become intractable, integral decimation can be a powerful alternative.
Authors: Karol Sajnok, Kacper Dębski
We introduce a covariant canonical quantization for a particle in curved spacetime that tracks operator-ordering ambiguities. Parameterizing spatial and temporal ordering, we derive a Hermitian Hamiltonian with leading quantum-relativistic corrections. In a uniformly accelerated frame, we show the semiclassical heat-capacity approximation misses these effects and then develop a perturbative quantum treatment using Airy-function modes to obtain analytical first- and second-order energy shifts. Including these shifts in the partition function yields nontrivial, ordering-dependent specific-heat corrections. Numerical studies for electrons in extreme electric fields and ultra-light particles in strong gravitational fields demonstrate that these corrections become significant at intermediate temperatures. Enforcing the Tolman-Ehrenfest relation for spatial temperature variation further modulates the heat-capacity profile. Our results suggest that precision calorimetry in laser-acceleration or analogue gravity setups could probe quantum-ordering effects in relativistic regimes.
Authors: Azhar Ikhtiarudin, Gagus Ketut Sunnardianto, Fadjar Fathurrahman, Mohammad Kemal Agusta, Hermawan Kresno Dipojono
The Adaptive Variational Quantum Eigensolver (ADAPT-VQE) is a promising approach for quantum algorithms in the Noisy Intermediate-Scale Quantum (NISQ) era, offering advantages over traditional VQE methods by reducing circuit depth and mitigating challenges in classical optimization. However, a major challenge in ADAPT-VQE is the high quantum measurement (shot) overhead required for circuit parameter optimization and operator selection. In this work, we propose two integrated strategies to reduce the shot requirements in ADAPT-VQE. First, we reuse Pauli measurement outcomes obtained during VQE parameter optimization in the subsequent operator selection step of the next ADAPT-VQE iteration, which involves operator gradient measurements. Second, we apply variance-based shot allocation to both Hamiltonian and operator gradient measurements. Our numerical results demonstrate that each method, individually and in combination, significantly reduces the number of shots needed to achieve chemical accuracy while maintaining result fidelity across the studied molecular systems.
Authors: Tian-Ming Li, Zheng-Hang Sun, Yun-Hao Shi, Zhen-Ting Bao, Yong-Yi Wang, Jia-Chi Zhang, Yu Liu, Cheng-Lin Deng, Yi-Han Yu, Zheng-He Liu, Chi-Tong Chen, Li Li, Hao Li, Hao-Tian Liu, Si-Yun Zhou, Zhen-Yu Peng, Yan-Jun Liu, Ziting Wang, Yue-Shan Xu, Kui Zhao, Yang He, Da'er Feng, Jia-Cheng Song, Cai-Ping Fang, Junrui Deng, Mingyu Xu, Yu-Tao Chen, Bozhen zhou, Gui-Han Liang, Zhong-Cheng Xiang, Guangming Xue, Dongning Zheng, Kaixuan Huang, Zheng-An Wang, Haifeng Yu, Piotr Sierant, Kai Xu, Heng Fan
Quantum many-body systems with sufficiently strong disorder can exhibit a non-equilibrium phenomenon, known as the many-body localization (MBL), which is distinct from conventional thermalization. While the MBL regime has been extensively studied in one dimension, its existence in higher dimensions remains elusive, challenged by the avalanche instability. Here, using a 70-qubit two-dimensional (2D) superconducting quantum simulator, we experimentally explore the robustness of the MBL regime in controlled finite-size 2D systems. We observe that the decay of imbalance becomes more pronounced with increasing system sizes, scaling up from 21, 42 to 70 qubits, with a relatively large disorder strength, and for the first time, provide an evidence for the many-body delocalization in 2D disordered systems. Our experimental results are consistent with the avalanche theory that predicts the instability of MBL regime beyond one spatial dimension. This work establishes a scalable platform for probing high-dimensional non-equilibrium phases of matter and their finite-size effects using superconducting quantum circuits.
Authors: Saeed A. Khan, Sridhar Prabhu, Logan G. Wright, Peter L. McMahon
Quantum computing has the potential to deliver large advantages on computational tasks, but advantages for practical tasks are not yet achievable with current hardware. Quantum sensing is an entirely separate quantum technology that can provide its own kind of a quantum advantage. In this Perspective, we explain how the merger of quantum sensing with quantum computing has recently given rise to the notion of quantum computational sensing, and a new kind of quantum advantage: a quantum computational-sensing advantage. This advantage can be realized with far lower hardware requirements than purely computational quantum advantage. We explain how several recent proposals and experiments can be understood as quantum computational sensing, and discuss categorizations of the general architectures that quantum-computational-sensing protocols can have. We conclude with an outlook on open questions and the prospects for quantum computational sensors and quantum computational-sensing advantage.
Authors: Jatin Ghildiyal, Shubhrangshu Dasgupta, Asoka Biswas
We present a theoretical study of synchronization between two strongly driven magnon modes indirectly coupled via a single-mode microwave cavity. Each magnon mode, hosted in separate Yttrium Iron Garnet spheres, interacts coherently with the cavity field, leading to cavity-mediated nonlinear coupling. We show, by using input-output formalism, that both classical and quantum synchronization emerge for appropriate choices of coupling, detuning, and driving. We find that thermal noise reduces quantum synchronization, highlighting the importance of low-temperature conditions. This study provides useful insights into tunable magnonic interactions in cavity systems, with possible applications in quantum information processing and hybrid quantum technologies.
Authors: Ishaan Ganti, Jianshu Cao
Cavity polaritons, quasiparticles formed by coherent light-matter coupling, are at the heart of fundamental concepts of quantum optics. The quintessential signature of this coherent coupling is the Rabi oscillation, which results from the neglect of the counter-rotating-wave (CRW) effect in the weak-coupling regime. The goal of this letter is to predict resonant beatings that envelop the Rabi oscillation on the second or higher excitation manifold. These polariton beatings arise from the CRW term in the Dicke or Pauli-Fierz model and are directly correlated with the asymmetry in polariton eigenenergies. Our findings highlight the relevance of the CRW effect even in the weak-coupling regime, offer novel perspectives about coherent polariton dynamics, and shed new light on experiments of coupled quantum systems.
Authors: Francisco D. Santillan, Andreas Hanke
The interaction between atoms and a quantized radiation field is fundamentally important in quantum optics and quantum information science. Due to their unusual properties, Rydberg atoms are promising building blocks for two-qubit gates and atom-light quantum interfaces, exploiting the Rydberg blockade interaction, which prevents two atoms at close distance from being simultaneously excited to Rydberg states. Recently, this effect was used to engineer quantum processors based on arrays of interacting Rydberg atoms illuminated by Raman lasers. Motivated by these experiments, we extend the Jaynes-Cummings model to study the interaction between two Rydberg atoms interacting by the Rydberg blockade and a quantized radiation field. We consider both number (Fock) states of the field and single-mode quantum coherent states. In particular, we discuss different types of entanglements between various components of the total system consisting of the two Rydberg-interacting atoms and coherent states of the field, and show that the behavior is significantly different compared to a system with non-interacting atoms corresponding to the two-atom Tavis-Cummings model. Our results are relevant in view of atom-light quantum interfaces as components for future long-distance quantum communication.
Authors: Salvatore Tirone, Gian Marcello Andolina, Giuseppe Calajò, Vittorio Giovannetti, Davide Rossini
Quantum batteries have demonstrated remarkable charging properties, showing that a quantum advantage is possible in the realm of quantum thermodynamics. However, finding an effective strategy to store energy for long periods remains crucial in these systems. Here, we investigate different configurations of a waveguide-QED system acting as a quantum battery and show that, in this context, collective effects can slow down the self-discharging time of the battery, thus improving the storage time. Specifically, when the artificial atoms of the array are arranged randomly, the energy and ergotropy of the optical system are shown to decay at a subexponential rate over long periods, in contrast to the energy decay of a single atom in a waveguide. In the case of atoms arranged in an ordered lattice, collective effects slow down dissipative discharging only for a specific lattice spacing. Thus, in both configurations, collective effects can be used to boost the energy-protection properties of optical systems.
Authors: Maritza Ahumada, Natalia Valderrama-Quinteros, Diego Tancara, Guillermo Romero
Controlling the propagation of quantum excitations in low-dimensional systems is pivotal for quantum technologies, including communication networks and quantum simulators. We propose a one-dimensional hybrid quantum lattice model, where each lattice unit integrates a single-mode resonator that interacts with a two-level system (TLS), featuring direct coupling between adjacent TLSs. This configuration enables the coherent propagation of polaritons, spin waves, and photons, depending on the interplay between light-matter coupling and qubit-qubit interactions. Employing the time-evolving block decimation (TEBD) algorithm, we simulate the dynamics of various excitation configurations and analyze their transport characteristics using local observables. Our analysis reveals the importance of matching impedance and resonance conditions via system parameters for the propagation of different types of excitations or swapping the nature of excitations along the hybrid lattice. These findings offer insights into designing controllable quantum links and single-excitation swaps in low-dimensional quantum systems.
Authors: Lars Meschede, Daniel D. A. Clarke, Ortwin Hess
The unprecedented pace of evolution in nanoscale architectures for cavity quantum electrodynamics (cQED) has posed crucial challenges for theory, where the quantum dynamics arising from the non-perturbative dressing of matter by cavity electric and magnetic fields, as well as the fundamentally non-hermitian character of the system are to be treated without significant approximation. The lossy electromagnetic resonances of photonic, plasmonic or magnonic nanostructures are described as quasinormal modes (QNMs), whose properties and interactions with quantum emitters and spin qubits are central to the understanding of dissipative nano-optics and magnetodielectric cQED. Despite recent advancements toward a fully quantum framework for QNMs, a general and universally accepted approach to QNM quantization for arbitrary linear media remains elusive. In this work, we introduce a unified theoretical framework, based on macroscopic QED and complex coordinate transformations, that achieves QNM quantization for a wide class of spatially inhomogeneous, dissipative (with possible gain components) and dispersive, linear, magnetodielectric resonators. The complex coordinate transformations equivalently convert the radiative losses into non-radiative material dissipation, and via a suitable transformation that reflects all the losses of the resonator, we define creation and annihilation operators that allow the construction of modal Fock states for the joint excitations of field-dressed matter. By directly addressing the intricacies of modal loss in a fully quantum theory of magnetodielectric cQED, our approach enables the exploration of modern, quantum nano-optical experiments utilizing dielectric, plasmonic, magnetic or hybrid cQED architectures, and paves the way towards a rigorous assessment of room-temperature quantum nanophotonic technologies without recourse to ad hoc quantization schemes.
Authors: Arash Azizi
We report a profound manifestation of quantum complementarity in the higher-order photon statistics of the ``Janus state,'' a coherent superposition of two squeezed vacua. We find that the state acts as a perfect quantum switch for multi-photon correlations, toggled by the availability of which-path information. Erasing this information activates quantum interference that can be tuned to be maximally destructive. This reveals a remarkable hierarchy of suppression: while two-photon correlations remain finite, we prove analytically and demonstrate numerically that it is possible to drive all higher-order correlations ($g^{(k)}$ for $k \ge 3$) to zero. This transition from the extreme bunching of the constituent states ($g^{(k)} \to \infty$) to a state of profound quantum order is visualized by the emergence of negativity in the state's Wigner function, an unambiguous signature of non-classicality. This work provides a foundational demonstration of quantum complementarity in multi-photon statistics and introduces a new paradigm for engineering highly ordered, non-classical light from Gaussian resources.
Authors: Sangwon Kim, Seongjin Ahn, Denis V. Seletskiy, Andrey S. Moskalenko
Recent realization of an intense quantum light, namely bright squeezed vacuum, opened a new perspective on quantum light-matter interaction. Several theoretical works have appeared based on coherent state expansions of quantum state of light to investigate non-classical driving of high-harmonic generation in atomic gases and solids, or free-electron dynamics, but their predictions surprisingly coincide with what one could expect from essentially classical interpretations of the light statistics. A deeper theoretical insight into the underlying physics is necessary for understanding of observed experimental findings and predicting emerging effects relying on this new configuration. Here we present a theoretical framework to describe tunneling driven by quantum light, where the properties of such light are captured by a statistical ensemble of classical fields via a hydrodynamic, also referred to as Bohmian, formulation. Generalizing the quasiclassical theory of non-adiabatic tunneling driven by classical light, a single tunneling event is described by a bundle of tunneling solutions, each driven by a classical field corresponding to one realization in the ensemble. Quantum statistics of light are thus imprinted on the measured current. Fully quantum description of light via the Bohmian trajectories of its field provides a perfect fit to the description of the electron (under-) above-barrier dynamics in terms of (complex quasiclassical) real classical trajectories, resulting in a consistent and elegant theoretical approach. To illustrate this, we consider BSV-induced electron transport from the tip to the surface in the tunneling microscope configuration demonstrating the transition from the multiphoton to the direct tunneling regime.
Authors: D V Tsarev, D V Ansimov, S A Podoshvedov, A P Alodjants
In this work, we consider the feasibility of Schr{ö}dinger cat (SC) and $N00N$ states formation by a convenient bosonic Josephson junction (BJJ) system in two-mode approximation. Starting with purely quantum description of two-mode Bose-Einstein condensate we investigate the effective potential approach that provides an accurate analytical description for the system with a large number of particles. We show that in the zero temperature limit SC states result from a quantum phase transition that occurs when the nonlinear strength becomes comparable with the Josephson coupling parameter. The Wigner function approach demonstrates the growth of the SC state halves separation and formation of $N00N$-like states (a Fock state superposition) with the particle number increase. We examine the possibility to attain the SC state at finite temperatures and a weak dissipation leading to appearing of some critical temperature; it defines the second-order phase transition from classical activation process to the SC state formation through the quantum tunneling phenomenon. Numerical estimations demonstrate that the critical temperature is sufficiently below the temperature of atomic condensation. The results obtained may be useful for experimental observation of SC states with small condensate Josephson junctions.
Authors: Felix Burt, Kuan-Cheng Chen, Kin K. Leung
Quantum computers face inherent scaling challenges, a fact that necessitates investigation of distributed quantum computing systems, whereby scaling is achieved through interconnection of smaller quantum processing units. However, connecting large numbers of QPUs will eventually result in connectivity constraints at the network level, where the difficulty of entanglement sharing increases with network path lengths. This increases the complexity of the quantum circuit partitioning problem, since the cost of generating entanglement between end nodes varies with network topologies and existing links. We address this challenge using a simple modification to existing partitioning schemes designed for all-to-all connected networks, that efficiently accounts for both of these factors. We investigate the performance in terms of entanglement requirements and optimisation time of various quantum circuits over different network topologies, achieving lower entanglement costs in the majority of cases than state-of-the-art methods. We provide techniques for scaling to large-scale quantum networks employing both network and problem coarsening. We show that coarsened methods can achieve improved solution quality in most cases with significantly lower run-times than direct partitioning methods.
Authors: Mahmoud A. Selim, Max Ehrhardt, Yuqiang Ding, Qi Zhong, Armando Perez Leija, Konstantinos G. Makris, Ramy El Ganainy, Sahin K. Ozdemir, Matthias Heinrich, Alexander Szameit, Demetrios N. Christodoulides, Mercedeh Khajavikhan
Engineering quantum bath networks through non-Hermitian subsystem Hamiltonians has recently emerged as a promising strategy for qubit cooling, state stabilization, and fault-tolerant quantum computation. However, scaling these systems while maintaining precise control over their complex interconnections, especially in the optical domain, poses significant challenges in both theoretical modeling and physical implementation. In this work, drawing on principles from quantum and mathematical physics, we introduce a systematic framework for constructing non-Hermitian subsystems within entirely Hermitian photonic platforms. In particular, controlled exponential decay without actual absorption loss is realized in finite 1-D waveguide chains through discrete-to-continuum coupling and Lanczos transformations. Using this new methodology, we implement parity-time symmetric quantum systems and experimentally demonstrate that these artificial bath environments accurately replicate the dynamics of non-Hermitian arrangements in both single- and multi-photon excitation regimes. Since the non-Hermitian subsystem response deterministically arises from an artificially built Hermitian bath, the quantum evolution can be monitored via post-selection in this fully conservative configuration. This approach bridges the gap between theoretical models and experimental realizations, thus paving the way for exploiting quantum bath engineering in advanced information processing and emerging quantum technologies.
Authors: A.Yu. Bazhenov, M. Nikitina, Alexander Alodjants
In the present work we propose a novel quantum material concept, which enables super- and/or ultrastrong interaction of two-level systems with the photonic field in a complex network. Within the mean field approximation we examine phase transition to superradiance that results in two excitation (polariton) branches and is accompanied by the appearance of non-zero macroscopic polarization of two-level systems. We characterize the statistical properties of networks by the first, ${\langle}k{\rangle}$, and second normalized, $\zeta\equiv{\langle}k^2{\rangle}/{\langle}k{\rangle}$, moments for node degree distribution. We have shown that the Rabi frequency is essentially enhanced due to the topology of the network within the anomalous domain where ${\langle}k{\rangle}$ and $\zeta$ sufficiently grow. The multichannel (multimode) structure of matter-field interaction leads superstrong coupling that provides primary behavior of the high temperature phase transition. The results obtained pave the way to design new photonic and polaritonic circuits, quantum networks for efficient processing quantum information at high (room) temperatures.
Authors: Yaqi Zhao, Kan He, Yongtao Zhang, Jinchuan Hou, Jianxi Gao, Shlomo Havlin, Xiangyi Meng
Quantum networks (QNs) have been predominantly driven by discrete-variable (DV) architectures. Yet, many optical platforms naturally generate Gaussian states--the common states of continuous-variable (CV) systems, making CV-based QNs an attractive route toward scalable, chip-integrated quantum computation and communication. To bridge the conceptual gap between well-studied DV entanglement percolation theories and their CV counterpart, we introduce a Gaussian-to-Gaussian entanglement distribution scheme that deterministically transports two-mode squeezed vacuum states across large CV networks. Analysis of the scheme's collective behavior using statistical-physics methods reveals a new form of entanglement percolation--negativity percolation theory (NegPT)--characterized by a bounded entanglement measure called the ratio negativity. We discover that NegPT exhibits a mixed-order phase transition, marked simultaneously by both an abrupt change in global entanglement and a long-range correlation between nodes. This distinctive behavior places CV-based QNs in a new universality class, fundamentally distinct from DV systems. Additionally, the abruptness of this transition introduces a critical vulnerability of CV-based QNs: conventional feedback mechanism becomes inherently unstable near the threshold, highlighting practical implications for stabilizing large-scale CV-based QNs. Our results not only unify statistical models for CV-based entanglement distribution but also uncover previously unexplored critical phenomena unique to CV systems, providing valuable insights and guidelines essential for developing robust, feedback-stabilized QNs.
Authors: Jie Qian, Qi Hong, Zi-Yuan Wang, Wen-Xin Wu, Yihao Yang, C.-M. Hu, J. Q. You, Yi-Pu Wang
Chiral coupling opens new avenues for controlling and exploiting light-matter interactions. We demonstrate that chiral coupling can be utilized to achieve unidirectional perfect absorption. In our experiments, chiral magnon-photon coupling is realized by coupling the magnon modes in yttrium iron garnet (YIG) spheres with spin-momentum-locked waveguide modes supported by spoof surface plasmon polaritons (SSPPs). These photon modes exhibit transverse spin, with the spin direction determined by the propagation direction. Due to the intrinsic spin properties of the magnon mode, it exclusively couples with microwaves traveling in one direction, effectively suppressing the reflection channel. Under the critical coupling condition, transmission is also eliminated, resulting in unidirectional perfect absorption. By incorporating additional YIG spheres, bidirectional and multi-frequency perfect absorption can be achieved. Our work introduces a novel platform for exploring and harnessing chiral light-matter interactions within spin-momentum locked devices, offering a paradigm for unidirectional signal processing and energy harvesting technologies.
Authors: Kai-Chi Chang, Xiang Cheng, Felix Ribuot-Hirsch, Murat Can Sarihan, Yujie Chen, Jaime Gonzalo Flor Flores, Mingbin Yu, Patrick Guo-Qiang Lo, Dim-Lee Kwong, Chee Wei Wong
Large-scale quantum computers possess the capacity to effectively tackle practical problems that can be insurmountable for classical computers. The main challenge in building these quantum computers is to realize scalable modules for remote qubits and entanglement. By assembling small, specialized parts into a larger architecture, the modular approach mitigates complexity and uncertainty. Such a distributed architecture requires non-local quantum gate operations between remote qubits. An essential method for implementing such operations, known as quantum gate teleportation, requires only local operations, classical communication, and shared entanglement. Till today, the quantum gate teleportation using a photonic chip has remained elusive. Here we experimentally demonstrate the quantum teleportation of an on-chip controlled-NOT (CNOT) gate, assisted with the scalable silicon chip platform, high-fidelity local quantum logic gates, linear optical components, post-selected entanglement, and coincidence measurements from photonic qubits. First, we measure and characterize our teleported chip-scale CNOT gate with an average truth table fidelity of 93.1 +- 0.3%. Second, for different input polarization states, we obtain an average quantum state fidelity of 87.0 +- 2.2% with our teleported on-chip CNOT gate. Third, we use our non-local CNOT gate for remote entanglement creation of four Bell states, with an average quantum state fidelity of 86.2 +- 0.8%. Fourthly, we fully characterize our teleported on-chip CNOT gate with a quantum process fidelity 83.1 +- 2.0%, and an average non-local CNOT gate fidelity of 86.5 +- 2.2%. Our teleported photonic on-chip quantum logic gate could be extended both to multiple qubits and chip-scale modules towards fault-tolerant and large-scale distributed quantum computation.
Authors: Shuo Zhang, Langxuan Chen, Pengfei Zhang
The rapid advancement of quantum science and technology has established Rydberg atom arrays as a premier platform for exploring quantum many-body physics with exceptional precision and controllability. Traditionally, each atom is modeled as a spin degree of freedom with its spatial motion effectively frozen. This simplification has facilitated the discovery of a rich variety of novel equilibrium and non-equilibrium phases, including $\mathbb{Z}_{\text{N}}$ symmetry-breaking orders and quantum scars. In this work, we investigate the consequences of incorporating atomic vibrations in optical tweezers, which give rise to spin-phonon coupling. For systems in thermal equilibrium, we find that this coupling leads to a new symmetry-breaking phase in the weak driving limit, as a result of induced three-spin interactions. Furthermore, we show that the violation of quantum thermalization in $\mathbb{Z}_2$-ordered states is suppressed when spin-phonon coupling is introduced. Our results are readily testable in state-of-the-art Rydberg atom array experiments.
Authors: Alberto J. B. Rosal, Diogo O. Soares-Pinto, Diego Paiva Pires
We discuss quantum speed limits (QSLs) for finite-dimensional quantum systems undergoing general physical processes. These QSLs were obtained using Schatten $\alpha$-norms, firstly exploiting geometric features of the quantum state space, and secondly by applying the Holder's inequality for matrix norms. For single-qubit states, we find that the geometric QSL is independent of the chosen Schatten norm, thus revealing universality behavior. We compare these QSLs with existing speed limits in literature, showing that the latter results represent particular cases of a general class of QSLs related to Schatten $\alpha$-norms. We address necessary and sufficient conditions for the tightness of the QSLs that depends on populations and coherences of the qubits, also addressing their geometric meaning. We compare the QSLs obtained for qubit dynamics, also exploring their geometrical meaning. Finally, we show that the geometric QSL is tighter for general qubit dynamics with initial pure states, which indicates a universal QSL.
Authors: Lauritz van Luijk, Alexander Stottmeister, Reinhard F. Werner, Henrik Wilming
Embezzlement of entanglement refers to the counterintuitive possibility of extracting entangled quantum states from a reference state of an auxiliary system (the "embezzler") via local quantum operations while hardly perturbing the latter. We uncover a deep connection between the operational task of embezzling entanglement and the mathematical classification of von Neumann algebras. Our result implies that relativistic quantum fields are universal embezzlers: Any entangled state of any dimension can be embezzled from them with arbitrary precision. This provides an operational characterization of the infinite amount of entanglement present in the vacuum state of relativistic quantum field theories.
Authors: Savannah Garmon, Gonzalo Ordonez, Kenichi Noba
A striking feature of cavity quantum electrodynamics is the existence of atom-photon bound states, which typically form when the coupling between the atom and its environment are strong enough that after de-excitation the atom can ``grab'' an emitted photon and re-absorb it, resulting in a virtual cloud surrounding the atom. Here we will demonstrate the existence of bound states that instead form in the case of weak coupling. Specifically, we show that when a quantum emitter is weakly coupled to a structured reservoir exhibiting topologically-protected surface states, hybridizations between these states and the emitter can form, resulting in mid-gap bound states. We illustrate this using a semi-infinite extension of the Su-Schrieffer-Heeger (SSH) model as our reservoir. First, we diagonalize the bare semi-infinite SSH chain and reveal a winding number that predicts only the edge state on the finite side of the chain survives the semi-infinite extension. Then, after coupling the quantum emitter to this end of the chain, we analyze the modified emitter spectrum and reveal the existence of bound states in three parameter regions. Two of these represent the usual strong-coupling bound states, while the third gives the weak-coupling bound states with eigenvalue appearing in the SSH band gap and which exhibit partial sublattice localization. We demonstrate that oscillations between the weak-coupling bound states can be used to transfer the particle from the emitter into the lattice in a predictable and reversible manner.
Authors: Jonathan Oppenheim, Zachary Weller-Davies
We introduce configuration space path integrals for quantum fields interacting with classical fields. We show that this can be done consistently by proving that the dynamics are completely positive directly, without resorting to master equation methods. These path integrals allow one to readily impose space-time symmetries, including Lorentz invariance or diffeomorphism invariance. They generalize and combine the Feynman-Vernon path integral of open quantum systems and the stochastic path integral of classical stochastic dynamics while respecting symmetry principles. We introduce a path integral formulation of general relativity where the space-time metric is treated classically. The theory is a candidate for a fundamental theory that reconciles general relativity with quantum mechanics. The theory is manifestly covariant, and may be inequivalent to the theory derived using master-equation methods. We prove that entanglement cannot be created via the classical field, reinforcing proposals to test the quantum nature of gravity via entanglement generation.
Authors: Pronoy Das, Sathwik Bharadwaj, Jungho Mun, Xueji Wang, Junsuk Rho, Zubin Jacob
The absence of inversion symmetry in chiral tellurium (Te) creates exotic spin textures within its electron waves. However, understanding textured optical waves within Te remains a challenge due to the semi-classical limitations of long-wavelength approximation. To unveil these textured optical waves, we develop a spin-resolved deep-microscopic optical bandstructure for Te analogous to its electronic counterpart. We demonstrate that the degeneracies in this optical bandstructure is lifted by the twisted lattice of Te, which induces optical gyrotropy. Our theory shows excellent agreement with experimental optical gyrotropy measurements. At the lattice level, we reveal that the chirality of Te manifests as deep-microscopic optical spin texture within the optical wave. Our framework uncovers the finite-momentum origin of optical activity and provides a microscopic basis for light-matter interactions in chiral crystalline materials.
Authors: Eugene V. Stolyarov, V. L. Andriichuk, Andrii M. Sokolov
We study the interactions mediated by symmetric superconducting quantum interference device (SQUID), their renormalizations, and applicability of the anharmonic oscillator model for a coupled phase qubit. The coupling SQUID can switch between single- or two-photon interaction in situ. We consider a coupled resonator and an rf SQUID. The latter dwells in the vicinity of its metastable well holding a number of anharmonic energy states and acts as an artificial atom known as the phase qubit. Apart from the linear and two-photon couplings, interactions of optomechanical type and a cross-Kerr coupling arise. Near the two-photon resonance, we calculate the renormalizations due to nonresonant interactions, which are more prominent with the higher Josephson energy of the coupler. We interpret the renormalizations by depicting some of the virtual processes involved. That also allows us to determine the minimal amount of metastable states in the phase qubit for the renormalization formulas to hold.
Authors: Enrico Di Benedetto, Xuejian Sun, Marcel A. Pinto, Luca Leonforte, Chih-Ying Chang, Vincent Jouanny, Léo Peyruchat, Pasquale Scarlino, Francesco Ciccarello
We investigate qubits coupled to the boundary of a two dimensional photonic lattice that supports dispersionless edge modes, unlike conventional edge modes that sustain propagating photons. As a case study, we consider a honeycomb lattice (photonic graphene) of coupled resonators with a zigzag edge, where the edge modes form a flat band defined only over a restricted region of momentum space. We show that light matter interactions are effectively captured by a dissipative cavity QED model, wherein the emitter coherently couples to a fictitious cavity mode emerging as a superposition of edge modes. This mode has support on only one sublattice and, most notably, displays an unconventional power law localization around the qubit, yet remaining normalizable in the thermodynamic limit, with a spatial range that can be tuned by introducing lattice anisotropy We predict occurrence of vacuum Rabi oscillations and efficient state transfer between distant emitters. An experimental demonstration using superconducting circuits is proposed.
Authors: Lin-Lin Jiang, Xuan Xie, Lin Tian, Jin-Feng Huang
We investigate the emergence of frustrated quantum phases in a Jaynes-Cummings (JC) trimer with complex hopping amplitudes between the cavities, which represents the smallest frustrated unit in light-matter systems. The complex hopping amplitudes that can be engineered via synthetic gauge fields introduce chiral effects and geometric frustration into the this http URL obtain analytic solutions in the semiclassical limit and map out the phase diagram of this model, featuring one normal and three distinct superradiant phases. Among these phases, a chirally frustrated superradiant phase emerges, characterized by broken chiral and translational symmetries and unidirectional photon flow. These results reveal how frustration and symmetry breaking can arise in JC systems with synthetic gauge fields and ultrastrong coupling.
Authors: Alberto García Martín-Caro, Javier Olmedo, Jose M. Sánchez Velázquez
In this work we explore the limitations and robustness of thermal radiation in dynamical Casimir systems serving as analogs for Hawking radiation. Through detailed numerical analysis, we characterize particle production spectra in cavities with moving boundaries under various configurations, including expanding, collapsing, and rigidly accelerating scenarios. We find that thermal signatures emerge in specific expanding cavity configurations but are highly dependent on frequency bands and acceleration parameters. In those configurations of the cavity where there is thermal production, we derive fitting expressions that quantify deviations from idealized thermal spectra through gray-body factors, revealing oscillatory behaviors tied to acceleration duration. Our results identify which experimental setups can reliably simulate gravitationally-induced phenomena and quantify how finite-size effects and transient dynamics modify the expected thermal distributions, providing a comprehensive framework for distinguishing genuine Hawking-like radiation from experimental artifacts.
Authors: Paul R. Berman, Peter W. Milonni
Spontaneous emission in dipole approximation is studied theoretically using both source-field theory and a Schrodinger picture approach. Using source-field theory we obtain formal equations for the Poynting vector and energy density without making the rotating wave approximation (RWA) and Weisskopf-Wigner approximation (WWA). The initial condition at t=0 is one in which the atom is in an excited state and the field in the vacuum state. The source-field expressions are evaluated within the the RWA and WWA and are found to satisfy Poynting's theorem. To explore the consequences of not making the RWA and WWA, the Poynting vector and energy density are calculated using perturbation theory. We use a Schrodinger picture approach and essentially reproduce and complement the results of Compagno, Passante, and Persico [J. Mod. Optics 37:8, 1377 (2007)] and those of Power and Thirunamachandran [Phys. Rev. A 45, 54 (1992)] obtained using a Heisenberg picture approach. The theory involves a sum over field mode frequencies and both finite cutoffs and convergence factors are used to carry out the sums. It is shown that the perturbation theory calculation leads to unphysical values for atomic state populations for all times when a sum over all field frequencies is taken, even if a convergence factor is used. It is also proved that the fields calculated using source-field theory always satisfy Poynting's theorem for ct not equal to R, where R is the distance from the atom.
Authors: Siwei Wang, Liang-Yan Hsu, Hsing-Ta Chen
Exciton transport in molecular aggregates with magic-angle orientation is expected to be strongly suppressed due to their negligible dipole-dipole interactions. However, recent reports show that light-matter interactions can significantly enhance exciton transport attributed to the effective long-range coupling mediated by the photonic fields. To elucidate their interplay, we employ the macroscopic quantum electrodynamics framework to simulate exciton transport within a chromophore array arranged in a magic-angle configuration in proximity to a silver surface. Our results show a significant enhancement of the exciton diffusion coefficient that is robust across variations in chromophore-surface separation, intermolecular distance, and molecular transition frequency. Furthermore, based on the image-dipole method, we derive analytical expressions that agree well with numerical simulations, revealing the enhancement's origin in the near-field coupling term as induced by the radiative scattering at the metallic surface. More importantly, we observe non-trivial differences in the diffusion coefficient's scaling near metallic surfaces compared to free space. Our findings highlight the potential to control exciton transport by designing coupled exciton-photon systems and engineering the dielectric environments.
Authors: Xuan Zhao, Yi Xu, Le-Man Kuang, Jie-Qiao Liao
Fock-state lattices (FSLs) are becoming an emerging research hotspot in quantum physics, not only because the FSLs provide a new perspective for studying atom-field interactions, but also because they build the connection between quantum optics and condensed matter physics. Owing to the multiple transition paths in the lattices, inherent quantum interference effect exists in these systems, and hence how to find new quantum coherent phenomena and exploit their applications becomes a significant and desired task in this field. In this work, we study the dark-state effect in the FSLs generated by the multimode Jaynes-Cummings (JC) models. By considering the FSLs in certain-excitation-number subspaces, we study the dark states with respect to the states associated with the atomic excited state using the arrowhead-matrix method. We find that there exist dark-state subspaces with the dimensions determined by the number of orthogonal dark states. When the dimension is larger than one, the forms of these dark-state bases are not unique. Further, we obtain the number and form of the orthogonal dark states in the two-, three-, and four-mode JC models. In addition, we find that for a general $N$-mode JC model, there are $C_{N+n-2}^{N-2}$ orthogonal dark states in the $n$-excitation subspace. We also build the relationship between the dark modes and dark states. Our work will pave the way for exploring quantum optical effects and quantum information processing based on the FSLs.
Authors: Edison S. Carrera, Harold Erbin, Grégoire Misguich
We present a quantum optimal control protocol to generate highly spin-squeezed states in Rydberg atom arrays coupled via Ising-type Van der Waals interactions. Using gradient-based optimization techniques we construct time-dependent pulse sequences that steer an initial product state toward highly entangled, spin-squeezed states with predefined magnetization and squeezing axes. We focus on the Wineland parameter $\xi_W^2$ to measure spin squeezing and our approach achieves near-optimal spin squeezing in one-dimensional ring arrays of up to $N=8$ spins, significantly outperforming conventional quench dynamics for all system sizes studied. Remarkably, optimized pulse sequences can be directly scaled to larger arrays without additional optimization, achieving a squeezing parameter as low as $\xi_W^2 = 0.227$ in systems containing $N=50$ spins. This work demonstrates the potential of quantum optimal control methods for preparing highly spin-squeezed states, opening pathways to enhanced quantum metrology.
Authors: Laura Blázquez Martínez, Changlong Zhu, Birgit Stiller
Strongly coupling two systems allows them to exchange coherent information before the systems decohere. This important regime in light-matter interactions has predominantly been reached in optical resonator configurations. In this work, we present the experimental realization of strong coupling between optical and acoustic fields within a continuum of modes in a cavity-less configuration after a single-pass through an optical waveguide. The underlying physical effect of anti-Stokes Brillouin-Mandelstam scattering in a highly nonlinear fiber at T = 4 K allows us to experimentally demonstrate strong coupling in a waveguide scenario. We show the splitting of the optoacoustic spectral response and introduce a novel technique to measure the avoided crossing of hybrid optoacoustic modes via forced detuning. This demonstration opens a path towards in-line acoustic-waves-based quantum signal processing in waveguide systems.
Authors: Sounak Bhowmik, Talita Perciano, Himanshu Thapliyal
Dementia is a devastating condition with profound implications for individuals, families, and healthcare systems. Early and accurate detection of dementia is critical for timely intervention and improved patient outcomes. While classical machine learning and deep learning approaches have been explored extensively for dementia prediction, these solutions often struggle with high-dimensional biomedical data and large-scale datasets, quickly reaching computational and performance limitations. To address this challenge, quantum machine learning (QML) has emerged as a promising paradigm, offering faster training and advanced pattern recognition capabilities. This work aims to demonstrate the potential of quantum transfer learning (QTL) to enhance the performance of a weak classical deep learning model applied to a binary classification task for dementia detection. Besides, we show the effect of noise on the QTL-based approach, investigating the reliability and robustness of this method. Using the OASIS 2 dataset, we show how quantum techniques can transform a suboptimal classical model into a more effective solution for biomedical image classification, highlighting their potential impact on advancing healthcare technology.
Authors: Ratun Rahman, Atit Pokharel, Dinh C. Nguyen
Quantum Federated Learning (QFL) is an emerging paradigm that combines quantum computing and federated learning (FL) to enable decentralized model training while maintaining data privacy over quantum networks. However, quantum noise remains a significant barrier in QFL, since modern quantum devices experience heterogeneous noise levels due to variances in hardware quality and sensitivity to quantum decoherence, resulting in inadequate training performance. To address this issue, we propose SpoQFL, a novel QFL framework that leverages sporadic learning to mitigate quantum noise heterogeneity in distributed quantum systems. SpoQFL dynamically adjusts training strategies based on noise fluctuations, enhancing model robustness, convergence stability, and overall learning efficiency. Extensive experiments on real-world datasets demonstrate that SpoQFL significantly outperforms conventional QFL approaches, achieving superior training performance and more stable convergence.
Authors: Silvie Illésová
Hybrid quantum-classical neural networks represent a promising frontier in the search for improved machine learning models. This thesis explores the integration of quantum layers within classical convolutional neural network architectures, aiming to leverage quantum entanglement and feature mapping to enhance learning capabilities. A detailed methodology for constructing and training such hybrid models is presented, using PyTorch and Qiskit Machine Learning frameworks. Experiments investigate the performance impact of inserting quantum layers at different stages of the neural network pipeline. The results suggest that quantum components can introduce meaningful transformations even with a limited number of qubits, motivating further research into scalable quantum machine learning. The full implementation is made publicly available, and future work will focus on expanding experimental evaluations and publishing additional findings.
Authors: Diego Ruiz, Jérémie Guillaud, Christophe Vuillot, Mazyar Mirrahimi
Magic state distillation enables universal fault-tolerant quantum computation by implementing non-Clifford gates via the preparation of high-fidelity magic states. However, it comes at the cost of substantial logical-level overhead in both space and time. In this work, we propose a very low-cost magic state distillation scheme for biased-noise qubits. By leveraging the noise bias, our scheme enables the preparation of a magic state with a logical error rate of $3 \times 10^{-7}$, using only 53 qubits and 5.5 error correction rounds, under a noise bias of $\eta \gtrsim 5 \times 10^6$ and a phase-flip noise rate of $0.1\%$. This reduces the circuit volume by more than one order of magnitude relative to magic state cultivation for unbiased-noise qubits and by more than two orders of magnitude relative to standard magic state distillation. Moreover, our scheme provides three key advantages over previous proposals for biased-noise qubits. First, it only requires nearest-neighbor two-qubit gates on a 2D lattice. Second, the logical fidelity remains nearly identical even at a more modest noise bias of $\eta \gtrsim 80$, at the cost of a slightly increased circuit volume. Third, the scheme remains effective even at high physical phase-flip rates, in contrast to previously proposed approaches whose circuit volume grows exponentially with the error rate. Our construction is based on unfolding the $X$ stabilizer group of the Hadamard 3D quantum Reed-Muller code in 2D, enabling distillation at the physical level rather than the logical level, and is therefore referred to as $\textit{unfolded}$ distillation.
Authors: Tsung-Cheng Lu, Sarang Gopalakrishnan, Yizhi You
Two prevalent approaches for preparing long-range entangled quantum states are (i) linear-depth sequential unitary (SU) circuits, which apply local unitary gates sequentially, and (ii) constant-depth measurement-feedback (MF) circuits, which employ mid-circuit measurements and conditional feedback based on measurement outcomes. Here, we establish that a broad class of SU and MF circuits are dual to each other under a spacetime rotation. We investigate this spacetime duality in the preparation of various long-range entangled states, including GHZ states, topologically ordered states, and fractal symmetry-breaking states. As an illustration, applying a spacetime rotation to a linear-depth SU circuit that implements a non-invertible Kramers-Wannier duality, originally used to prepare a 1D GHZ state, yields a constant-depth MF circuit that implements a $\mathbb{Z}_2$ symmetry gauging map, which equivalently prepares the GHZ state. Leveraging this duality, we further propose experimental protocols that require only a constant number of qubits to measure unconventional properties of 1D many-body states. These include (i) measurement of disorder operators, which diagnose the absence of spontaneous symmetry breaking, and (ii) postselection-free detection of measurement-induced long-range order, which emerges in certain symmetry-protected topological phases. We also show that measurement-induced long-range order provides a lower bound for strange correlators, which may be of independent interest.
Authors: Ivan Rojkov, Elias Zapusek, Florentin Reiter
Dissipative quantum error correction (QEC) autonomously protects quantum information using engineered dissipation and offers a promising alternative to error correction via measurement and feedback. However, scalability remains a challenge, as correcting high-weight errors typically requires increasing dissipation rates and exponentially many correction operators. Here, we present a scalable dissipative QEC protocol for discrete-variable codes, correcting multi-qubit errors via a trickle-down mechanism that sequentially reduces errors weight. Our construction exploits redundancy in the Knill-Laflamme conditions to design correction operators that act on multiple error subspaces simultaneously, thereby reducing the overhead from exponential to polynomial in the number of required operators. We illustrate our approach with repetition codes under biased noise, showing a fourfold improvement in the exponential suppression factor at realistic physical error rates. Our approach connects autonomous QEC for discrete-variable codes with demonstrated dissipative protocols for bosonic codes and opens up new avenues for traditional measurement-feedback QEC and fault-tolerant quantum operations.
Authors: Yanxuan Shao, Saikat Guha, Adilson E. Motter
Quantum networks hold promise for key distribution, private and distributed computing, and quantum sensing, among other applications. The scale of such networks for ground users is currently limited by one's ability to distribute entanglement between distant locations. This can in principle be carried out by transmitting entangled photons through optical fibers or satellites. The former is limited by fiber optic attenuation while the latter is limited by atmospheric extinction and diffraction. Here, we propose a hybrid network and protocol that outperform both ground- and satellite-based designs and lead to high-fidelity entanglement at a continental or even global scale.
Authors: Matteo Votto, Marko Ljubotina, Cécilia Lancien, J. Ignacio Cirac, Peter Zoller, Maksym Serbyn, Lorenzo Piroli, Benoît Vermersch
We present and test a protocol to learn the matrix-product operator (MPO) representation of an experimentally prepared quantum state. The protocol takes as an input classical shadows corresponding to local randomized measurements, and outputs the tensors of a MPO which maximizes a suitably-defined fidelity with the experimental state. The tensor optimization is carried out sequentially, similarly to the well-known density matrix renormalization group algorithm. Our approach is provably efficient under certain technical conditions which are expected to be met in short-range correlated states and in typical noisy experimental settings. Under the same conditions, we also provide an efficient scheme to estimate fidelities between the learned and the experimental states. We experimentally demonstrate our protocol by learning entangled quantum states of up to $N = 96$ qubits in a superconducting quantum processor. Our method upgrades classical shadows to large-scale quantum computation and simulation experiments.
Authors: Gubio G. de Lima, Iann Cunha, Leonardo Kleber Castelano
Accurate characterization of quantum systems is essential for the development of quantum technologies, particularly in the noisy intermediate-scale quantum (NISQ) era. While traditional methods for Hamiltonian learning and noise characterization often require extensive measurements and scale poorly with system size, machine learning approaches offer promising alternatives. In this work, we extend the inverse physics-informed neural network (referred to as PINNverse) framework to open quantum systems governed by Lindblad master equations. By incorporating both coherent and dissipative dynamics into the neural network training, our method enables simultaneous identification of Hamiltonian parameters and decay rates from noisy experimental data. We demonstrate the effectiveness and robustness of the approach through numerical simulations of two-qubit open systems. Our results show that PINNverse provides a scalable and noise-resilient framework for quantum system identification, with potential applications in quantum control and error mitigation.
Authors: Rohan Joshi, Michael Meth, Jan C. Louw, Jesse J. Osborne, Kevin Mato, Martin Ringbauer, Jad C. Halimeh
A major challenge in the burgeoning field of quantum simulation for high-energy physics is the realization of scalable $2+1$D lattice gauge theories on state-of-the-art quantum hardware, which is an essential step towards the overarching goal of probing $3+1$D quantum chromodynamics on a quantum computer. Despite great progress, current experimental implementations of $2+1$D lattice gauge theories are mostly restricted to relatively small system sizes and two-level representations of the gauge and electric fields. Here, we propose a resource-efficient method for quantum simulating $2+1$D spin-$S$ $\mathrm{U}(1)$ quantum link lattice gauge theories with dynamical matter using qudit-based quantum processors. By integrating out the matter fields through Gauss's law, we reformulate the quantum link model in a purely spin picture compatible with qudit encoding across arbitrary spatial dimensions, eliminating the need for ancillary qubits and reducing resource overhead. Focusing first on the spin-$1/2$ case, we construct explicit circuits for the full Hamiltonian and demonstrate through numerical simulations that the first-order Trotterized circuits accurately capture the quench dynamics even in the presence of realistic noise levels. Additionally, we introduce a general method for constructing coupling-term circuits for higher-spin representations $S>1/2$. Compared to conventional qubit encodings, our framework significantly reduces the number of quantum resources and gate count. Our approach significantly enhances scalability and fidelity for probing nonequilibrium phenomena in higher-dimensional lattice gauge theories, and is readily amenable to implementation on state-of-the-art qudit platforms.
Authors: Rohan Joshi, Jan C. Louw, Michael Meth, Jesse J. Osborne, Kevin Mato, Guo-Xian Su, Martin Ringbauer, Jad C. Halimeh
An overarching goal in the flourishing field of quantum simulation for high-energy physics is the first-principles study of the microscopic dynamics of scattering processes on a quantum computer. Currently, this is hampered by small system sizes and a restriction to two-level representations of the gauge fields in state-of-the-art quantum simulators. Here, we propose efficient experimentally feasible digital qudit quantum circuits for far-from-equilibrium quench dynamics of a $\mathrm{U}(1)$ quantum link lattice gauge theory, where the electric and gauge fields are represented as spin-$1$ operators. Using dedicated numerical simulations, we probe scattering processes in this model on these proposed circuits, focusing on meson-meson and meson-antimeson collisions. The latter are not possible with a two-level representation of the fields, highlighting the suitability of qudits in exploring scattering processes relevant to quantum electrodynamics. The probed scattering dynamics showcases rich physics, including meson flipping and a reflection-transmission transition in meson-antimeson collisions as a function of the gauge coupling strength. Our simulations, which include realistic noise models of dephasing and depolarization, show very good agreement with the exact noiseless dynamics, signaling the readiness of current qudit platforms to observe microscopic scattering dynamics with significantly shallower circuit depths than their qubit counterparts.
Authors: Dennis Lima, Saif Al-Kuwari
Chemical vapor deposition (CVD) is the most efficient process to synthesize graphene sheets using methane as precursor, making it a strategic alternative route for the Liquefied Natural Gas market. In this reaction, tetracyclic aromatic hydrocarbons (TAH) are produced as residual and intermediary molecules. Sorting a combinatorial space of variants of TAHs by energy is a poorly studied problem needed to optimize CVD, while it is also a candidate for quantum advantage in quantum computers. We extend on Sridhara's polynomial root formula to perform block-diagonalization (hence SBD) of six TAHs using Hartree-Fock Hamiltonians with STO-3G basis set and active orbital space growing from 2 to 6 orbitals, with equal numbers for the number of active electrons. We show that the proposed compression algorithm followed by Variational Quantum Eigensolver (VQE) allows for sorting of the molecules by ground state energy, while speeding up the VQE simulation up to tenfold and reducing its error to the $10^{-1}$ scale. The compression capability of $(1-2^{-k})\cdot 100\%$ in matrix size for $k$ qubits allows VQEs to have a broader set of applications, providing a new and necessary tool to overcome the circuit size and quantum noise limitations of large quantum processing units.
Authors: M. Y. Abd-Rabbou, Amr M. Abdallah, Ahmed A. Zahia, Ashraf A. Gouda, Cong-Feng Qiao
Reliable detection and quantification of quantum entanglement, particularly in high-spin or many-body systems, present significant computational challenges for traditional methods. This study examines the effectiveness of ensemble machine learning models as a reliable and scalable approach for estimating entanglement, measured by negativity, in quantum systems. We construct an ensemble regressor integrating Neural Networks (NNs), XGBoost (XGB), and Extra Trees (ET), trained on datasets of pure states and mixed Werner states for various spin dimensions. The ensemble model with stacking meta-learner demonstrates robust performance by CatBoost (CB), accurately predicting negativity across different dimensionalities and state types. Crucially, visual analysis of prediction scatter plots reveals that the ensemble model exhibits superior predictive consistency and lower deviation from true entanglement values compared to individual strong learners like NNs, even when aggregate metrics are comparable. This enhanced reliability, attributed to error cancellation and variance reduction inherent in ensembling, underscores the potential of this approach to bypass computational bottlenecks and provide a trustworthy tool for characterizing entanglement in high-dimensional quantum physics. An empirical formula for estimating data requirements based on system dimensionality and desired accuracy is also derived.
Authors: Hari Timsina, Yi-Ming Ding, Emanuele Tirrito, Poetri Sonya Tarabunga, Bin-Bin Mao, Mario Collura, Zheng Yan, Marcello Dalmonte
We study the behavior of magic as a bipartite correlation in the quantum Ising chain across its quantum phase transition, and at finite temperature. In order to quantify the magic of partitions rigorously, we formulate a hybrid scheme that combines stochastic sampling of reduced density matrices via quantum Monte Carlo, with state-of-the-art estimators for the robustness of magic - a {\it bona fide} measure of magic for mixed states. This allows us to compute the mutual robustness of magic for partitions up to 8 sites, embedded into a much larger system. We show how mutual robustness is directly related to critical behaviors: at the critical point, it displays a power law decay as a function of the distance between partitions, whose exponent is related to the partition size. Once finite temperature is included, mutual magic retains its low temperature value up to an effective critical temperature, whose dependence on size is also algebraic. This suggests that magic, differently from entanglement, does not necessarily undergo a sudden death.
Authors: Shiwen He, Zi-Long Yang, Sitong Jin, Feng-Yang Zhang, Chong Li
Four-component Schrödinger cat states, superpositions of four coherent states symmetrically arranged in phase space, offer rich nonclassical features and enhanced resilience to decoherence, making them promising resources for quantum information science. We propose a Floquet-engineered scheme to deterministically generate four-component Schrödinger cat states in a hybrid quantum system composed of two superconducting qubits, a microwave cavity, and a magnon mode. By applying periodic transverse drives to the qubits, we derive an effective Hamiltonian that conditionally displaces the magnon mode depending on the joint qubit state. Conditional measurements in the $\sigma_x$ basis project the magnon mode into nonclassical four-component cat states with high fidelity. The Wigner functions in the original frame verified the non-classicality of the four-component Schrödinger cat states. Results of systematically analyzing the impacts of qubit decoherence, magnon loss, and cavity dissipation demonstrate the robustness to the dissipation. The results show that this scheme can realize the generation of multi-component quantum superposition states in a scalable solid-state platform, providing a new approach for hybrid quantum information processing based on nonclassical magnon this http URL.
Authors: Javier Argüello-Luengo, Javier Rivera-Dean, Philipp Stammer, Marcelo F. Ciappina, Maciej Lewenstein
The classical Kramers-Henneberger transformation connects, via a series of unitary transformations, the dynamics of a quantum particle of mass $m$ located in a trap at position $\alpha(t)$, with the dynamics of a charge $e$ moving in an electric field $e{\cal{E}}(t)=-m\ddot{\alpha}(t)$ within the dipole approximation. In this paper, we extend the classical Kramers-Henneberger transformation to the quantum electrodynamic and quantum optical realm, by explicitly treating the trap location quantum mechanically, thus taking into account the quantum fluctuations of the time-dependent displacement force. Compared to the classical case, we show that quantum electrodynamic corrections appear, and we propose an optomechanical realization for the quantized position of the trap to show that such corrections can manifest in state-of-the-art experiments. These results open the path to novel quantum simulation of quantum electrodynamics and quantum optics of attoscience and ultrafast physics by using ultracold trapped atoms and ions.
Authors: Stefano Longhi
Preserving entanglement in the presence of decoherence remains a major challenge for quantum technologies. Recent proposals [M.A. Selim et al., Science 387, 1424 (2025)] have employed photonic filters based on anti-parity-time symmetry to recover certain entangled states, but these approaches require intricate, symmetry-constrained waveguide architectures and precise bath engineering. In this work, we show that such strict non-Hermitian symmetry constraints are not necessary for entanglement filtering. Instead, we identify post-selection and the emergence of dark states -- arising naturally through destructive interference in simple photonic settings -- as the essential mechanisms. By avoiding the need for special bath engineering or non-Hermitian symmetries, our approach significantly simplifies the design and architecture, enhances universality, and extends applicability beyond previously studied dimer configurations. We demonstrate this concept using minimal waveguide network designs, offering a broadly accessible route to robust entanglement filtering.
Authors: D. A. Saltykova, A. V. Yulin, I. A. Shelykh
Cavity polaritons, the elementary excitations appearing in quantum microcavities in the strong-coupling regime, reveal clear signatures of quantum collective behavior. The combination of unique spin structure and strong nonlinear response opens the possibility of direct experimental observation of a plethora of nontrivial optical polarization phenomena. Spin relaxation processes are of crucial importance here. However, a mathematical formalism for their coherent description is still absent. In the present paper, based on the quantum hydrodynamics approach for a two-component liquid, we derive the set of the corresponding equations where both energy and spin relaxation terms appear naturally. We analyze in detail how these terms affect the dynamics of spinor polariton droplets in the external magnetic field and the dispersion of elementary excitations of a uniform polariton condensate. Although we focus on the case of cavity polaritons, our approach can be applied to other cases of spinor bosonic condensates, where the processes of spin relaxation play a major role.
Authors: So Chigusa, Taiyo Kasamaki, Toshi Kusano, Takeo Moroi, Kazunori Nakayama, Naoya Ozawa, Yoshiro Takahashi, Atsuhiro Umemoto, Amar Vutha
A new scheme for detecting wave-like dark matter (DM) using Rydberg atoms is proposed. Recent advances in trapping and manipulating Rydberg atoms make it possible to use Rydberg atoms trapped in optical tweezer arrays for DM detection. We present a simple and innovative experimental procedure that searches for excitations of trapped Rydberg atoms due to DM-induced electric field. A scan over DM mass is enabled with the use of the Zeeman and diamagnetic shifts of energy levels under an applied external magnetic field. Taking dark photon DM as an example, we demonstrate that our proposed experiment can have high sensitivity enough to probe previously unexplored regions of the parameter space of dark photon coupling strengths and masses.
Authors: Muhammad Ahsan
We present experimental quantum computation of the ground-state energy in a 103-site flat Kagome lattice under the antiferromagnetic Heisenberg model (KAFH), with IBM's Heron r1 and Heron r2 quantum processors. For spin-1/2 KAFH, our per-site ground-state energy estimate is $-0.417\,J$, which, under open-boundary corrections, matches the energy in the thermodynamic limit, i.e., $-0.4386\,J$. To achieve this, we used a hybrid approach that splits the conventional Variational Quantum Eigensolver (VQE) into local (classical) and global (quantum) components for efficient hardware utilization. More importantly, we introduce a Hamiltonian engineering strategy that increases coupling on defect triangles to mimic loop-flip dynamics, allowing us to simplify the ansatz while retaining computational accuracy. Using a single-repetition, hardware-efficient ansatz, we entangle up to 103 qubits with high fidelity to determine the Hamiltonian's lowest eigenvalue. This work demonstrates the scalability of VQE for frustrated 2D systems and lays the foundation for future studies using deeper ansatz circuits and larger lattices on utility quantum processors.
Authors: Yueyang Min, Ziliang Li, Yi Zhong, Jia-An Xuan, Jian Lin, Lei Feng, Xiaopeng Li
We demonstrate a reinforcement learning (RL) based control framework for optimizing evaporative cooling in the preparation of strongly interacting degenerate Fermi gases of Li6. Using a Soft Actor-Critic (SAC) algorithm, the system autonomously explores a high-dimensional parameter space to learn optimal cooling trajectories. Compared to conventional exponential ramps, our method achieves up to 130% improvement in atomic density within a 0.5 second, revealing non-trivial control strategies that balance fast evaporation and thermalization. While our current optimization focuses on the evaporation stage, future integration of other cooling stages, such as grey molasses cooling, could further extend RL to the full preparation pipeline. Our result highlights the promise of RL as a general tool for closed-loop quantum control and automated calibration in complex atomic physics experiments.
Authors: Muhammad Faryad
These notes begin in Chapter 1 with a review of linear algebra and the postulates of quantum mechanics, leading to an explanation of single- and multi-qubit gates. Chapter 2 explores the challenge of constructing arbitrary quantum states from given initial states, and introduces circuits for building oracles. Chapter 3 presents foundational algorithms such as entanglement creation, quantum teleportation, Deutsch-Jozsa, Bernstein-Vazirani, and Simon's algorithm. Chapters 4 and 5 cover algorithms based on the quantum Fourier transform, including phase estimation, period finding, factoring, and logarithm computation. These chapters also include complexity analysis and detailed quantum circuits suitable for implementation in code. Chapter 6 introduces Grover's algorithm for quantum search and amplitude amplification, including its realization via Hamiltonian simulation and a method for derandomization. Chapter 7 discusses basic techniques for Hamiltonian simulation, such as Lie-Trotter decomposition, sparse Hamiltonians, and the linear combination of unitaries. It also provides example circuits for simulating Hamiltonians expressed as linear combinations of Pauli operators. Chapter 8 introduces variational quantum algorithms, and Chapter 9 presents an algorithm for simulating fermionic many-particle systems, with an emphasis on molecular Hamiltonians. It also outlines the key transformations needed to map a molecular Hamiltonian to a form suitable for simulation on a quantum computer.
Authors: Xiaogang Li, Qiming Ding, Kecheng Liu, Xiao Yuan
Non-Hermitian quantum systems exhibit unique properties and hold significant promise for diverse applications, yet their dynamical simulation poses a particular challenge due to intrinsic openness and non-unitary evolution. Here, we introduce a hybrid classical-quantum algorithm based on Quantum Monte Carlo (QMC) for simulating the dynamics of arbitrary time-dependent non-Hermitian systems. Notably, this approach constitutes a natural extension of the quantum imaginary-time evolution (QITE) algorithm. This algorithm combines the advantages of both classical and quantum computation and exhibits good applicability and adaptability, making it promising for simulating arbitrary non-Hermitian systems such as PT-symmetric systems, non-physical processes, and open quantum systems. To validate the algorithm, we applied it to the dynamic simulation of open quantum systems and achieved the desired results.
Authors: Nishan Amgain, Mahir Rahman, Umar Arshad, Fernando Romero Consuegra, Emil Sayahi, Imran M. Mirza
We numerically investigate the generation and dynamics of tripartite entanglement among qubits (quantum emitters or atoms) in multimode cavity quantum electrodynamics (cQED). Our cQED architecture features three initially unentangled excited two-level quantum emitters confined within a triangle-shaped multimode optical cavity, which later become entangled due to a Jaynes-Cummings-like interaction. Using the tripartite negativity measure of entanglement and fidelity with respect to the genuine tripartite entangled state (Greenberger-Horne-Zeilinger (or GHZ) state, to be precise), we analyze the impact of the number of cavity modes, qubit locations, and losses (spontaneous emission from qubits and photon leakage from the cavity mirrors) on the generated entanglement. Our key results include the presence of two kinds of retardation effects: one resulting from the time it takes for photons to propagate from one qubit to another, and the other to complete one round trip in the cavity. We observed these retardation effects only in multimode cavities, with the exciting possibility of controlling the collapse and revival patterns of tripartite entanglement by altering the qubit locations in the cavity. Furthermore, the impact of losses on the generated entanglement and the dependence of maximum entanglement on the total number of modes yield results that surpass those reported for single and two excitations. With recent advances in circuit quantum electrodynamics, these findings hold promise for the development of entanglement-based quantum networking protocols and quantum memories.
Authors: Jianmin Wang, Rong Zhu, Yue Li, Z. Y. Ou
Any amplifier requires coupling to its internal degrees of freedom for energy gain. This coupling introduces extra quantum noise to the output. On the other hand, if the internal degree of the amplifier can be accessed and manipulated, we can manage and even reduce the quantum noise of the amplifier's output. In this paper, we present an experiment to reduce the quantum noise of a Raman amplifier by preparing the atomic medium in a correlated state with the Stokes light field. We report an observation of quantum noise reduction of more than 3.5 dB in the atomic Raman amplification process. From another perspective, the Raman amplifier at high gain in turn serves as a measurement tool for the quantum correlation between the atom and light. Furthermore, such a scheme, when viewed as a whole, also forms a quantum-entangled atom-light hybrid interferometer that can lead to quantum-enhanced sensors.
Authors: Zi-Xu Lu, Xuan Zuo, Zhi-Yuan Fan, Jie Li
The capability of magnons to coherently couple with various quantum systems makes them an ideal candidate to build hybrid quantum systems. The optomagnonic coupling is essential for constructing a hybrid magnonic quantum network, as the transmission of quantum information among remote quantum nodes must be accomplished using light rather than microwave field. Here we provide an optomagnonic continuous-variable quantum teleportation protocol, which enables the transfer of an input optical state to a remote magnon mode. To overcome the currently relatively weak coupling in the experiment, we introduce non-Gaussian distillation operations to enhance the optomagnonic entanglement and thus the fidelity of the teleportation. An auxiliary microwave cavity is adopted to realize the non-Gaussian and displacement operations on magnons. We show that a series of optical states, such as coherent, single-photon, squeezed and cat states, can be teleported to the magnon mode. The work provides guidance for the experimental realization of magnonic quantum repeaters and quantum networks and a new route to prepare diverse magnonic quantum states exploiting the photon-to-magnon quantum teleportation.
Authors: Maritza Ahumada, Natalia Valderrama-Quinteros, Diego Tancara, Guillermo Romero
Controlling the propagation of quantum excitations in low-dimensional quantum systems is pivotal for advancing quantum technologies, including communication networks and quantum simulators. We propose a one-dimensional hybrid quantum lattice model comprising coupled cavity quantum electrodynamics (QED) units. Each unit integrates a single-mode cavity that interacts with a two-level system (TLS), featuring direct coupling between adjacent TLLs. This configuration enables the coherent propagation of polaritons, spin waves, and photons, depending on the interplay between light-matter coupling and spin-spin interactions. Employing the time-evolving block decimation (TEBD) algorithm, we simulate the dynamics of various excitation configurations and analyze their transport characteristics using local observables. Our analysis reveals the importance of matching impedance and resonance conditions via system parameters for the propagation of different types of excitations or swapping the nature of excitations along the hybrid lattice. These findings offer insight into designing controllable quantum links and single-excitation swaps in low-dimensional quantum systems.
Authors: Kenan Qu, Nathaniel J. Fisch
Quantum squeezed states enable precision measurements beyond the standard quantum limit, but conventional solid-state media fundamentally limit pump intensities to the ionization threshold. We demonstrate that plasma waves can mediate ultra-strong two-mode squeezing through stimulated Raman scattering, achieving up to ultrastrong squeezing using $10^{16}{Wcm^{-2}}$ pump lasers. Employing two copropagating pump beams with frequency difference matching twice the plasma frequency, we generate quantum-correlated photon pairs through phonon-mediated four-wave mixing. The process exhibits remarkable thermal noise tolerance, allowing strong squeezing even with large thermal phonon numbers. This plasma-based approach produces squeezed states with ultrahigh photon numbers, opening new possibilities for strong-field applications across optical to X-ray wavelengths.
Authors: Alexander Schuckert, Eleanor Crane, Alexey V. Gorshkov, Mohammad Hafezi, Michael J. Gullans
Simulating the dynamics of electrons and other fermionic particles in quantum chemistry, materials science, and high-energy physics is one of the most promising applications of fault-tolerant quantum computers. However, the overhead in mapping time evolution under fermionic Hamiltonians to qubit gates renders this endeavor challenging. We introduce fermionic fault-tolerant quantum computing, a framework which removes this overhead altogether. Using native fermionic operations we first construct a repetition code which corrects phase errors only. Within a fermionic color code, which corrects for both phase and loss errors, we then realize a universal fermionic gate set, including transversal fermionic Clifford gates. Interfacing with qubit color codes we introduce qubit-fermion fault-tolerant computation, which allows for qubit-controlled fermionic time evolution, a crucial subroutine in state-of-the-art quantum algorithms. As an application, we consider simulating crystalline materials, finding an exponential improvement in circuit depth for a single time step from $\mathcal{O}(N)$ to $\mathcal{O}(\log(N))$ with respect to lattice site number $N$ while retaining a site count of $\tilde{\mathcal{O}}(N)$, implying a linear-in-$N$ end-to-end gate depth for simulating materials, as opposed to quadratic in previous approaches. We also introduce a fermion-inspired qubit algorithm with $O(\mathrm{log}(N)$ depth, but a prohibitive number of additional ancilla qubits. We show how our framework can be implemented in neutral atoms, overcoming the apparent inability of neutral atoms to implement non-number-conserving gates. Our work opens the door to fermion-qubit fault-tolerant quantum computation in platforms with native fermions such as neutral atoms, quantum dots and donors in silicon, with applications in quantum chemistry, material science, and high-energy physics.
Authors: Matthieu Sarkis, Alexandre Tkatchenko
Isolated atoms as well as molecules at equilibrium are presumed to be simple from the point of view of quantum computational complexity. Here we show that the process of chemical bond formation is accompanied by a marked increase in the quantum complexity of the electronic ground state. By studying the hydrogen dimer H$_{2}$ as a prototypical example, we demonstrate that when two hydrogen atoms form a bond, a specific measure of quantum complexity exhibits a pronounced peak that closely follows the behavior of the binding energy. This measure of quantum complexity, known as magic in the quantum information literature, reflects how difficult it is to simulate the state using classical methods. This observation suggests that regions of strong bonding formation or breaking are also regions of enhanced intrinsic quantum complexity. This insight suggests a connection of quantum information measures to chemical reactivity and advocates the use of stretched molecules as a quantum computational resource.
Authors: Vikash Mittal, Yi-Ping Huang
Quantum magic, which accounts for the non-stabilizer content of a state, is essential for universal quantum computation beyond classically simulable resources. We investigate the generation and evolution of quantum magic in discrete-time quantum walks (DTQWs) using the Stabilizer Renyi Entropy as a measure of quantum magic. We investigate single- and two-walker quantum walks on a one-dimensional lattice, considering a wide range of initial coin states. Our results reveal that DTQWs can dynamically generate significant magic, with the amount and structure strongly dependent on the initial state of the coin. In the case of a single walker, the relationship between magic and entanglement is found to be nontrivial and complementary at long times. These findings position DTQWs as accessible and controllable platforms for producing quantum magic, offering a new perspective on their role in quantum information processing and reliable quantum computation.
Authors: Jeremy T. Young, Ron Belyansky, Kang-Kuen Ni, Alexey V. Gorshkov
We propose a method to sympathetically cool polar molecules with Rydberg atoms without destroying the quantum information encoded in the polar molecules. While the interactions between the two are usually state-dependent, we show how to engineer state-insensitive interactions between the hot molecules and the cold atoms with a suitable choice of internal states and the application of external fields. The resulting interactions, which may be van der Waals or dipolar, induce a phonon swap interaction between the two species, thereby coherently cooling the polar molecules without affecting the internal state, a process which can be repeated if the atoms are cooled again or new cold atoms are brought in. Our cooling schemes open the possibility of extending quantum computation and simulation times in emerging hybrid tweezer arrays of polar molecules and neutral atoms.
Authors: Sam Alterman, Peter J. Love
In the light-front (LF) formulation of quantum field theory (QFT), physics is formulated from the perspective of a massless observer necessarily traveling at the speed of light. The LF formulation provides an alternative computational approach to lattice gauge theory, and has recently been investigated as a future application of quantum computers. A natural question is how quantum resources such as entanglement and contextuality amongst physical qubits in the laboratory are utilized in LF simulations of QFTs. We use the (1+1)D transverse-field Ising model to explore this question. We derive the LF energy operator that generates the LF dynamics of the system, which is distinct from the instant-form (IF) Hamiltonian. We find that while the eigenstates of the IF Hamiltonian exhibit pairwise entanglement between positive and negative momenta in IF momentum-space, the eigenstates of the LF Hamiltonian are separable in LF momentum-space. We then calculate the momentum-space magic of the IF-momentum-space ground state and show that it always requires more magic to prepare than the LF-momentum-space ground state. At the quantum critical point, corresponding to a massless free fermion, both LF and IF ground states are stabilizers, but the LF ground state is separable in LF momentum-space while the IF ground state is a product of maximally entangled pairs in IF momentum-space. These results show that quantum resources such as entanglement and magic are utilized differently by quantum simulations formulated in LF and IF, and that the simplicity of the LF ground state results in fewer required quantum resources.
Authors: Tanay Pathak, Masaki Tezuka
Understanding how quantum chaotic systems generate entanglement can provide insight into their microscopic chaotic dynamics and can help distinguish between different classes of chaotic behavior. Using von Neumann entanglement entropy, we study a nonentangled state evolved under three variants of the Sachdev-Ye-Kitaev (SYK) model with a finite number of Majorana fermions $N$. All the variants exhibit linear entanglement growth at early times, which at late times saturates to a universal value consistent with random matrix theory (RMT), but their growth rates differ. We interpret this as a large-$N$ effect, arising from the enhanced non-locality of fermionic operators in SYK and binary SYK, absent in spin operators of the spin-SYK model. Numerically, we find that these differences emerge gradually with increasing $N$. Although all variants are quantum chaotic, their entanglement dynamics reflect varying degrees of chaos and indicate that the entanglement production rate serves as a fine-grained probe of chaos beyond conventional measures. To probe its effect on thermalization properties of these models, we study the two-point autocorrelation function, finding no differences between the SYK variants, but deviations from RMT predictions for $N \geq 24$, particularly near the crossover from exponential decay to saturation regime.
Authors: Ye-jun Xu, Long-hua Zhai, Peng Fu, Shou-jing Cheng, Guo-Qiang Zhang
We investigate the nonreciprocal quantum phase transition in a cavity magnonic system driven by a parametric field, where an yttrium iron garnet (YIG) sphere is placed in a spinning microwave resonator. The system exhibits a rich phase diagram due to both magnon Kerr nonlinearity in YIG and parametric drive on the resonator. Especially, Sagnac-Fizeau shift caused by the spinning of the resonator brings about a significant modification in the critical driving strengths for second- and first-order quantum phase transitions, which means that the highly controllable quantum phase can be realized by the spinning speed of the resonator. More importantly, based on the difference in the detunings of the counterclockwise and clockwise modes induced by spinning direction of the resonator, we show that the phase transition in this system is nonreciprocal, that is, the quantum phase transition occurs when the cavity is driven in one direction but not the other. Our work offers an alternative path to engineer and design nonreciprocal magnonic devices.
Authors: Edoardo Ballini, Julius Mildenberger, Matteo M. Wauters, Philipp Hauke
Non-Abelian gauge theories underlie our understanding of fundamental forces of modern physics. Simulating them on quantum hardware is an outstanding challenge in the rapidly evolving field of quantum simulation. A key prerequisite is the protection of local gauge symmetries against errors that, if unchecked, would lead to unphysical results. While an extensive toolkit devoted to identifying, mitigating, and ultimately correcting such errors has been developed for Abelian groups, non-commuting symmetry operators complicate the implementation of similar schemes in non-Abelian theories. Here, we discuss two techniques for error mitigation through symmetry verification, tailored for non-Abelian lattice gauge theories implemented in noisy qudit hardware: dynamical post-selection (DPS), based on mid-circuit measurements without active feedback, and post-processed symmetry verification (PSV), which combines measurements of correlations between target observables and gauge transformations. We illustrate both approaches for the discrete non-Abelian group $D_3$ in 2+1 dimensions, explaining their usefulness for current NISQ devices even in the presence of fast fluctuating noise. Our results open new avenues for robust quantum simulation of non-Abelian gauge theories, for further development of error-mitigation techniques, and for measurement-based control methods in qudit platforms.
Authors: Adrien Bensemhoun, Silvia Cassina, Carlos Gonzalez-Arciniegas, Mohamed Fauzi Melalkia, Giuseppe Patera, Jonathan Faugier-Tovar, Quentin Wilmart, Ségolène Olivier, Alessandro Zavatta, Anthony Martin, Jean Etesse, Laurent Labonté, Olivier Pfister, Virginia D'Auria, Sébastien Tanzilli
This experimental work demonstrates multipartite quantum correlation in bright frequency combs out of a microresonator integrated on silicon nitride operating above its oscillation threshold. Multipartite features, going beyond so far reported two-mode correlation, naturally arise due to a cascade of non-linear optical processes, making a single-color laser pump sufficient to initiate their generation. Our results show the transition from two-mode to multipartite correlation, witnessed by noise reductions as low as $-2.5$\,dB and $-2$\,dB, respectively, compared to corresponding classical levels. A constant of the movement of the non-linear interaction Hamiltonian is identified and used to asses the multipartite behavior. Reported demonstrations pave the way to next generation on-chip multipartite sources for quantum technologies applications.
Authors: Mason C. Marshall, Daniel A. Rodriguez Castillo, Willa J. Arthur-Dworschack, Alexander Aeppli, Kyungtae Kim, Dahyeon Lee, William Warfield, Joost Hinrichs, Nicholas V. Nardelli, Tara M. Fortier, Jun Ye, David R. Leibrandt, David B. Hume
We report a single-ion optical atomic clock with fractional frequency uncertainty of $5.5\times10^{-19}$ and fractional frequency stability of $3.5 \times10^{-16}/\sqrt{\tau/\mathrm{s}}$, based on quantum logic spectroscopy of a single $^{27}$Al$^+$ ion. A co-trapped $^{25}$Mg$^+$ ion provides sympathetic cooling and quantum logic readout of the $^{27}$Al$^+$ $^1$S$_0\leftrightarrow^3$P$_0$ clock transition. A Rabi probe duration of 1 s, enabled by laser stability transfer from a remote cryogenic silicon cavity across a 3.6 km fiber link, results in a threefold reduction in instability compared to previous $^{27}$Al$^+$ clocks. Systematic uncertainties are lower due to an improved ion trap electrical design, which reduces excess micromotion, and a new vacuum system, which reduces collisional shifts. We also perform a direction-sensitive measurement of the ac magnetic field due to the RF ion trap, eliminating systematic uncertainty due to field orientation.
Authors: Tiemo Pedergnana, Florian Kogelbauer
We derive the analogues of the Dirac and Pauli equations from a spatially fourth-order Klein--Gordon equation with a universal length scale. Starting from a singularly perturbed variant of Maxwell's equations, we deduce a 32-dimensional variant of the Dirac equation for spin-$1/2$ particles through an algebraic factorization procedure. We illustrate an experimental test of the theory from the split lines of the electron beam in a Stern--Gerlach experiment. This hyperfine splitting leads to four distinct eigenvalues of the spin operator, which can be grouped into two pairs centered around the classic values of $\pm\hbar/2$. The modified electrodynamic framework features particle-antiparticle asymmetry and an oriented, micropolar spacetime.
Authors: Wolfgang Dür
We propose a quantum computation architecture based on geometries with nearest-neighbor interactions, including e.g. planar structures. We show how to efficiently split the role of qubits into data and entanglement-generation qubits. Multipartite entangled states, e.g. 2D cluster states, are generated among the latter, and flexibly transformed via mid-circuit measurements to multiple, long-ranged Bell states, which are used to perform several two-qubit gates in parallel on data qubits. We introduce planar architectures with $n$ data and $n$ auxiliary qubits that allow one to perform $O(\sqrt n)$ long-ranged two-qubit gates simultaneously, with only one round of nearest neighbor gates and one round of mid-circuit measurements. We also show that our approach is applicable in existing superconducting quantum computation architectures, with only a constant overhead.
Authors: Ammar Ali, Joe Gibbs, Keerthi Kumaran, Varadharajan Muruganandam, Bo Xiao, Paul Kairys, Gábor Halász, Arnab Banerjee, Phillip C. Lotshaw
Kitaev's honeycomb model is a paradigmatic exactly solvable system hosting a quantum spin liquid with non-Abelian anyons and topologically protected edge modes, offering a platform for fault-tolerant quantum computation. However, real candidate Kitaev materials invariably include complex secondary interactions that obscure the realization of spin-liquid behavior and demand novel quantum computational approaches for efficient simulation. Here we report quantum simulations of a 22-site Kitaev honeycomb lattice on a trapped-ion quantum processor, without and with non-integrable Heisenberg interactions that are present in real materials. We develop efficient quantum circuits for ground-state preparation, then apply controlled perturbations and measure time-dependent spin correlations along the system's edge. In the non-Abelian phase, we observe chiral edge dynamics consistent with a nonzero Chern number -- a hallmark of topological order -- which vanishes upon transition to the Abelian toric code phase. Extending to the non-integrable Kitaev-Heisenberg model, we find that weak Heisenberg interactions preserve chiral edge dynamics, while stronger couplings suppress them, signaling the breakdown of topological protection. Our work demonstrates a viable route for probing dynamical signatures of topological order in quantum spin liquids using programmable quantum hardware, opening new pathways for quantum simulation of strongly correlated materials.
Authors: Rui-Hao Li, Hakan Doga, Bryan Raubenolt, Sarah Mostame, Nicholas DiSanto, Fabio Cumbo, Jayadev Joshi, Hanna Linn, Maeve Gaffney, Alexander Holden, Vinooth Kulkarni, Vipin Chaudhary, Kenneth M. Merz Jr, Abdullah Ash Saki, Tomas Radivoyevitch, Frank DiFilippo, Jun Qin, Omar Shehab, Daniel Blankenberg
In this work, we present the first implementation of the face-centered cubic (FCC) lattice model for protein structure prediction with a quantum algorithm. Our motivation to encode the FCC lattice stems from our observation that the FCC lattice is more capable in terms of modeling realistic secondary structures in proteins compared to other lattices, as demonstrated using root mean square deviation (RMSD). We utilize two quantum methods to solve this problem: a polynomial fitting approach (PolyFit) and the Variational Quantum Eigensolver with constraints (VQEC) based on the Lagrangian duality principle. Both methods are successfully deployed on Eagle R3 (ibm_cleveland) and Heron R2 (ibm_kingston) quantum computers, where we are able to recover ground state configurations for the 6-amino acid sequence KLVFFA under noise. A comparative analysis of the outcomes generated by the two QPUs reveals a significant enhancement (reaching nearly a two-fold improvement for PolyFit and a three-fold improvement for VQEC) in the prediction and sampling of the optimal solution (ground state conformations) on the newer Heron R2 architecture, highlighting the impact of quantum hardware advancements for this application.
Authors: Arseny Kovyrshin, Dilhan Manawadu, Edoardo Altamura, George Pennington, Benjamin Jaderberg, Sebastian Brandhofer, Anton Nykänen, Aaron Miller, Walter Talarico, Stefan Knecht, Fabijan Pavošević, Alberto Baiardi, Francesco Tacchino, Ivano Tavernelli, Stefano Mensa, Jason Crain, Lars Tornberg, Anders Broo
Proton transfer reactions are fundamental to many chemical and biological systems, where quantum effects such as tunneling, delocalization, and zero-point motion play key kinetic control roles. However, classical methods capable of accurately capturing these phenomena scale prohibitively with system size. Here, we develop and demonstrate quantum computing algorithms based on the Nuclear-Electronic Orbital framework, treating the transferring proton quantum mechanically. We assess the potential of current quantum devices for simulating proton transfer kinetics with high accuracy. We first construct a deep initial ansätze within a truncated orbital space by employing the frozen natural orbital approximation. Then, to balance circuit depth against state fidelity, we implement an adaptive form of approximate quantum compiling. Using resulting circuits at varying compression levels transpiled for the ibm_fez device, we compute barrier heights and delocalised proton densities along the proton transfer pathway using a realistic hardware noise model. We find that, although current quantum hardware introduces significant noise relative to the demanding energy tolerances involved, our approach allows substantial circuit simplification while maintaining energy barrier estimates within 13% of the reference value. Despite present hardware limitations, these results offer a practical means of approximating key circuit segments in near-term devices and early fault-tolerant quantum computing systems.
Authors: Abdolreza Pasharavesh, Sai Sreesh Venuturumilli, Michal Bajcsy
We investigate the operation of a photon-number-resolving (PNR) detector consisting of a cascade of waveguide-coupled lambda-type emitters, where each waveguide-coupled emitter extracts a single photon from the input light and sends it to a single-photon detector. Using Green's function and input-output formalisms, we derive the scattering matrices and photon-photon correlators for individual scatterers. By cascading these results, we obtain a closed-form expression for the detector's precision in the linear regime and predict how correlations generated by nonlinear photon-photon interactions influence this precision. To evaluate the performance of this PNR detector in the nonlinear regime, we apply the quantum trajectory method to the cascaded setup, calculating the achievable precision and analyzing its dependence on key system parameters, such as the number of emitters and their coupling strength to the waveguide. We compare the performance of the proposed PNR detector with that of a conventional PNR scheme based on spatial demultiplexing via beamsplitters. Our results indicate that the proposed scheme can outperform conventional detectors under realistic conditions, making it a promising candidate for next-generation PNR detection.
Authors: Zhenghao Zhang, Qingtian Miao, G. S. Agarwal
Nonequilibrium dissipative phase transition, arising from the competition of cooperative behavior and coherent field driving, discovered in the 1970s by Narducci et al. and Walls et al., has been found to exhibit time-crystal behavior when the driving field exceeds the cooperative decay rate. This was seen through the study of the eigenvalues of the Liouvillian superoperator that describes the joint effect of drive and cooperativity. The cooperative decay depends on the nature of the reservoir correlations. If the reservoir correlations have phase-sensitive behavior, then the eigenvalues of the Liouvillian will be different. We investigate the time-crystal behavior of the nonequilibrium dissipative phase transitions under the influence of a squeezed vacuum reservoir. We analyze the steady-state phase diagram as a function of the control parameter and demonstrate that increasing the squeezing strength sharpens the dissipative phase transition. Spectral analysis of the Liouvillian reveals gap closings and the emergence of purely imaginary eigenvalues in the thermodynamic limit, indicating the time-crystal phase. We find that the real parts of subleading eigenvalues exhibit nonmonotonic behavior with increasing squeezing, reflecting the sensitivity of relaxation dynamics to the reservoir properties. Time-domain simulations confirm that the oscillation frequencies correspond to the imaginary parts of the Liouvillian eigenvalues. We also present results on quantum fluctuations in the time-crystal phase. Our results call attention to the study of time crystals in models of cooperativity based on engineered environments.
Authors: Cong-Gang Song, Qing-yu Cai
Temporal resolution is a critical figure of merit in quantum sensing. This study combines the distinguishable condition of quantum states with quantum speed limits to establish a lower bound on interrogation time. When the interrogation time falls below this bound, the output state becomes statistically indistinguishable from the input state, and the information will inevitably be lost in noise. Without loss of generality, we extend these conclusions to time-dependent signal Hamiltonian. In theory, leveraging certain quantum control techniques allows us to calculate the minimum interrogation time for arbitrary signal Hamiltonian. Finally, we illustrate the impact of quantum speed limits on magnetic field measurements and temporal resolution.
Authors: Shiran Even-Haim, Ethan Nussinson, Roni Ben-Maimon, Alexey Gorlach, Ron Ruimy, Ephraim Shahmoon, Osip Schwartz, Ido Kaminer
Quantum metrology experiments in atomic physics and quantum optics have demonstrated measurement accuracy beyond the shot-noise limit via multi-particle entanglement. At the same time, electron microscopy, an essential tool for high-resolution imaging of biological systems, is severely constrained in its signal-to-noise ratio (SNR) by shot noise, due to the dose limit imposed by electron beam-induced damage. Here, we show theoretically that spin squeezing, a form of quantum metrology based on entanglement, is a natural fit for improving the SNR in electron microscopy. We investigate the generation of the necessary entangled states through electron-electron Coulomb interactions and quantum non-demolition measurements. Our results connect the fields of quantum metrology and electron interferometry, paving the way toward electron microscopy with SNR beyond the shot-noise limit.
Authors: Hai Wang
Superposition is an essential feature of quantum mechanics. From the Schrodinger's cat to quantum algorithms such as Deutsch-Jorsza algorithm, quantum superposition plays an important role. It is one fundamental and crucial question how to quantify superposition. Until now, the framework of coherence has been well established as one typical instance of quantum resource theories. And the concept of coherence has been generalized into linearly independent basis, projective measurements and POVMs. In this work, we will focus on coherence measures for projective measurements or orthogonal subspaces. One new condition, order-preserving condition, is proposed for such measures. This condition is rooted in the mathematical structure of Hilbert spaces' orthogonal decomposition. And by generalizing the 1/2-affinity of coherence into subspace cases, we verify that this generalized coherence measure satisfies the order-preserving condition. And it also satisfies other reasonable conditions to be a good coherence measure. As the partial order relationship exists for not only projective measurements, but also POVMs, it's natural to study the order-preserving condition in POVM cases, which will be the last part of this work.
Authors: Alba Torras-Coloma, Luca Cozzolino, Ariadna Gómez-del-Pulgar-Martínez, Elia Bertoldo, P. Forn-Díaz
We present an ultrastrong superinductor-based coupling consisting of a flux qubit galvanically coupled to a resonator. The coupling inductor is fabricated in granular Aluminum, a superinductor material able to provide large surface inductances. Spectroscopy measurements on the qubit-resonator system reveal a Bloch-Siegert shift of \SI{23}{\mega\hertz} and a coupling fraction of $g/\omega_r \simeq 0.13$, entering the perturbative ultrastrong coupling (USC) regime. We estimate the inductance of the coupler independently by low-temperature resistance measurements providing $L_c = (0.74\pm0.14)\,\mathrm{nH}$, which is compatible with $g/\omega \gtrsim 0.1$. Our results show that superinductors are a promising tool to study USC physics in high-coherence circuits using flux qubits with small loop areas and low persistent currents.
Authors: Ramaseshan R, Prateek P. Kulkarni, Sharanya Madhusudhan, Kaustav Bhowmick
Entanglement is a cornerstone of quantum technology, playing a key role in quantum computing, cryptography, and information processing. Conventional methods for generating entanglement via optical setups rely on beam splitters, nonlinear media, or quantum dots, which often require bulky configurations and precise phase control. In contrast, metasurfaces - ultrathin, engineered optical interfaces - offer a compact and tunable alternative for quantum photonics. In this work, we demonstrate that metasurfaces can serve as a promising platform for generating Bell states through a Hamiltonian-driven spin-entanglement mechanism. By analyzing the system's evolution under a metasurface interaction Hamiltonian, we show that an initially separable spin state evolves into a maximally entangled Bell state. We further study classical and quantum correlations, evaluate the impact of environmental decoherence, and compute quantum discord to quantify correlation robustness beyond entanglement. Our analysis shows that metasurfaces can generate Bell states with a concurrence of about 0.995 and maintain quantum discord for up to 29 microseconds. These results establish metasurfaces as scalable, high-fidelity components for next-generation quantum photonic architectures.
Authors: Noura Chabar, Mohamed Amazioug
We propose an experimental scheme for enhancing entanglement, achieving asymmetric Einstein-Podolsky-Rosen (EPR) steering, and creating nonreciprocal quantum correlations within a hybrid system. This system integrates a yttrium iron garnet (YIG) sphere, which exhibits magnon-phonon coupling via magnetostriction, with a silica sphere featuring optomechanical whispering-gallery modes. By tuning the Barnett effect through the magnetic field direction, our system enables controllable asymmetric EPR steering and nonreciprocal entanglement between both directly and indirectly coupled modes. We demonstrate that adjusting the reflectivity of a beam splitter can boost stationary quantum steering and entanglement, effectively countering thermal noise. This approach allows for the generation of multipartite entanglement and both one-way and two-way steering. The proposed system is experimentally feasible and holds significant promise for various quantum information applications.
Authors: Pei-Kun Yang
Structure-based virtual screening (SBVS) is a key computational strategy for identifying potential drug candidates by estimating the binding free energies (delta G_bind) of protein-ligand complexes. The immense size of chemical libraries, combined with the need to account for protein and ligand conformations as well as ligand translations and rotations, makes these tasks computationally intensive on classical hardware. This study proposes a quantum convolutional neural network (QCNN) framework to estimate delta G_bind efficiently. Using the PDBbind v2020 dataset, we trained QCNN models with 9 and 12 qubits, with the core set designated as the test set. The best-performing model achieved a Pearson correlation coefficient of 0.694 on the test set. To assess robustness, we introduced quantum noise under two configurations. While noise increased the root mean square deviation, the Pearson correlation coefficient remained largely stable. These results demonstrate the feasibility and noise tolerance of QCNNs for high-throughput virtual screening and highlight the potential of quantum computing to accelerate drug discovery.
Authors: Gal G. Shaviner, Ziv Chen, Steven H. Frankel
This work presents a quantum algorithm for solving linear systems of equations of the form $\mathbf{A}{\frac{\mathbf{\partial f}}{\mathbf{\partial x}}} = \mathbf{B}\mathbf{f}$, based on the Quantum Singular Value Transformation (QSVT). The algorithm uses block-encoding of $A$ and applies an 21st-degree polynomial approximation to the inverse function $f(x) = 1/x$, enabling relatively shallow quantum circuits implemented on 9 qubits, including two ancilla qubits, corresponding to a grid size of 128 points. Phase angles for the QSVT circuit were optimized classically using the Adagrad gradient-based method over 100 iterations to minimize the solution cost. This approach was simulated in PennyLane and applied to solve a 1D benchmark case of Maxwell's equations in free space, with a Gaussian pulse as the initial condition, where the quantum-computed solution showed high fidelity of more than 99.9% when compared to the normalized classical solution. Results demonstrate the potential of QSVT-based linear solvers on simulators with full quantum state access. However, practical hardware implementations face challenges because accessing the complete quantum state is infeasible. This limitation restricts applicability to cases where only $O({poly}(n))$ observables are needed. These findings highlight both the promise and current limitations of using quantum algorithms, such as QSVT, to solve linear systems of equations, and they point to the need for the development of measurement-efficient algorithms for near-term quantum devices.
Authors: Hanfeng Wang, Kurt Jacobs, Donald Fahey, Yong Hu, Dirk R. Englund, Matthew E. Trusheim
Phase transitions can dramatically alter system dynamics, unlocking new behavior and improving performance. Exceptional points (EPs), where the eigenvalues and corresponding eigenvectors of a coupled linear system coalesce, are particularly relevant for sensing applications as they can increase sensor response to external perturbations to a range of phenomena from optical phase shifts to gravitational waves. However, the coalescence of eigenstates at linear EPs amplifies noise, negating the signal-to-noise ratio (SNR) enhancement. Here, we overcome this limitation using nonlinearity, which exhibits exceptional SNR around a bistable transition point (BP). We couple a state-of-the-art diamond quantum sensor to a nonlinear Van der Pol oscillator, forming a self-oscillating hybrid system that exhibits both a single-valued and bistable phase. The boundaries between these phases are marked by both adiabatic and deterministic non-adiabatic transitions that enable chiral state switching and state coalescence at the BP. Crucially, NV magnetometry performed near the BP exhibits a 17x enhancement in SNR, achieving a record sensitivity of 170 fT/\sqrt{Hz}. This result surpasses the sensitivity limit of an ideal, thermally-limited electron magnetometer and resolves a long-standing debate regarding EP-like physics in advanced quantum sensing.
Authors: Asma Al-Othni, Saif Al-Kuwari, Mohammad Mahdi Nasiri Fatmehsari, Kamila Zaman, Ebrahim Ardeshir Larijani
Generative Adversarial Networks (GANs) have demonstrated immense potential in synthesizing diverse and high-fidelity images. However, critical questions remain unanswered regarding how quantum principles might best enhance their representational and computational capacity. In this paper, we investigate hybrid quantum-classical GAN architectures supplemented by transfer learning to systematically examine whether incorporating Variational Quantum Circuits (VQCs) into the generator, the discriminator, or both improves performance over a fully classical baseline. Our findings indicate that fully hybrid models, which incorporate VQCs in both the generator and the discriminator, consistently produce images of higher visual quality and achieve more favorable quantitative metrics compared to their fully classical counterparts. In particular, VQCs in the generator accelerate early feature learning, whereas those in the discriminator, despite exhibiting slower initial convergence, ultimately facilitate more refined synthetic outputs. Moreover, the model sustains near-comparable performance even when the dataset size is drastically reduced, suggesting that transfer learning and quantum enhancements mitigate the problem of data scarcity. Overall, the results underscore that carefully integrating quantum computing with classical adversarial training and pretrained feature extraction can considerably enrich GAN-based image synthesis. These insights open avenues for future work on higher-resolution tasks, alternative quantum circuit designs, and experimentation with emerging quantum hardware.
Authors: Mustafa Bakr, Mohammed Alghadeer, Simon Pettersson Fors, Simone D. Fasciati, Shuxiang Cao, Atharv Mahajan, Smain Amari, Anton Frisk Kockum, Peter Leek
Decoherence due to radiative decay remains an important consideration in scaling superconducting quantum processors. We introduce a passive, interference-based methodology for suppressing radiative decay using only the intrinsic multi-mode structured environment of superconducting circuits. By taking into account the full electromagnetic mode-mode couplings within the device, we derive analytic conditions that enable destructive interference. These conditions are realized by introducing controlled geometric asymmetries -- such as localized perturbations to the transmon capacitor -- which increase mode hybridization and activate interference between multiple decay pathways. We validate this methodology using perturbation theory, full-wave electromagnetic simulations, and experimental measurements of a symmetry-broken transmon qubit with improved coherence times.
Authors: Mirco A. Mannucci
We investigate the sequential composition of weak values in the framework of time-symmetric quantum mechanics. Specifically, we consider a forward'' weak measurement from a preselected state $\ket{\psi}$ to a post-selected state $\ket{\phi}$, followed by a reverse'' weak measurement. We show that the product of these two weak values corresponds to the normalized expectation value of a strong, state-conditioned observable $B = A P_\psi A$, where $P_\psi = \ket{\psi}\bra{\psi}$ is the projector onto the preselected state. Analyzing the structure of $B$, we demonstrate how it encodes interference information, particularly when $\ket{\psi}$ is a superposition rather than an eigenstate of $A$. This formulation extends naturally to mixed states by replacing $P_\psi$ with a generic density matrix $\rho$, linking the construction to the formalism of generalized quantum measurements. We illustrate practical applications in quantum information, including state-specific error witnessing in quantum computing, and show how the phase of a weak value can be inferred via strong measurements in the pure-state case.
Authors: Nahid Binandeh Dehaghani, A. Pedro Aguiar, Rafal Wisniewski
Quantum entanglement is a key resource for quantum technologies, yet its efficient and high-fidelity generation remains a challenge due to the complexity of quantum dynamics. This paper presents a quantum optimal control framework to maximize bipartite entanglement within a fixed time horizon, under bounded control inputs. By leveraging Pontryagin's Minimum Principle, we derive a set of necessary conditions that guide the design of time-dependent control fields to steer a two-qubit system toward maximally entangled Bell states. The entanglement is quantified using concurrence, and the control objective is formulated as maximizing this measure at the terminal time. Our approach is validated through numerical simulations of Liouville-von Neumann dynamics. The results demonstrate the effectiveness of switching-based control strategies in achieving robust entanglement, offering insights into practical implementations of quantum control for entanglement generation in quantum networks.
Authors: Yoshiharu Kawamura
Taking as a model the fact that Heisenberg's matrix mechanics was derived from Hamiltonian mechanics using the correspondence principle, we explore a class of dynamical systems involving discrete variables, with Nambu mechanics as the starting point. Specifically, we reconstruct an extended version of matrix mechanics that describes dynamical systems possessing physical quantities expressed through generalized matrices. Furthermore, we reconfirm that a multiple commutator involving generalized matrices can serve as a discrete (quantized) version of the Nambu bracket or the Jacobian.
Authors: Koki Nagamachi, Hiroki Yamashita, Mikio Fujiwara, Shigehito Miki, Hirotaka Terai, Takafumi Ono
We evaluated a multi-color two-photon entangled state generated in silicon via spontaneous four-wave mixing (SFWM) as a potential source for bosonic integrated circuits. Spatially entangled photon states were created using a pair of silicon waveguides that produced signal and idler photons through SFWM, allowing us to observe quantum interference between them. Assuming that the frequencies of the multi-color photons were nearly identical, we characterized the generated quantum state by performing quantum state tomography on the bosonic system using a linear optical circuit. This study demonstrates the feasibility of using photon-pair sources generated in silicon via SFWM in bosonic optical circuits and highlights their potential for a wide range of applications in silicon-based optical quantum technologies.
Authors: M. D. Urmey, S. Dickson, K. Adachi, S. Mittal, L. G. Talamo, A. Kyle, N. E. Frattini, S.-X. Lin, K. W. Lehnert, C. A. Regal
A microwave-optical transducer of sufficiently low noise and high signal transfer rate would allow entanglement to be distributed between superconducting quantum processors at a rate faster than the lifetimes of the quantum memories being linked. Here we present measurements of a membrane-based opto-electromechanical transducer with high signal throughput, as quantified by an efficiency-bandwidth-duty-cycle product of 7 kHz, approaching quantum-enabled operation in upconversion as well as downconversion, with input-referred added noise of 3 photons. In downconversion, throughput of this magnitude at the few-photon noise level is unprecedented. Using the quantum channel capacity, we also find an expression for the maximum rate at which quantum information can be transduced, providing insight into the importance of improving both a transducer's throughput and noise performance. With feasible improvements, the high throughput achieved with this device positions membrane-based transducers as a strategic choice for demonstrations of a quantum network with reasonable averaging times.
Authors: Jiaxin Huang, Yan Zhu, Giulio Chiribella, Ya-Dong Wu
Characterization of quantum systems from experimental data is a central problem in quantum science and technology. But which measurements should be used to gather data in the first place? While optimal measurement choices can be worked out for small quantum systems, the optimization becomes intractable as the system size grows large. To address this problem, we introduce a deep neural network with a sequence model architecture that searches for efficient measurement choices in a data-driven, adaptive manner. The model can be applied to a variety of tasks, including the prediction of linear and nonlinear properties of quantum states, as well as state clustering and state tomography tasks. In all these tasks, we find that the measurement choices identified by our neural network consistently outperform the uniformly random choice. Intriguingly, for topological quantum systems, our model tends to recommend measurements at the system's boundaries, even when the task is to predict bulk properties. This behavior suggests that the neural network may have independently discovered a connection between boundaries and bulk, without having been provided any built-in knowledge of quantum physics.
Authors: Bachtiar Rifai, Dwi Satya Palupi, Muhammad Farchani Rosyid
The ontological status of the quantum wavefunction remains one of the most debated questions in quantum theory. While epistemic interpretations regard the wavefunction as a reflection of our knowledge or beliefs, ontic interpretations treat it as a real physical object. In this paper, we argue that epistemic approaches struggle to explain the universality and precision of the uncertainty principle, a core feature of quantum mechanics. By contrast, treating the wave-function as ontic allows a consistent and natural derivation of quantum uncertainty from the mathematical structure of Hilbert space. We examine key interpretations on both sides and highlight why the epistemic view falls short in addressing constraints that appear to be intrinsic to nature.
Authors: Ignatius William Primaatmaja, Hong Jie Ng, Koon Tong Goh
Device-independent quantum random number generation (DIQRNG) is the gold standard for generating truly random numbers, as it can produce certifiably random numbers from untrusted devices. However, the stringent device requirements of traditional DIQRNG protocols have limited their practical applications. Here, we introduce Device-Independent Private Quantum Randomness Beacon (DIPQRB), a novel approach to generate random numbers from untrusted devices based on routed Bell tests. This method significantly relaxes the device requirements, enabling a more practical way of generating randomness from untrusted devices. By distributing the device requirements across a network of servers and clients, our proposal allows the server to operate high-performance devices while the clients can be equipped with more cost-effective devices. Moreover, the outputs of the client's device are also private, even against the server, which is essential in cryptographic applications. Therefore, DIPQRB provides a cost-effective method to generate secure and private random numbers from untrusted devices.
Authors: Anant Vijay Varma, Doron Cohen
Heat and work in thermodynamics refer to the measurement of changes in energy content of external bodies (baths and agents). We discuss the implications of quantum mechanics on the possibility to measure work in a mesoscopic context. The agent is a quantum entity (say an oscillator) that is used to drive the system. An obvious limitation is related to back-reaction, leading to a classical-like restriction. We find that in order to resolve fingerprints of interference an additional quantum uncertainty limitation should be taken into account in the design of the agent. The quantum limitation is fundamental, and cannot be relaxed by super-resolution techniques.
Authors: Petr O. Jedlička, Šimon Kos, Martin Šmíd, Jiří Vomlel, Jan Slavík
As we approach the centennial anniversary of modern quantum mechanics this paper revisits the foundational debates through a new poll within the research community. Inspired by the survey by Schlosshauer, Kofler, and Zeilinger at the specialized 2011 Quantum Physics and the Nature of Reality conference, we expanded our recruitment to include a more representative sample of the broader community of physicists with the aim to reveal potential shifts in scientists' views and compare our findings with those from several previous polls. While quantum foundations still lack a consensus interpretation, our results indicate a persistent preference for the Copenhagen interpretation. This enduring support likely reflects both the educational emphasis on the Copenhagen interpretation and its pragmatic appeal in avoiding complex metaphysical questions and introducing new notions (e.g., other worlds or the pilot wave). Our findings thus underscore the relative stability of interpretational preferences over the past decades.
Authors: Jan van Neerven, Marijn Waaijer
In this paper we develop a mathematical framework for indiscernibility of quantum states, arguing that, given a set of observables, the ``distinguishable objects'' are the equivalence classes modulo indiscernibility relative to the observables. The structure of the set of distinguishable objects - called the Holevo space - is investigated in detail, and it is shown that the observables admit a natural lift to continuous functions on the Holevo space. The theory is illustrated by several examples where the ``distinguishable objects'' can be described explicitly. Among other things, the Holevo spaces and the lifted functions are described for position measurements on a free particle and for spin measurements in the EPR and Bell experiments.
Authors: Wenxuan Tao, Fen Zuo
The maximum energy of the EPR model on a weighted graph is known to be upper-bounded by the sum of the total weight and the value of maximum-weight fractional matching~(MWFM). Recently, Apte, Parekh and Sud~(APS) conjecture that the bound could be strengthened by replacing MWFM with maximum weight matching~(MWM). Here we test this conjecture on a special class of regular graphs that Henning and Yeo constructed many years ago. On this class of regular graphs, MWMs achieve tight lower bounds. As for the maximum energy of the EPR model, we have recently devised a new algorithm called Fractional Entanglement Distribution~(FED) based on quasi-homogeneous fractional matchings, which could achieve rather high accuracy. Applying the FED algorithm to the EPR model on Henning-Yeo graphs, we could thus obtain energy as high as possible and matching value as low as possible, and then make high-precision tests of the APS conjecture. Nevertheless, our numerical results do not show any evidence that the APS conjecture could be violated.
Authors: Koki Shiraishi, Masaya Nakagawa, Takashi Mori
We derive a thermodynamically consistent quantum master equation that satisfies locality for quadratic systems coupled to independent and identical baths at each site. We show that the quasi-local Redfield equation coincides exactly with the Davies equation, which satisfies the detailed-balance condition, due to cancellation of quantum coherence generated by each bath. This derivation does not rely on the secular approximation, which fails in systems with vanishing energy-level spacings. We discuss generalizations of our result to slowly driven quadratic systems and generic quantum many-body systems. Our result paves the way to a thermodynamically consistent description of quantum many-body systems.
Authors: Xuan Liu, Shaoping Shi, Yimiao Wu, Xuan Wang, Long Tian, Wei Li, Yajun Wang, Yaohui Zheng
Constructing large-scale quantum resources is an important foundation for further improving the efficiency and scalability of quantum communication. Here, we present an efficient extraction and stable control scheme of 40 pairs of entangled sideband modes from the squeezed light by specially designing optical parametric oscillator. Utilizing the low-loss optical frequency comb control technology and the local cross-correlation algorithm, we model and manage the efficient separation process of the entangled sidebands modes facilitated by the optical filtering cavities, a maximum entanglement level of 6.5 dB is achieved. The feasibility of large-capacity quantum dense coding based on these entangled sideband modes is proved experimentally, which is of great significance for optimizing the utilization of quantum resources, thereby contributing to the advancement of large-capacity quantum communication networks and enabling the realization of more secure and efficient quantum communication systems.
Authors: Silvie Illésová, Tomasz Rybotycki, Piotr Gawron, Martin Beseda
We present a systematic study of how quantum circuit design, specifically the depth of the variational ansatz and the choice of quantum feature mapping, affects the performance of hybrid quantum-classical neural networks on a causal classification task. The architecture combines a convolutional neural network for classical feature extraction with a parameterized quantum circuit acting as the quantum layer. We evaluate multiple ansatz depths and nine different feature maps. Results show that increasing the number of ansatz repetitions improves generalization and training stability, though benefits tend to plateau beyond a certain depth. The choice of feature mapping is even more critical: only encodings with multi-axis Pauli rotations enable successful learning, while simpler maps lead to underfitting or loss of class separability. Principal Component Analysis and silhouette scores reveal how data distributions evolve across network stages. These findings offer practical guidance for designing quantum circuits in hybrid models. All source codes and evaluation tools are publicly available.
Authors: Artur Czerwinski
Quantum money represents an innovative approach to currency by encoding economic value within the quantum states of physical systems, utilizing the principles of quantum mechanics to enhance security, integrity, and transferability. This perspective article explores the definition and properties of quantum money. We analyze the process of transferring quantum money via quantum teleportation, using terrestrial and satellite-based quantum networks. Furthermore, we consider the impact of quantum money on the modern banking system, particularly in money creation. Finally, we conduct an analysis to assess the strengths and weaknesses of quantum money, as well as opportunities and threats associated with this emerging concept.
Authors: Debnath Ghosh, Soumit Roy, Prithwi Bagchi, Indranil Chakrabarty, Ashok Kumar Das
Quantum digital signatures ensure unforgeable message authenticity and integrity using quantum principles, offering unconditional security against both classical and quantum attacks. They are crucial for secure communication in high-stakes environments, ensuring trust and long-term protection in the quantum era. Nowadays, the majority of arbitrated quantum signature (AQS) protocols encrypt data qubit by qubit using the quantum one-time pad (QOTP). Despite providing robust data encryption, QOTP is not a good fit for AQS because of its susceptibility to many types of attacks. In this work, we present an efficient AQS protocol to encrypt quantum message ensembles using a distinct encryption technique, the chained controlled unitary operations. In contrast to existing protocols, our approach successfully prevents disavowal and forgery attacks. We hope this contributes to advancing future investigations into the development of AQS protocols.
Authors: Kaifeng Bu, Bikun Li
Non-Gaussian states are essential resources in quantum information processing. In this work, we investigate methods for quantifying bosonic non-Gaussianity in many-body systems. Building on recent theoretical insights into the self-convolution properties of bosonic pure states, we introduce non-Gaussian entropy as a new measure to characterize non-Gaussianity in bosonic pure states. We further propose a practical protocol for measuring non-Gaussian entropy using three beam splitters and four copies of the input state. In addition, we extend this framework to mixed states, providing a general approach to quantifying non-Gaussianity. Our results offer a convenient and efficient method for characterizing bosonic non-Gaussianity, paving the way for its implementation on near-term experimental platforms.
Authors: Luca Magazzù, Elisabetta Paladino, Jukka P. Pekola, Milena Grifoni
A qubit-oscillator junction connecting as a series two bosonic heat baths at different temperatures can display heat valve and diode effects. In particular, the rectification can change in magnitude and even in sign, implying an inversion of the preferential direction for the heat current with respect to the temperature bias. We perform a systematic study of these effects in a circuit QED model of qubit-oscillator system and find that the features of current and rectification crucially depend on the qubit-oscillator coupling. While at small coupling, transport occurs via a resonant mechanism between the sub-systems, in the ultrastrong coupling regime the junction is a unique, highly hybridized system and the current becomes largely insensitive to the detuning. Correspondingly, the rectification undergoes a change of sign. In the nonlinear transport regime, the coupling strength determines whether the current scales sub- or super-linearly with the temperature bias and whether the rectification, which increases in magnitude with the bias, is positive or negative. We also find that steady-state coherence largely suppresses the current and enhances rectification. An insight on these behaviors with respect to changes in the system parameters is provided by analytical approximate formulas.
Authors: Xiantao Li
Quantum speed-ups for dynamical simulation usually demand unitary time-evolution, whereas the large ODE/PDE systems encountered in realistic physical models are generically non-unitary. We present a universal moment-fulfilling dilation that embeds any linear, non-Hermitian flow $\dot x = A x$ with $A=-iH+K$ into a strictly unitary evolution on an enlarged Hilbert space: \[ ( (l| \otimes I ) \mathcal T e^{-i \int ( I_A\otimes H +i F\otimes K) dt} ( |r) \otimes I ) = \mathcal T e^{\int A dt}, \] provided the triple $( F, (l|, |r) )$ satisfies the compact moment identities $(l| F^{k}|r) =1$ for all $k\ge 0$ in the ancilla space. This algebraic criterion recovers both \emph{Schrödingerization} [Phys. Rev. Lett. 133, 230602 (2024)] and the linear-combination-of-Hamiltonians (LCHS) scheme [Phys. Rev. Lett. 131, 150603 (2023)], while also unveiling whole families of new dilations built from differential, integral, pseudo-differential, and difference generators. Each family comes with a continuous tuning parameter \emph{and} is closed under similarity transformations that leave the moments invariant, giving rise to an overwhelming landscape of design space that allows quantum dilations to be co-optimized for specific applications, algorithms, and hardware. As concrete demonstrations, we prove that a simple finite-difference dilation in a finite interval attains near-optimal oracle complexity. Numerical experiments on Maxwell viscoelastic wave propagation confirm the accuracy and robustness of the approach.
Authors: Douglas Hendry, Alessandro Sinibaldi, Giuseppe Carleo
Excited states play a central role in determining the physical properties of quantum matter, yet their accurate computation in many-body systems remains a formidable challenge for numerical methods. While neural quantum states have delivered outstanding results for ground-state problems, extending their applicability to excited states has faced limitations, including instability in dense spectra and reliance on symmetry constraints or penalty-based formulations. In this work, we rigorously formalize the framework introduced by Pfau et al.~\cite{pfau2024accurate} in terms of Grassmann geometry of the Hilbert space. This allows us to generalize the Stochastic Reconfiguration method for the simultaneous optimization of multiple variational wave functions, and to introduce the multidimensional versions of operator variances and overlaps. We validate our approach on the Heisenberg quantum spin model on the square lattice, achieving highly accurate energies and physical observables for a large number of excited states.
Authors: Takanao Ishii, Masahito Ueda
Understanding how a quantum many-body state is maintained stably as a nonequilibrium steady state is of fundamental and practical importance for exploration and exploitation of open quantum systems. We establish a general equivalent condition for an open quantum many-body system governed by the Gorini-Kossakowski-Sudarshan-Lindblad dynamics under local drive and/or dissipation to have a quantum independent and identically distributed (i.i.d.) steady state. We present a sufficient condition for a system to have a quantum i.i.d. steady state by identifying a set of operators that commute with arbitrary quantum i.i.d. states. In particular, a set of quantum i.i.d. states is found to be an invariant subset of time evolution superoperators for systems that satisfy the sufficient condition. These findings not only identify a class of models with exactly solvable steady states but also lead to a no-go theorem that precludes quantum entanglement and spatial correlations in a broad class of quantum many-body steady states in a dissipative environment.
Authors: Gina Warttmann, Florian Meinert, Hans Peter Büchler, Sebastian Weber
We present a method to suppress crosstalk from implementing controlled-Z gates via local addressing in neutral atom quantum computers. In these systems, a fraction of the laser light that is applied locally to implement gates typically leaks to other atoms. We analyze the resulting crosstalk in a setup of two gate atoms and one neighboring third atom. We then perturbatively derive a spin-echo-inspired gate protocol that suppresses the leading order of the amplitude error, which dominates the crosstalk. Numerical simulations demonstrate that our gate protocol improves the fidelity by two orders of magnitude across a broad range of experimentally relevant parameters. To further reduce the infidelity, we develop a circuit to cancel remaining phase errors. Our results pave the way for using local addressing for high-fidelity quantum gates on Rydberg-based quantum computers.
Authors: Jan Krause, Nino Walenta
Current count-rate models for single-photon avalanche diodes (SPADs) typically assume an instantaneous recovery of the quantum efficiency following dead-time, leading to a systematic overestimation of the effective detection efficiency for high photon flux. To overcome this limitation, we introduce a generalized analytical count-rate model for free-running SPADs that models the non-instantaneous, exponential recovery of the quantum efficiency following dead-time. Our model, framed within the theory of non-homogeneous Poisson processes, only requires one additional detector parameter -- the exponential-recovery time constant $\tau_\mathrm{r}$. The model accurately predicts detection statistics deep into the saturation regime, outperforming the conventional step-function model by two orders of magnitude in terms of the impinging photon rate. For extremely high photon flux, we further extend the model to capture paralyzation effects. Beyond photon flux estimation, our model simplifies SPAD characterization by enabling the extraction of quantum efficiency $\eta_0$, dead-time $\tau_\mathrm{d}$, and recovery time constant $\tau_\mathrm{r}$ from a single inter-detection interval histogram. This can be achieved with a simple setup, without the need for pulsed lasers or externally gated detectors. We anticipate broad applicability of our model in quantum key distribution (QKD), time-correlated single-photon counting (TCSPC), LIDAR, and related areas. Furthermore, the model is readily adaptable to other types of dead-time-limited detectors. A Python implementation is provided as supplementary material for swift adoption.
Authors: Zvika Brakerski, Nir Magrafta, Tomer Solomon
Classical Shadow Tomography (Huang, Kueng and Preskill, Nature Physics 2020) is a method for creating a classical snapshot of an unknown quantum state, which can later be used to predict the value of an a-priori unknown observable on that state. In the short time since their introduction, classical shadows received a lot of attention from the physics, quantum information, and quantum computing (including cryptography) communities. In particular there has been a major effort focused on improving the efficiency, and in particular depth, of generating the classical snapshot. Existing constructions rely on a distribution of unitaries as a central building block, and research is devoted to simplifying this family as much as possible. We diverge from this paradigm and show that suitable distributions over \emph{states} can be used as the building block instead. Concretely, we create the snapshot by entangling the unknown input state with an independently prepared auxiliary state, and measuring the resulting entangled state. This state-based approach allows us to consider a building block with arguably weaker properties that has not been studied so far in the context of classical shadows. Notably, our cryptographically-inspired analysis shows that for \emph{efficiently computable} observables, it suffices to use \emph{pseudorandom} families of states. To the best of our knowledge, \emph{computational} classical shadow tomography was not considered in the literature prior to our work. Finally, in terms of efficiency, the online part of our method (i.e.\ the part that depends on the input) is simply performing a measurement in the Bell basis, which can be done in constant depth using elementary gates.
Authors: Alejandro Martínez-Méndez, Jesús Moreno-Meseguer, Mariusz Mrózek, Adam Wojciechowski, Priya Balasubramanian, Fedor Jelezko, Javier Prior
In this work we present a comprehensive method of characterization and optimization of laser irradiation within a confocal microscope tailored to quantum sensing experiments using nitrogen-vacancy (NV) centres. While confocal microscopy is well-suited for such experiments, precise control and understanding of several optical parameters are essential for reliable single-emitter studies. We investigate the laser beam intensity profile, single-photon emission statistics, fluorescence response under varying polarization and saturation conditions, spectral characteristics, and the temporal profiles of readout and reinitialization pulses. The beam quality is assessed using the beam propagation factor $M^2$, determined via the razorblade technique. Optical fluorescence spectrum is recorded to confirm NV centre emission. To confirm single-emitter operation, we measure second-order autocorrelation function $g^{(2)}(\tau)$. Saturation behaviour is analysed by varying laser power and recording the corresponding fluorescence, while polarization dependence is studied using a half-wave ($\lambda/2$) plate. Temporal laser pulse profile is examined by modulating the power of an acousto-optic modulator. After optimizing all relevant parameters, we demonstrate the microscope's capabilities in driving spin transitions of a single NV centre. This work establishes a straightforward and effective protocol for laser excitation optimization, enhancing the performance and reliability of NV-based quantum sensors.
Authors: C. Huerta Alderete, Anubhav Kumar Srivastava, Andrew T. Sornborger
We investigate the response of a Ramsey interferometric quantum sensor based on a frustrated, three-spin system (a Kitaev trimer) to a classical time-dependent field (signal). The system eigenspectrum is symmetric about a critical point, $|\vec{b}| = 0$, with four of the spectral components varying approximately linearly with the magnetic field and four exhibiting a nonlinear dependence. Under the adiabatic approximation and for appropriate initial states, we show that the sensor's response to a zero-mean signal is such that below a threshold, $|\vec{b}| < b_\mathrm{th}$, the sensor does not respond to the signal, whereas above the threshold, the sensor acts as a detector that the signal has occurred. This thresholded response is approximately omnidirectional. Moreover, when deployed in an entangled multisensor configuration, the sensor achieves sensitivity at the Heisenberg limit. Such detectors could be useful both as standalone units for signal detection above a noise threshold and in two- or three-dimensional arrays, analogous to a quantum bubble chamber, for applications such as particle track detection and long-baseline telescopy.
Authors: Javier Lopez-Cerezo
This work provides a rigorous and self-contained introduction to numerical methods for Hamiltonian simulation in quantum computing, with a focus on high-order product formulas for efficiently approximating the time evolution of quantum systems. Aimed at students and researchers seeking a clear mathematical treatment, the study begins with the foundational principles of quantum mechanics and quantum computation before presenting the Lie-Trotter product formula and its higher-order generalizations. In particular, Suzuki's recursive method is explored to achieve improved error scaling. Through theoretical analysis and illustrative examples, the advantages and limitations of these techniques are discussed, with an emphasis on their application to $k$-local Hamiltonians and their role in overcoming classical computational bottlenecks. The work concludes with a brief overview of current advances and open challenges in Hamiltonian simulation.
Authors: Shival Dasu, Simon Burton
This work classifies stabilizer codes by the set of diagonal Clifford gates that can be implemented transversally on them. We show that, for any stabilizer code, its group of diagonal transversal Clifford gates on $\ell$ code blocks must be one of six distinct families of matrix groups. We further develop the theory of classifying stabilizer codes by via matrix algebras of endomorphisms first introduced by Rains, and give a complete classification of the diagonal Clifford symmetries of $\ell$ code blocks. A number of corollaries are given in the final section.
Authors: Ariel Edery
Decay rates in quantum field theory (QFT) are typically calculated assuming the particles are represented by momentum eigenstates (i.e. plane waves). However, strictly speaking, localized free particles should be represented by wave packets. This yields width corrections to the decay rate and to the clock behaviour under Lorentz boosts. We calculate the decay rate of a particle of mass $M$ modeled as a Gaussian wavepacket of width $a$ and centered at zero momentum. We find the decay rate to be $\Gamma_0 \big[1- \frac{3 a^2}{4 M^2} +\mathcal{O}\big(\tfrac{a^4}{M^4}\big)\big]$ where $\Gamma_0$ is the decay rate of the particle at rest treated as a plane wave. The leading correction is then of order $\tfrac{a^2}{M^2}$. We then perform a Lorentz boost of velocity $v$ on the above Gaussian and find that its decay rate does not decrease \textit{exactly} by the Lorentz factor $\sqrt{1-v^2}$. There is a correction of order $\tfrac{a^2v^2}{M^2}$. Therefore, the decaying wave packet does not act exactly like a typical clock under Lorentz boosts and we refer to it is a "WP clock" (wave packet clock). A WP clock does not move with a single velocity relative to an observer but has a spread in velocities (more specifically, a spread in momenta). Nonetheless, it is best viewed as a single clock as the wave packet represents a one-particle state in QFT. WP clocks do not violate Lorentz symmetry and are not based on new physics: they are a consequence of the combined requirements of special relativity, quantum mechanics and \textit{localized} free particles.
Authors: Ariel Edery
It is known that perturbative expansions in powers of the coupling in quantum mechanics (QM) and quantum field theory (QFT) are asymptotic series. This can be useful at weak coupling but fails at strong coupling. In this work, we present two types of series expansions valid at strong coupling. We apply the series to a basic integral as well as a QM path integral containing a quadratic and quartic term with coupling constant $\lambda$. The first series is the usual asymptotic one, where the quartic interaction is expanded in powers of $\lambda$. The second series is an expansion of the quadratic part where the interaction is left alone. This yields an absolutely convergent series in inverse powers of $\lambda$ valid at strong coupling. For the basic integral, we revisit the first series and identify what makes it diverge even though the original integral is finite. We fix the problem and obtain, remarkably, a series in powers of the coupling which is absolutely convergent and valid at strong coupling. We explain how this series avoids Dyson's argument on convergence. We then consider the QM path integral (discretized with time interval divided into $N$ equal segments). As before, the second series is absolutely convergent and we obtain analytical expressions in inverse powers of $\lambda$ for the $n$th order terms by taking functional derivatives of generalized hypergeometric functions. The expressions are functions of $N$ and we work them out explicitly up to third order. The general procedure has been implemented in a Mathematica program that generates the expressions at any order $n$. We present numerical results at strong coupling for different values of $N$ starting at $N=2$. The series matches the exact numerical value for a given $N$ (up to a certain accuracy). The continuum is formally reached when $N\to \infty$ but in practice this can be reached at small $N$.
Authors: Carsten Henkel
The sensory perceptions of vision and sound may be considered as complementary doorways towards interpreting and understanding physical phenomena. We provide a few selected samples where scientific data of systems usually not directly accessible to humans may be listened to. The examples are chosen close to the regime where quantum mechanics is applicable. Visual and auditory renderings are compared with some connections to music, illustrating in particular a kind of fractal complexity along the time axis.
Authors: Bek Herz, Ivan Contreras
In this paper, we develop the groundwork for a graph theoretic toy model of supersymmetric quantum mechanics. Using discrete Witten-Morse theory, we demonstrate that finite graphs have a natural supersymmetric structure and use this structure to incorporate supersymmetry into an existing model of graph quantum mechanics. We prove that although key characteristics of continuum supersymmetric systems are preserved on finite unweighted graphs, supersymmetry cannot be spontaneously broken. Finally, we prove new results about the behavior of supersymmetric graph quantum systems under edge rewiring.
Authors: Glenn Wagner, Titus Neupert
Anyons are quasiparticles with fractional charge and statistics that arise in strongly correlated two-dimensional systems such as the fractional quantum Hall (FQH) effect and fractional Chern insulators (FCI). Interactions between anyons can lead to emergent phenomena, such as anyon superconductivity as well as anyon condensation which allows for a hierarchical construction of quantum Hall states. In this work, we study how quasihole anyons in a $\nu=1/3$ Laughlin fractional quantum Hall state can be bound together by a sufficiently strong attractive impurity potential. The competition between the repulsive interaction between the quasiholes themselves and the attractive interaction between the quasiholes and the impurity leads to states with different numbers of quasiholes bound to the impurity. Tuning the chemical potential via gating while remaining within a quantum Hall plateau changes the number of quasiholes bound to the impurity. We propose methods for studying these states experimentally, for example using scanning tunneling microscopy and exciton spectroscopy. While the impurities in traditional platforms such as GaAs heterostructures are typically too weak to observe the binding of anyons, the recently discovered zero-field fractional Chern insulators in twisted MoTe$_2$ offer a platform which may realize the strong-impurity regime.
Authors: Nader Mostaan, Nathan Goldman, Ataç İmamoğlu, Fabian Grusdt
The study of anyons in topologically ordered quantum systems has mainly relied on edge-state interferometry. However, realizing controlled braiding of anyons necessitates the ability to detect and manipulate individual anyons within the bulk. Here, we propose and theoretically investigate a first step toward this goal by demonstrating that a long-lived, optically generated interlayer exciton can bind to a quasihole in a fractional quantum Hall state, forming a composite excitation we term an anyon-trion. Using exact diagonalization, we show that mobile anyon-trions possess a binding energy of approximately 0.5 meV, whereas static anyon-trions exhibit a binding energy of about 0.9 meV, that is linearly proportional to the quasiholes fractional charge. An experimental realization based on photoluminescence from localized interlayer excitons in a quantum twisting microscope setup should allow for a direct optical observation of anyon-trions.
Authors: Thibault Charpentier, Anton Khvalyuk, Lev Ioffe, Mikhail Feigel'man, Nicolas Roch, Benjamin Sacépé
Improving the coherence of superconducting qubits is essential for advancing quantum technologies. While superconductors are theoretically perfect conductors, they consistently exhibit residual energy dissipation when driven by microwave currents, limiting coherence times. Here, we report a universal scaling between microwave dissipation and the superfluid density, a bulk property of superconductors related to charge carrier density and disorder. Our analysis spans a wide range of superconducting materials and device geometries, from highly disordered amorphous films to ultra-clean systems with record-high quality factors, including resonators, 3D cavities, and transmon qubits. This scaling reveals an intrinsic bulk dissipation channel, independent of surface dielectric losses, that originates from a universal density of nonequilibrium quasiparticles trapped within disorder-induced spatial variations of the superconducting gap. Our findings define a fundamental limit to coherence set by intrinsic material properties and provide a predictive framework for selecting materials and the design of next-generation superconducting quantum circuits.
Authors: Sazzad Hossain, Ponkrshnan Thiagarajan, Shashank Pathrudkar, Stephanie Taylor, Abhijeet S. Gangan, Amartya S. Banerjee, Susanta Ghosh
Machine Learning (ML) models for electronic structure rely on large datasets generated through expensive Kohn-Sham Density Functional Theory simulations. This study reveals a surprisingly high level of redundancy in such datasets across various material systems, including molecules, simple metals, and complex alloys. Our findings challenge the prevailing assumption that large, exhaustive datasets are necessary for accurate ML predictions of electronic structure. We demonstrate that even random pruning can substantially reduce dataset size with minimal loss in predictive accuracy, while a state-of-the-art coverage-based pruning strategy retains chemical accuracy and model generalizability using up to 100-fold less data and reducing training time by threefold or more. By contrast, widely used importance-based pruning methods, which eliminate seemingly redundant data, can catastrophically fail at higher pruning factors, possibly due to the significant reduction in data coverage. This heretofore unexplored high degree of redundancy in electronic structure data holds the potential to identify a minimal, essential dataset representative of each material class.
Authors: Fabrizio Tafuri, Carmine Antonio Perroni, Giulio De Filippis
In these notes, we present a rigorous and self-contained introduction to the fundamental concepts and methods of quantum many-body theory. The text is designed to provide a solid theoretical foundation for the study of interacting quantum systems, combining clarity with mathematical precision. Core topics are developed systematically, with detailed derivations and comprehensive proofs that aim to make the material accessible to graduate students and beginning PhD students. Special attention is given to formal consistency and pedagogical structure, so as to guide the reader through both the conceptual and technical aspects of the subject. This work is intended as a reliable starting point for further exploration and research in modern quantum many-body physics.
Authors: Julian Schwingel, Michael Turaev, Johann Kroha, Sayak Ray
We study the spatio-temporal dynamics of interacting bosons on a two-dimensional Hubbard lattice in the strongly interacting regime, taking into account the dynamics of condensate amplitude as well as the direct transport of non-condensed fluctuations. To that end we develop a selfconsistent density-matrix approach which goes beyond the standard Gutzwiller mean-field theory. Starting from the Liouville-von-Neumann equation we derive a quantum master equation for the time evolution of the system's local density matrix at each lattice site, with a dynamical bath that represents the rest of the system. We apply this method to the expansion dynamics of an initially prepared cloud of interacting bosons in an optical lattice. We observe a ballistic expansion of the condensate, as expected, followed by slow, diffusive transport of the normal bosons. We discuss, in particular, the robustness of the Mott insulator phase as well as its melting due to incoherent transport. The method should be applicable to various models of lattice bosons in the strongly correlated regime.
Authors: Konghao Sun, Haiping Hu
Non-Bloch band theory serves as a cornerstone for understanding intriguing non-Hermitian phenomena, such as the skin effect and extreme spectral sensitivity to boundary conditions. Yet this theory hinges on translational symmetry and thus breaks down in disordered systems. Here, we develop a real-space Lyapunov formulation of band theory that governs the spectra and eigenstates of disordered non-Hermitian systems. This framework yields universal non-Hermitian Thouless relations linking spectral density and localization to Lyapunov exponents under different boundary conditions. We further identify an exact topological criterion: skin modes and Anderson-localized modes correspond to nonzero and zero winding numbers, respectively, revealing the topological nature of the skin-Anderson transition. This transition is dictated by an essential Lyapunov exponent and gives rise to novel unidirectional critical states. Our formulation provides a unified and exact description of spectra and localization in generic one-dimensional non-Hermitian systems without translational symmetry, offering new insights into the interplay among non-Hermiticity, disorder, and topology.
Authors: Miloslav Znojil
A family of discrete Schrödinger equations with imaginary potentials $V(x)$ is studied. Inside the domain ${\cal D}$ of unitarity-compatible values of $V(x)$, the reality of all of the bound-state energies survives up to the ``exceptional-point'' (EP) maximally non-Hermitian spectral-degeneracy boundaries $\partial {\cal D}$. The computer-assisted localization of the EP limits is performed showing that the complexity of the task grows quickly with the number $N$ of grid points $x$.
Authors: Carlos Magno O. Pereira, Edilberto O. Silva
We predict a new class of quantum Hall phenomena in completely neutral systems, demonstrating that the interplay between radial electric fields and dipole moments induces exact $e^2/h$ quantization without the need for Landau levels or external magnetic fields. Contrary to conventional wisdom, our theory reveals that: (i) the singularity of line charges does not destroy topological protection, (ii) spin-control of quantization emerges from boundary conditions alone, and (iii) the effect persists up to 25 K, surpassing typical neutral systems. These findings establish electric field engineering as a viable route to topological matter beyond magnetic paradigms.
Authors: Victor Berenstein, Giacomo Valtolina, Zohar Amitay, Nimrod Moiseyev
The gas-phase hydrogen exchange reaction (HER) is the most fundamental chemical process for benchmarking quantum reaction dynamics. In this Letter, we focus on controlling HER by means of strong light-matter coupling inside a resonant cavity, an approach often called polariton chemistry. In particular, we focus on the isotopic variation of HER involving collisions between molecular hydrogen H$_2$ and deuterium atom D, i.e., H$_2$+D$\to$HD+H. We find that the asymmetry introduced by the different isotopes, despite being small, enables strong cavity-induced modifications of reaction rates. Outside of the cavity the reaction is as usual D+H${_2}$$\to$DH+H. However, inside the cavity another type of reactions take place where D+H$_2$$\to$DH+H+E$_{photon}$, where E$_{photon}$=$\hbar\omega_{cav}$. Our results show that HER is an ideal platform to make a significant step toward closing the gap between theory and experiment in polariton chemistry.
Authors: Beau Leighton-Trudel
The possibility that spatial geometry may emerge from the entanglement structure of a quantum many-body system is a subject of fundamental interest. Here, we propose and numerically test a candidate distance metric in 1D, d_E, defined purely from quantum mutual information (I) via the relation d_E = K_0 / sqrt(I). Using large-scale density-matrix renormalization group (DMRG) simulations, we compute this emergent distance for the ground state of the 1D spin-1/2 XXZ chain, a canonical model system. Our simulations show that in the quantum critical phase at anisotropy $\Delta = 1.0$, the mutual information exhibits a power-law decay consistent with the emergence of a valid metric space. In stark contrast, within the gapped, antiferromagnetic phase ($\Delta = 2.0$), where mutual information decays exponentially, the emergent distance grows exponentially, a behavior inconsistent with the triangle inequality. These results provide numerical evidence that this information-theoretic definition can yield a well-behaved geometry in critical systems, offering a quantitative tool for probing quantum phases and motivating further analytical investigation into the foundations of emergent space.
Authors: Soumya Chakraborty, Pooja Sudha, Hemant Arora, Daniel Moraru, Arup Samanta
Multi-donor architecture developed on the base of silicon technology holds significant potential towards room-temperature qubit and other single-electron tunneling (SET) functionalities. However, within such architecture, the overlap of multiple donor wave-functions results in a complex internal electronic configuration with discrete energy levels. Probing these discrete states, observed as multiple conductance peaks, is essential for understanding inter-donor coupling and exchange interactions towards coherent electron transfer. In this direction, we have experimentally demonstrated one-by-one electron filling within multiple-donor molecules with the fundamental analysis of clear and sustained quantum Coulomb blockade (QCB) effect. Moreover, the underlying physics of molecular orbitals, where the increasing energy leads to a larger spatial extent of the corresponding orbital, has been reflected by the systematic decrement of the respective charging-energies. The molecular energy levels, resulting from the orbital hybridization of individual donors, are also confirmed through first-principles simulations using density functional theory (DFT). Furthermore, Monte Carlo simulations based on the orthodox theory of Coulomb blockade support the observed QCB characteristics.
Authors: Jiu Hui Wu, Jiamin Niu, Kejiang Zhou
Based on the general thermodynamic analysis of Polanyi adsorption potential, the adsorption potential condition for superconductors is obtained exactly by using the quantum state equation we presented. Because this adsorption potential results in changes of electron concentration, temperature and pressure in a certain volume (adsorption space) adjacent to the surface of the lattice, the composition and structure of superconductors are of course decisive for the adsorption potential. Then we calculate the molar adsorption potentials for those typical superconductors, and find that it is positively correlated to the superconductivity temperature , which reveals that those high-superconductors are mainly determined by the higher molar adsorption potentials. In addition, the adsorption potential at still works despite the disappearance of the energy gap of the BCS theory. This shows that beyond the electron-phonon interaction mechanism, the Cooper-paired electrons are mainly formed by this physical adsorption potential for high-superconductors. This adsorption potential theory could explain almost all common facts about high-temperature superconductors, including many anomalies of the normal and superconducting states.
Authors: Alexander Fritzsche, Riccardo Sorbello, Ronny Thomale, Alexander Szameit
We explore the effect of self-orthogonality at exceptional points (EPs) in non-Hermitian Parity-Time-symmetric systems. Using a driven three-band lattice model, we show that the Rabi frequency diverges as the system approaches an EP due to the coalescence of eigenstates. We demonstrate that this divergence manifests in experimentally accessible power oscillations, establishing a direct observable for self-orthogonality. Our results provide a pathway for probing EP physics in various metamaterial platforms.
Authors: Koustabh Gogoi, Tanay Nag, Arnob Kumar Ghosh
Periodic drive is an intriguing way of creating topological phases in a non-topological setup. However, most systems are often studied as a closed system, despite being always in contact with the environment, which induces dissipation. Here, we investigate a periodically driven Rashba nanowire in proximity to an $s$-wave superconductor in a dissipative background. The system's dynamics is governed by a periodic Liouvillian operator, from which we construct the Liouvillian time-evolution operator and use the third-quantization method to obtain the `Floquet damping matrix', which captures the spectral and topological properties of the system. We show that the system exhibits edge-localized topological Majorana $0$-modes (MZMs) and $\pi$-modes (MPMs). Additionally, the system also supports a trivial $0$-modes (TZMs) and $\pi$-modes (TPMs), which are also localized at the edges of the system. The MZMs and the MPMs are connected to the bulk topology and carry a bulk topological invariant, while the emergence of TZMs and TPMs is primarily tied to exceptional points and is topologically trivial. We study the topological phase diagrams in terms of the topological invariants and show that the dissipation can modify the topological phase diagram substantially and even induce topological phases in the system. Our work extends the understanding of a driven-dissipative topological superconductor.
Authors: Kristine V. Ung, Connor A. Roncaioli, Ronald L. Walsworth, Sean M. Blakley
The negatively charged diamond nitrogen-vacancy ($\mathrm{{NV}^-}$) center plays a central role in many cutting edge quantum sensing applications; despite this, much is still unknown about the energy levels in this system. The ionization energy of the $\mathrm{^{1}E}$ singlet state in the $\mathrm{{NV}^-}$ has only recently been measured at between 2.25 eV and 2.33 eV. In this work, we further refine this energy by measuring the $\mathrm{^{1}E}$ energy as a function of laser wavelength and diamond temperature via magnetically mediated spin-selective photoluminescence (PL) quenching; this PL quenching indicating at what wavelength ionization induces population transfer from the $\mathrm{^{1}E}$ into the neutral $\mathrm{{NV}^0}$ charge configuration. Measurements are performed for excitation wavelengths between 450 nm and 470 nm and between 540 nm and 566 nm in increments of 2 nm, and for temperatures ranging from about 50 K to 150 K in 5 K increments. We determine the $\mathrm{^{1}E}$ ionization energy to be between 2.29 and 2.33 eV, which provides about a two-fold reduction in uncertainty of this quantity. Distribution level: A. Approved for public release; distribution unlimited.
Authors: Nathaniel Johnston, Sarah Plosker, Charles Torrance, Luis M. B. Varona
We explore and generalize a Cauchy-Schwarz-type inequality originally proved in [Electronic Journal of Linear Algebra 35, 156-180 (2019)]: $\|\mathbf{v}^2\|\|\mathbf{w}^2\| - \langle\mathbf{v}^2,\mathbf{w}^2\rangle \leq \|\mathbf{v}\|^2\|\mathbf{w}\|^2 - \langle\mathbf{v},\mathbf{w}\rangle^2$ for all $\mathbf{v},\mathbf{w} \in \mathbb{R}^n$. We present three new proofs of this inequality that better illustrate "why" it is true and generalize it in several different ways: we generalize from vectors to matrices, we explore which exponents other than 2 result in the inequality holding, and we derive a version of the inequality involving three or more vectors.
Authors: Yuan Wang, Stefano Scali, Oleksandr Kyriienko
Photonic and polaritonic systems offer a fast and efficient platform for accelerating machine learning (ML) through physics-based computing. To gain a computational advantage, however, polaritonic systems must: (1) exploit features that specifically favor nonlinear optical processing; (2) address problems that are computationally hard and depend on these features; (3) integrate photonic processing within broader ML pipelines. In this letter, we propose a polaritonic machine learning approach for solving graph-based data problems. We demonstrate how lattices of condensates can efficiently embed relational and topological information from point cloud datasets. This information is then incorporated into a pattern recognition workflow based on convolutional neural networks (CNNs), leading to significantly improved learning performance compared to physics-agnostic methods. Our extensive benchmarking shows that photonic machine learning achieves over 90\% accuracy for Betti number classification and clique detection tasks - a substantial improvement over the 35\% accuracy of bare CNNs. Our study introduces a distinct way of using photonic systems as fast tools for feature engineering, while building on top of high-performing digital machine learning.
Authors: Florian Lorenz, Jan Robert Schmidt, Roland Ketzmerick
For chaotic scattering systems we investigate the left-right Husimi representation, which combines left and right resonance states. We demonstrate that the left-right Husimi representation is invariant in the semiclassical limit under the corresponding closed classical dynamics, which we call quantum invariance. Furthermore, we show that it factorizes into a classical multifractal structure times universal quantum fluctuations. Numerical results for a dielectric cavity, the three-disk scattering system, and quantum maps confirm both the quantum invariance and the factorization.
Authors: Urjit A. Yajnik
Relativistic quantum mechanics can be considered to have begun with a search for wave equations corresponding to each intrinsic spin. However, relativistic quantum physics differs fundamentally from the non-relativistic wave mechanics. It requires a formalism allowing \ creation and destruction of particles. This gets proper treatment only in a framework called quantum field theory. This article is a semi-historic account of the intriguing new features which emerge as a part of quantum field theory. Such a discussion is impossible without a basic presentation of the formalism itself. Hence some mathematics is included in finer print. The article is directed mostly to those familiar with essential classical mechanics and basic quantum mechanics, though I strive to provide a flavour of the subject to the keenly interested non-physics reader.
Authors: Zhao Zhang
The Hubbard model at $U\to\infty$ has recently been shown to have resonant valence bond (RVB) ground states on the corner-sharing sawtooth and pyrochlore lattices in the dilute doping limit of a single vacancy. In an effort to further generalize those results, I study how the ground state is modified when not all corners are shared between two tetrahedra as in the quasi-1D lattices of a pyrochlore stripe, and how to approach the problem in the case of finite doping. Using a non-Abelian version of the flux inequality, the tetrahedron chain is shown to have degenerate RVB-like ground states. The Bethe ansatz (BA) is adapted to solve the sawtooth chain with spinless or spin-polarized fermions and multiple holons, which is the first example of applying BA to a quasi-1D lattice.
Authors: Sebastian Pucher, Sofus Laguna Kristensen, Ronen M. Kroeze
Strontium-88 is a versatile atomic species often used in quantum optics, precision metrology, and quantum computing. Consolidated atomic data is essential for the planning, execution, and evaluation of experiments. In this reference, we present physical and optical properties of neutral $^{88}$Sr relevant to these applications. Here we focus on experimental results and supplement these with theoretical values. We present equations to convert values and derive important parameters. Tabulated results include key parameters for commonly used transitions in $^{88}$Sr ($^1\mathrm{S}_0 \rightarrow \, ^1\mathrm{P}_1$, $^1\mathrm{S}_0 \rightarrow \, ^3\mathrm{P}_{0,1,2}$, and $^3\mathrm{P}_{0,1,2} \rightarrow \, ^3\mathrm{S}_1$). This dataset serves as an up-to-date reference for studies involving bosonic $^{88}$Sr.
Authors: Kun Fang, Gilad Gour, Xin Wang
Distinguishability is fundamental to information theory and extends naturally to quantum systems. While quantum state discrimination is well understood, quantum channel discrimination remains challenging due to the dynamic nature of channels and the variety of discrimination strategies. This work advances the understanding of quantum channel discrimination and its fundamental limits. We develop new tools for quantum divergences, including sharper bounds on the quantum hypothesis testing relative entropy and additivity results for channel divergences. We establish a quantum Stein's lemma for memoryless channel discrimination, and link the strong converse property to the asymptotic equipartition property and continuity of divergences. Notably, we prove the equivalence of exponentially strong converse properties under coherent and sequential strategies. We further explore the interplay among operational regimes, discrimination strategies, and channel divergences, deriving exponents in various settings and contributing to a unified framework for channel discrimination. Finally, we recast quantum communication tasks as discrimination problems, uncovering deep connections between channel capacities, channel discrimination, and the mathematical structure of channel divergences. These results bridge two core areas of quantum information theory and offer new insights for future exploration.
Authors: Denys I. Bondar, Llorenc Balada Gaggioli, Georgios Korpas, Jakub Marecek, Jiri Vala, Kurt Jacobs
Optimization of constrained quantum control problems powers quantum technologies. This task becomes very difficult when these control problems are non-convex and plagued with dense local extrema. For such problems current optimization methods must be repeated many times to find good solutions, each time requiring many simulations of the system. Here, we present Quantum Control via Polynomial Optimization (QCPOP), a method that eliminates this problem by directly finding globally optimal solutions. The resulting increase in speed, which can be a thousandfold or more, makes it possible to solve problems that were previously intractable. This remarkable advance is due to global optimization methods recently developed for polynomial functions. We demonstrate the power of this method by showing that it obtains an optimal solution in a single run for a problem in which local extrema are so dense that gradient methods require thousands of runs to reach a similar fidelity. Since QCPOP is able to find the global optimum for quantum control, we expect that it will not only enhance the utility of quantum control by making it much easier to find the necessary protocols, but provide a key tool for understanding the precise limits of quantum technologies. Finally, we note that the ability to cast quantum control as polynomial optimization resolves an open question regarding the computability of exact solutions to quantum control problems.
Authors: Iris Paparelle, Faezeh Mousavi, Francesco Scazza, Angelo Bassi, Matteo Paris, Alessandro Zavatta
Quantum secure direct communication (QSDC) is a rapidly developing quantum communication approach, where secure information is directly transmitted, providing an alternative to key-based (de)encryption processes via Quantum Key Distribution (QKD). During the last decade, optical QSDC protocols based on discrete variable encodings have been successfully realized. Recently, continuous-variable (CV) QSDC schemes have been proposed, benefiting from less-sophisticated implementations with proven security. Here, we report the first table-top experimental demonstration of a CV-QSDC system and assess its security. For this realization, we analyze the security of different configurations, including coherent and squeezed sources, with Wyner wiretap channel theory in presence of a beam splitter attack. This practical protocol not only demonstrates the principle of QSDC systems based on CV encoding, but also showcases the advantage of squeezed states over coherent ones in attaining enhanced security and reliable communication in lossy and noisy channels. Our realization, which is founded on mature telecom components, paves the way into future threat-less quantum metropolitan networks, compatible with coexisting advanced wavelength division multiplexing (WDM) systems.
Authors: Jiong Li, Yi-Cheng Wang, Li-Wei Duan, Qing-Hu Chen
The $\mathcal{PT}$-symmetric non-Hermitian quantum Rabi model (QRM) with imaginary coupling is solved using the Bogoliubov operators approach. A transcendental function responsible for the exact solutions is derived, with its zeros yielding the regular spectrum. We find two types of intersections: One is the exceptional point (EP), which is widely studied in the non-Hermitian system; another one is due to doubly degenerate states caused by the conserved QRM parity, which is well-known in the Hermitian QRM. These intersections are identified through this transcendental function. EPs emerge between pairs of adjacent excited energy levels, shifting toward lower coupling strengths as energy levels increase. The fidelity susceptibility diverges to negative infinity at the EPs, consistent with recent findings in non-Hermitian systems, while it diverges to positive infinity at the doubly degenerate points. The EPs are further confirmed by the vanishing c-product in the biorthogonal basis. All eigenstates are characterized by conserved energy and QRM parity. We conclude that the non-Hermitian QRM is integrable, analogous to its Hermitian counterpart.
Authors: Vibhu Mishra, Salvatore Manmana, Stefan Kehrein
The quantum adiabatic theorem is a fundamental result in quantum mechanics, with a multitude of applications, both theoretical and practical. Here, we investigate the dynamics of adiabatic processes for quantum many-body systems %in detail by analysing the properties of observable-free, intensive quantities. In particular, we study the adiabatic rate function $f(T, \Delta \lambda)$ in dependence of the ramp time $T$, which gives us a complete characterization of the many-body adiabatic fidelity as a function of $T$ and the strength of the parameter displacement $\Delta \lambda$. $f(T, \Delta \lambda)$ quantifies the deviation from adiabaticity for a given process and therefore allows us to control and define the notion of adiabaticity in many-body systems. First we study $f(T, \Delta \lambda)$ for the 1D transverse field Ising model and the Luttinger liquid, both of which are quadratic systems and therefore allow us to look at the thermodynamic limit. For ramps across gapped phases, we relate $f(T, \Delta \lambda)$ to the transition probability of the system and for ramps across a gapless point, or gapless phase we relate it to the excitation density of the relevant quasiparticles. Then we investigate the XXZ model which allows us to see the qualitative features that survive when interactions are turned on. Several key results in the literature regarding the interplay of the thermodynamic and the adiabatic limit are obtained as inferences from the properties of $f(T, \Delta \lambda)$ in the large $T$ limit.
Authors: Nouhaila Innan, Alberto Marchisio, Mohamed Bennai, Muhammad Shafique
This study introduces the Quantum Federated Neural Network for Financial Fraud Detection (QFNN-FFD), a cutting-edge framework merging Quantum Machine Learning (QML) and quantum computing with Federated Learning (FL) for financial fraud detection. Using quantum technologies' computational power and the robust data privacy protections offered by FL, QFNN-FFD emerges as a secure and efficient method for identifying fraudulent transactions within the financial sector. Implementing a dual-phase training model across distributed clients enhances data integrity and enables superior performance metrics, achieving precision rates consistently above 95%. Additionally, QFNN-FFD demonstrates exceptional resilience by maintaining an impressive 80% accuracy, highlighting its robustness and readiness for real-world applications. This combination of high performance, security, and robustness against noise positions QFNN-FFD as a transformative advancement in financial technology solutions and establishes it as a new benchmark for privacy-focused fraud detection systems. This framework facilitates the broader adoption of secure, quantum-enhanced financial services and inspires future innovations that could use QML to tackle complex challenges in other areas requiring high confidentiality and accuracy.
Authors: Koki Shiraishi, Masaya Nakagawa, Takashi Mori, Masahito Ueda
The local Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) quantum master equation is a powerful tool for the study of open quantum many-body systems. However, its microscopic derivation applicable to many-body systems is available only in limited cases of weak internal couplings, and it has yet to be fully understood under what microscopic conditions the local GKSL equation is valid. We derive the local GKSL equation on the basis of the Lieb-Robinson bound, which provides an upper bound of the propagation of information in quantum many-body systems. We numerically test the validity of the derived local GKSL equation for a one-dimensional tight-binding fermion chain.
Authors: Javier Robledo-Moreno, Mario Motta, Holger Haas, Ali Javadi-Abhari, Petar Jurcevic, William Kirby, Simon Martiel, Kunal Sharma, Sandeep Sharma, Tomonori Shirakawa, Iskandar Sitdikov, Rong-Yang Sun, Kevin J. Sung, Maika Takita, Minh C. Tran, Seiji Yunoki, Antonio Mezzacapo
A universal quantum computer can simulate diverse quantum systems, with electronic structure for chemistry offering challenging problems for practical use cases around the hundred-qubit mark. While current quantum processors have reached this size, deep circuits and large number of measurements lead to prohibitive runtimes for quantum computers in isolation. Here, we demonstrate the use of classical distributed computing to offload all but an intrinsically quantum component of a workflow for electronic structure simulations. Using a Heron superconducting processor and the supercomputer Fugaku, we simulate the ground-state dissociation of N$_2$ and the [2Fe-2S] and [4Fe-4S] clusters, with circuits up to 77 qubits and 10,570 gates. The proposed algorithm processes quantum samples to produce upper bounds for the ground-state energy and sparse approximations to the ground-state wavefunctions. Our results suggest that, for current error rates, a quantum-centric supercomputing architecture can tackle challenging chemistry problems beyond sizes amenable to exact diagonalization.
Authors: Brian García Sarmina, Guo-Hua Sun, Shi-Hai Dong
This study investigates the efficacy of Stochastic Hill Climbing with Random Restarts (SHC-RR) compared to Local Search (LS) strategies within the Quantum Approximate Optimization Algorithm (QAOA) framework across various problem models. Employing uniform parameter settings, including the number of restarts and SHC steps, we analyze LS with two distinct perturbation operations: multiplication and summation. Our comparative analysis encompasses multiple versions of max-cut and random Ising model (RI) problems, utilizing QAOA models with depths ranging from $1L$ to $3L$. These models incorporate diverse mixing operator configurations, which integrate $RX$ and $RY$ gates, and explore the effects of an entanglement stage within the mixing operator. Our results consistently show that SHC-RR outperforms LS approaches, showcasing superior efficacy despite its ostensibly simpler optimization mechanism. Furthermore, we observe that the inclusion of entanglement stages within mixing operators significantly impacts model performance, either enhancing or diminishing results depending on the specific problem context.
Authors: Tobias Haug, Leandro Aolita, M.S. Kim
Nonstabilizerness or `magic' is a key resource for quantum computing and a necessary condition for quantum advantage. Non-Clifford operations turn stabilizer states into resourceful states, where the amount of nonstabilizerness is quantified by resource measures such as stabilizer Rényi entropies (SREs). Here, we show that SREs saturate their maximum value at a critical number of non-Clifford operations. Close to the critical point SREs show universal behavior. Remarkably, the derivative of the SRE crosses at the same point independent of the number of qubits and can be rescaled onto a single curve. We find that the critical point depends non-trivially on Rényi index $\alpha$. For random Clifford circuits doped with T-gates, the critical T-gate density scales independently of $\alpha$. In contrast, for random Hamiltonian evolution, the critical time scales linearly with qubit number for $\alpha>1$, while is a constant for $\alpha<1$. This highlights that $\alpha$-SREs reveal fundamentally different aspects of nonstabilizerness depending on $\alpha$: $\alpha$-SREs with $\alpha<1$ relate to Clifford simulation complexity, while $\alpha>1$ probe the distance to the closest stabilizer state and approximate state certification cost via Pauli measurements. As technical contributions, we observe that the Pauli spectrum of random evolution can be approximated by two highly concentrated peaks which allows us to compute its SRE. Further, we introduce a class of random evolution that can be expressed as random Clifford circuits and rotations, where we provide its exact SRE. Our results opens up new approaches to characterize the complexity of quantum systems.
Authors: Quoc Hoan Tran, Yasuhiro Endo, Hirotaka Oshima
Quantum machine learning (QML) requires significant quantum resources to address practical real-world problems. When the underlying quantum information exhibits hierarchical structures in the data, limitations persist in training complexity and generalization. Research should prioritize both the efficient design of quantum architectures and the development of learning strategies to optimize resource usage. We propose a framework called quantum curriculum learning (Q-CurL) for quantum data, where the curriculum introduces simpler tasks or data to the learning model before progressing to more challenging ones. Q-CurL exhibits robustness to noise and data limitations, which is particularly relevant for current and near-term noisy intermediate-scale quantum devices. We achieve this through a curriculum design based on quantum data density ratios and a dynamic learning schedule that prioritizes the most informative quantum data. Empirical evidence shows that Q-CurL significantly enhances training convergence and generalization for unitary learning and improves the robustness of quantum phase recognition tasks. Q-CurL is effective with physical learning applications in physics and quantum chemistry.
Authors: Stefano Mangini, Daniel Cavalcanti
Accurately estimating the properties of quantum systems is a central challenge in quantum computing and quantum information. We propose a method to obtain unbiased estimators of multiple observables with low statistical error by post-processing informationally complete measurements using tensor networks. Compared to other observable estimation protocols based on classical shadows and measurement frames, our approach offers several advantages: (i) it can be optimised to provide lower statistical error, resulting in a reduced measurement budget to achieve a specified estimation precision; (ii) it scales to a large number of qubits due to the tensor network structure; (iii) it can be applied to any measurement protocol with measurement operators that have an efficient tensor-network representation. We benchmark the method through various numerical examples, including spin and chemical systems, and show that our method can provide statistical error that are orders of magnitude lower than the ones given by classical shadows.
Authors: Kaiyuan Ji, Hemant K. Mishra, Milán Mosonyi, Mark M. Wilde
Quantum state exclusion is an operational task that has significance in studying foundational questions related to interpreting quantum theory. In such a task, one is given a system whose state is randomly selected from a finite set, and the goal is to identify a state from the set that is not the true state of the system. An error, i.e., an unsuccessful exclusion, occurs if and only if the state identified is the true state. In this paper, we study the optimal error probability of quantum state exclusion and its error exponent -- the rate at which the error probability decays asymptotically -- from an information-theoretic perspective. Our main finding is a single-letter upper bound on the error exponent of state exclusion given by the multivariate log-Euclidean Chernoff divergence, and we prove that this improves upon the best previously known upper bound. We also extend our analysis to the more complicated task of quantum channel exclusion, and we establish a single-letter and efficiently computable upper bound on its error exponent, even assuming the use of adaptive strategies. We derive both upper bounds, for state and channel exclusion, based on one-shot analysis and formulate them as a type of multivariate divergence measure called a barycentric Chernoff divergence. Moreover, our result on channel exclusion has implications in two important special cases. First, for the special case of two hypotheses, our upper bound provides the first known efficiently computable upper bound on the error exponent of symmetric binary channel discrimination. Second, for the special case of classical channels, we show that our upper bound is achievable by a parallel strategy, thus solving the exact error exponent of classical channel exclusion and generalising a similar result on symmetric binary classical channel discrimination.
Authors: Nakshatra Gangopadhay, Sayan Choudhury
Controlling the dynamics of quantum many-body systems is crucial for developing quantum technologies. This work demonstrates that counter-diabatic (CD) driving provides a powerful tool for steering collective spin systems along entangled trajectories for a long time. In particular, CD driving leads to approximate stroboscopic freezing and eternal entanglement oscillations for a large class of initial states in the periodically driven Lipkin-Meshkov-Glick model. Intriguingly, CD driving generates spin squeezing and its associated metrologically useful multipartite entanglement at the mid-point of every drive cycle, when the system is initially prepared in a fully x-polarized state. The CD driving induced non-ergodic dynamics is accompanied by a decrease in the average eigenstate entanglement and inverse participation ratio, thereby signalling greater eigenstate localization. Our work opens a new route to evade Floquet heating and control entanglement generation in collective spin systems.
Authors: Vaisakh Mannalath, Víctor Zapatero, Marcos Curty
The performance of quantum key distribution (QKD) heavily depends on statistical inference. For a broad class of protocols, the central statistical task is a random sampling problem, customarily addressed using a hypergeometric tail bound due to Serfling. Here, we provide an alternative solution for this task of unprecedented tightness among QKD security analyses. As a by-product, confidence intervals for the average of nonidentical Bernoulli parameters follow too. These naturally fit in statistical analyses of decoy-state QKD and also outperform standard tools. Last, we show that, in a vast parameter regime, the use of tail bounds is not enforced because the cumulative mass function of the hypergeometric distribution is accurately computable. This sharply decreases the minimum block sizes necessary for QKD, and reveals the tightness of our analytical bounds when moderate-to-large blocks are considered.
Authors: Takuya Hatomura
Local counterdiabatic driving is a method of improving the performance of adiabatic control and digital implementation of quantum annealing with local counterdiabatic driving has been discussed. In this paper, we propose a decomposition formula which enables us to reduce digitization errors and the number of gate operations in digitized quantum annealing with local counterdiabatic driving.
Authors: Robert E. Thomas, Cole E. Wolfram, Noah B. Warren, Isaac J. Fouch, Boris B. Blinov, Maxwell F. Parsons
We describe an inexpensive and accessible instructional setup that explores particle trapping with a planar linear ion trap. The planar trap is constructed using standard printed circuit board manufacturing and is designed to trap macroscopic charged particles in air. Trapping, shuttling, and splitting are demonstrated to students using these particles, which are visible to the naked eye. Students control trap voltages and can compare properties of particle motion with an analytic model of the trap using a computer vision program for particle tracking. Learning outcomes include understanding the design considerations for planar AC traps, mechanisms underpinning particle ejection, the physics of micromotion, and methods of data analysis using standard computer vision libraries.
Authors: Vaibhav Sharma, Erich J Mueller
We develop a method for visualizing the internal structure of multipartite entanglement in pure stabilizer states. Our algorithm graphically organizes the many-body correlations in a hierarchical structure. This provides a rich taxonomy from which one can simultaneously extract many quantitative features of a state including some traditional quantities such as entanglement depth, k-uniformity and entanglement entropy. Our method also presents an alternative computational tool for extracting the exact entanglement depth and all separable partitions of a stabilizer state. Our construction is gauge invariant and goes beyond traditional entanglement measures by visually revealing how quantum information and entanglement is distributed. We use this tool to analyze the internal structures of prototypical stabilizer states (GHZ state, cluster state, stabilizer error correction codes) and are able to contrast the complexity of highly entangled volume law states generated by random unitary operators and random projective measurements.
Authors: Krzysztof Urbanowski
Analyzing Heisenberg--Robertson (HR) and Schrödinger uncertainty relations we found, that there can exist a large set of states of the quantum system under considerations, for which the lower bound of the product of the standard deviations of a pair of non--commuting observables, $A$ and $B$, is zero, and which differ from those described in the literature. These states are not eigenstates of either the observable $A$ or $B$. The correlation function for these observables in such states is equal to zero. We have also shown that the so--called "sum uncertainty relations" also do not provide any information about lower bounds on the standard deviations calculated for these states. We additionally show that the uncertainty principle in its most general form has two faces: one is that it is a lower bound on the product of standard deviations, and the other is that the product of standard deviations is an upper bound on the modulus of the correlation function of a pair of the non--commuting observables in the state under consideration.
Authors: Mircea Bejan, Benjamin Béri, Max McGinley
We study the ensemble of states generated by performing projective measurements on the output of a random matchgate (or free-fermionic) quantum circuit. We rigorously show that this `projected ensemble' exhibits deep thermalization: For large system sizes, it converges towards a universal ensemble that is uniform over the manifold of Gaussian fermionic states. As well as proving moment-wise convergence of these ensembles, we demonstrate that the full distribution of any physical observable in the projected ensemble is close to its universal form in Wasserstein-1 distance, which we argue is an appropriate and efficiently computable measure of convergence when studying deep thermalization. Using this metric, we also numerically find that local matchgate circuits deeply thermalize after a timescale $t \sim L^2$ set by the diffusive spreading of quantum information. Our work opens up new avenues to experimentally accessible protocols to probe the emergence of quantum statistical mechanics and benchmark quantum simulators.
Authors: Nouhaila Innan, Alberto Marchisio, Mohamed Bennai, Muhammad Shafique
Predicting loan eligibility with high accuracy remains a significant challenge in the finance sector. Accurate predictions enable financial institutions to make informed decisions, mitigate risks, and effectively adapt services to meet customer needs. However, the complexity and the high-dimensional nature of financial data have always posed significant challenges to achieving this level of precision. To overcome these issues, we propose a novel approach that employs Quantum Machine Learning (QML) for Loan Eligibility Prediction using Quantum Neural Networks (LEP-QNN). Our innovative approach achieves an accuracy of 98% in predicting loan eligibility from a single, comprehensive dataset. This performance boost is attributed to the strategic implementation of a dropout mechanism within the quantum circuit, aimed at minimizing overfitting and thereby improving the model's predictive reliability. In addition, our exploration of various optimizers leads to identifying the most efficient setup for our LEP-QNN framework, optimizing its performance. We also rigorously evaluate the resilience of LEP-QNN under different quantum noise scenarios, ensuring its robustness and dependability for quantum computing environments. This research showcases the potential of QML in financial predictions and establishes a foundational guide for advancing QML technologies, marking a step towards developing advanced, quantum-driven financial decision-making tools.
Authors: Yaoling Yang, Victor Montenegro, Abolfazl Bayat
The simultaneous estimation of multiple unknown parameters is the most general scenario in quantum sensing. Quantum multi-parameter estimation theory provides fundamental bounds on the achievable precision of simultaneous estimation. However, these bounds can become singular (no finite bound exists) in multi-parameter sensing due to parameter interdependencies, limited probe accessibility, and insufficient measurement outcomes. Here, we address the singularity issue in quantum sensing through a simple mechanism based on a sequential measurement strategy. This sensing scheme overcomes the singularity constraint and enables the simultaneous estimation of multiple parameters with a local and fixed measurement throughout the sensing protocol. This is because sequential measurements, involving consecutive steps of local measurements followed by probe evolution, inherently produce correlated measurement data that grows exponentially with the number of sequential measurements. Finally, through two different examples, namely a strongly correlated probe and a light-matter system, we demonstrate how such singularities are reflected when inferring the unknown parameters through Bayesian estimation.
Authors: Wen-Zheng Ling, Zhao-Fan Cai, Tao Liu
In contrast to the conventional (first-order) non-Hermitian skin effect (NHSE) in a $d$-dimensional system with linear size $L$, the $n$th-order (higher-order) NHSE is characterized by skin modes localized at lower-dimensional boundaries of dimension $(d-n)$. The total number of these modes scales linearly with the system size $L$. Significant progress has been made in understanding higher-order NHSE in non-interacting systems. In this work, we demonstrate the many-body interaction induced second-order skin effect in a two-dimensional non-Hermitian bosonic system. Specifically, we construct a square lattice that incorporates nonreciprocal single-boson hopping, onsite many-body interactions and two-boson pairing hopping. In the absence of interactions, no second-order NHSE is observed. However, with the inclusion of interactions, we identify interaction-induced skin modes for in-gap doublon states (i.e., bound pairs of bosons) localized at the corners of the lattice, while the bulk doublon states remain extended. These corner-localized skin modes arise from the interplay between interaction-induced edge states, localized along one-dimensional boundaries, and the nonreciprocal hopping along these boundaries. Furthermore, the number of corner skin modes scales linearly with the system size, confirming the presence of second-order NHSE in this interacting system. Our findings introduce a novel approach to realizing higher-order skin effects by leveraging interactions.
Authors: Christoph L. Bock, J. C. Rivera Hernández, Fabio Lingua, David B. Haviland
We investigate nonreciprocal scattering within the modes of a microwave frequency comb. Adjusting the pump frequencies, amplitudes, and phases of a Josephson parametric oscillator, we control constructive interference for the $m \longrightarrow \ell$ scattering processes, while concurrently achieving destructive interference for the inverse process $\ell \longrightarrow m$. We outline the methodology for realizing nonreciprocity in the context of two-mode isolation and a three-mode circulation, which we extend to multiple modes. We find good agreement between the experiments and a linearized theoretical model. Nonreciprocal scattering expands the toolset for parametric control, with the potential to engineer alternative quantum correlations.
Authors: Hyunho Cha, Jungwoo Lee
As quantum computers require highly specialized and stable environments to operate, expanding their capabilities within a single system presents significant technical challenges. By interconnecting multiple quantum processors, distributed quantum computing can facilitate the execution of more complex and larger-scale quantum algorithms. End-to-end heuristics for the distribution of quantum circuits have been developed so far. In this work, we derive an exact integer programming approach for the Distributed Quantum Circuit (DQC) problem, assuming fixed module allocations. Since every DQC algorithm necessarily yields a module allocation function, our formulation can be integrated with it as a post-processing step. This improves on the hypergraph partitioning formulation, which finds a module allocation function and an efficient distribution at once. We also show that a suboptimal heuristic to find good allocations can outperform previous methods. In particular, for quantum Fourier transform circuits, we conjecture from experiments that the optimal module allocation is the trivial one found by this method.
Authors: Cong-Gang Song, Qing-yu Cai
Precision and accuracy, as two crucial criteria for quantum metrology, have previously lacked rigorous definitions and distinctions. In this paper, we provide a unified definition of precision and accuracy from the perspective of distinguishing neighboring quantum states. Using the quantum Cramér-Rao bound as a lower bound for precision, we find that the corresponding accuracy will fall short of expectations, because the bias of the parameter estimation cannot be ignored. Given that probability estimation is unbiased, defining precision from the perspective of probability distributions provides a more comprehensive approach. This leads to a correction of the traditional precision lower bound by a factor of 2. The trade-off between precision and accuracy shows that precision can be further improved by sacrificing accuracy, while it should be restricted by inherent precision limit. The inherent precision limit, determined by the number of sampling, can reach the Heisenberg scaling even without entanglement resources, which, however, comes at the cost of significantly reduced accuracy. We show that accuracy may actually decrease with increasing sampling when one pursues excessive precision, which indicates the trade-off should be considered even with unlimited resources.
Authors: Choon-Lin Ho
Kramers escape from a metastable state in the presence of both thermal and quantum fluctuations under strong damping is treated as a thermally activated process in a quantum modified semiclassical potential. Dirac's time-dependent variational method together with the Jackiw-Kerman function is employed to derive the semiclassical potential. Quantum correction is incorporated in the drift potential, and is determined by quasi-stationary conditions and minimal uncertainty relation. The semiclassical rate obtained here is consistent in form with those from the quantum Smoluchowski equations deduced heuristically by modifying the diffusion coefficient using the path-integral method. Unlike approaches using the path-integral, which involves continuation into imaginary time, the approach here is simpler and more easily understood in terms of classical picture.
Authors: Gaurang Agrawal, Pritam Halder, Aditi Sen De
We explore the performance of the metrology scheme by employing a quantum time flip during encoding, a specific case of processes with indefinite time direction, which we refer to as indefinite time directed metrology (ITDM). In the case of single parameter estimation of a unitary, we demonstrate that our protocol can achieve Heisenberg scaling (1/N) with product probe states, surpassing the standard quantum limit (1/\sqrt{N}), where N is the number of particles in the probe. We establish this by computing the quantum Fisher information (QFI) which is a lower bound on the root mean square error occurred during parameter estimation. Although we analytically prove the optimality of the symmetric product probe state in ITDM, entangled probe states produce a higher QFI than optimal product probes without enhancing scaling, highlighting the non-essentiality of entanglement. For phase estimation, we propose a single-qubit measurement on the control qubit that accomplishes near-optimal Fisher information and eventually reaches Heisenberg scaling. Our findings reveal the best orientation of product probe states in every pertinent situation, emphasizing its independence from the parameter to be estimated in the limiting case. Furthermore, we illustrate the benefits of ITDM in noisy metrology, outperforming existing techniques in some situations.
Authors: Evan Sutcliffe, Alejandra Beghelli
We consider the problem of distributing entangled multipartite states across a quantum network with improved distribution rate and fidelity. For this, we propose fidelity-aware multi-path routing protocols, assess their performance in terms of the rate and fidelity of the distributed Greenberger-Horne-Zeilinger (GHZ) states, and compare such performance against that of single-path routing. Simulation results show that the proposed multi-path routing protocols select routes that require more Bell states compared to single-path routing, but also require fewer rounds of Bell state generation. We also optimised the trade-off between distribution rate and fidelity by selecting an appropriate cutoff to the quantum memory storage time. Using such a cutoff technique, the proposed multi-path protocols can achieve up to an 8.3 times higher distribution rate and up to a 28% improvement in GHZ state fidelity compared to single-path routing. These results show that multi-path routing both improves the distribution rates and enhances fidelity for multipartite state distribution.
Authors: Robert Leonard, Spencer E. Olson
A lattice beam configuration which results in an isotropic 3D trap near the surface of an atom chip is described. The lattice is formed near the surface of a reflectively coated atom chip, where three incident beams and three reflected beams intersect. The coherent interference of these six beams form a phase-stable optical lattice which extends to the surface of the atom chip. The lattice is experimentally realized and the trap frequency is measured. Degenerate Raman sideband cooling is performed in the optical lattice, cooling 80 million atoms to 1.1 $\mu$K.
Authors: S. B. Kožić, G. Torre, K. Delić, F. Franchini, S. M. Giampaolo
The study of entanglement and magic properties in topologically frustrated systems suggests that, in the thermodynamic limit, these quantities decompose into two distinct contributions. One is determined by the specific nature of the model and its Hamiltonian, and another arises from topological frustration itself, resulting in being independent of the Hamiltonian's parameters. In this work, we test the generality of this picture by investigating an additional quantum resource, namely quantum coherence, in two different models where topological frustration is induced through an appropriate choice of boundary conditions. Our findings reveal a perfect analogy between the behavior of quantum coherence and that of other quantum resources, particularly magic, providing further evidence in support of the universality of this picture and the topological nature of this source of frustration.
Authors: Jakob Murauer, Rajiv Krishnakumar, Sabine Tornow, Michaela Geierhos
Existing approaches to quantum reservoir computing can be broadly categorized into restart-based and continuous protocols. Restart-based methods require reinitializing the quantum circuit for each time step, while continuous protocols use mid-circuit measurements to enable uninterrupted information processing. A gap exists between these two paradigms: while restart-based methods naturally have high execution times due to the need for circuit reinitialization, they can employ novel feedback connections to enhance performance. In contrast, continuous methods have significantly faster execution times but typically lack such feedback mechanisms. In this work, we investigate a novel quantum reservoir computing scheme that integrates feedback connections, which can operate within the coherence time of a qubit. We demonstrate our architecture using a minimal example and evaluate memory capacity and predictive capabilities. We show that the correlation coefficient for the short-term memory task on past inputs is nonzero, indicating that feedback connections can effectively operate during continuous processing to allow the model to remember past inputs.
Authors: Zhaohui Yang, Dawei Ding, Chenghong Zhu, Jianxin Chen, Yuan Xie
Variational quantum algorithms (VQA) based on Hamiltonian simulation represent a specialized class of quantum programs well-suited for near-term quantum computing applications due to its modest resource requirements in terms of qubits and circuit depth. Unlike the conventional single-qubit (1Q) and two-qubit (2Q) gate sequence representation, Hamiltonian simulation programs are essentially composed of disciplined subroutines known as Pauli exponentiations (Pauli strings with coefficients) that are variably arranged. To capitalize on these distinct program features, this study introduces PHOENIX, a highly effective compilation framework that primarily operates at the high-level Pauli-based intermediate representation (IR) for generic Hamiltonian simulation programs. PHOENIX exploits global program optimization opportunities to the greatest extent, compared to existing SOTA methods despite some of them also utilizing similar IRs. Experimental results demonstrate that PHOENIX outperforms SOTA VQA compilers across diverse program categories, backend ISAs, and hardware topologies.
Authors: Archisman Ghosh, Avimita Chatterjee, Swaroop Ghosh
Quantum Error Correction (QEC) is the cornerstone of practical Fault-Tolerant Quantum Computing (FTQC), but incurs enormous resource overheads. Circuits must decompose into Clifford+T gates, and the non-transversal T gates demand costly magic-state distillation. As circuit complexity grows, sequential T-gate layers ("T-depth") increase, amplifying the spatiotemporal overhead of QEC. Optimizing T-depth is NP-hard, and existing greedy or brute-force strategies are either inefficient or computationally prohibitive. We frame T-depth reduction as a search optimization problem and present a Genetic Algorithm (GA) framework that approximates optimal layer-merge patterns across the non-convex search space. We introduce a mathematical formulation of the circuit expansion for systematic layer reordering and a greedy initial merge-pair selection, accelerating the convergence and enhancing the solution quality. In our benchmark with ~90-100 qubits, our method reduces T-depth by 79.23% and overall T-count by 41.86%. Compared to the reversible circuit benchmarks, we achieve a 2.58x improvement in T-depth over the state-of-the-art methods, demonstrating its viability for near-term FTQC.
Authors: Massimo Giovannini
Within an inherently classical perspective, there is always an unavoidable energy cost associated with the information deletion and this common lore is at the heart of the Landauer's conjecture that does not impose, per se, any relevant limit on the information acquisition. Although such a mindset should generally apply to systems of any size, its quantum mechanical implications are particularly intriguing and, for this reason, we examine here a minimal physical structure where the system and the environment are described, respectively, by a pair of quantum oscillators coupled by an appropriate Hermitian interaction able to amplify the entropy of the initial state. Since at the onset of the dynamical evolution the system is originally in a pure state, its entropy variation is always positive semidefinite and the Landauer's conjecture should not impose any constraint. Nonetheless, provided the quantum amplification is effective, it turns out that the entropy variation of the system always undershoots the heat transferred to the environment. When the initial thermal state of the environment is characterized by a chemical potential, the entropy growth is bounded both by the particles and by the heat flowing to the environment. The limits deduced in the quantum thermodynamical framework are also scrutinized from a field theory standpoint where species of different spins are copiously produced (especially in a cosmological context) thanks to the rapid variation of the space-time curvature.
Authors: Hsin-Yi Lin, Huan-Hsin Tseng, Samuel Yen-Chi Chen, Shinjae Yoo
Conventional Variational Quantum Circuits (VQCs) for Quantum Machine Learning typically rely on a fixed Hermitian observable, often built from Pauli operators. Inspired by the Heisenberg picture, we propose an adaptive non-local measurement framework that substantially increases the model complexity of the quantum circuits. Our introduction of dynamical Hermitian observables with evolving parameters shows that optimizing VQC rotations corresponds to tracing a trajectory in the observable space. This viewpoint reveals that standard VQCs are merely a special case of the Heisenberg representation. Furthermore, we show that properly incorporating variational rotations with non-local observables enhances qubit interaction and information mixture, admitting flexible circuit designs. Two non-local measurement schemes are introduced, and numerical simulations on classification tasks confirm that our approach outperforms conventional VQCs, yielding a more powerful and resource-efficient approach as a Quantum Neural Network.
Authors: Heyang Peng, Seid Koudia, Leonardo Oleynik, Symeon Chatzinotas
Measurement-device-independent quantum key distribution (MDI-QKD), enhances quantum cryptography by mitigating detector-side vulnerabilities. This study analyzes MDI-QKD performance in thermal-loss and phase noise channels, modeled as depolarizing and dephasing channels to capture thermal and phase noise effects. Based on this channel framework, we derive analytical expressions for Bell state measurement probabilities, quantum bit error rates (QBER), and secret key rates (SKR) of MDI-QKD. Our simulations reveal that SKR decreases exponentially with transmission distance, with performance further degraded by increasing thermal noise and phase noise, particularly under high thermal noise conditions. These findings offer insights into enhancing MDI-QKD's noise resilience, supporting secure key generation in practical, noisy environments.
Authors: Boris Ragula, Eduardo Martín-Martínez
Quantum energy teleportation (QET) exploits the existence of correlations to enable remote energy transfer without the need for physical energy carriers between emitter and receiver. This paper presents a review of the thermodynamic foundations of QET and reviews its first experimental demonstration (performed using Nuclear Magnetic Resonance), along with its implementation on publicly available superconducting quantum hardware. Additionally, we review an application of QET in the field of quantum thermodynamics as an efficient algorithmic cooling technique to cool down individual parts of interacting systems. Finally, we will review how QET can be employed to optimally generate exotic quantum states characterized by negative average stress-energy densities, offering a new operational approach to engineering such states which are promising in the context of semiclassical gravity.
Authors: Erik L. Connerty, Ethan N. Evans, Gerasimos Angelatos, Vignesh Narayanan
Recent advances in artificial intelligence have highlighted the remarkable capabilities of neural network (NN)-powered systems on classical computers. However, these systems face significant computational challenges that limit scalability and efficiency. Quantum computers hold the potential to overcome these limitations and increase processing power beyond classical systems. Despite this, integrating quantum computing with NNs remains largely unrealized due to challenges posed by noise, decoherence, and high error rates in current quantum hardware. Here, we propose a novel quantum echo-state network (QESN) design and implementation algorithm that can operate within the presence of noise on current IBM hardware. We apply classical control-theoretic response analysis to characterize the QESN, emphasizing its rich nonlinear dynamics and memory, as well as its ability to be fine-tuned with sparsity and re-uploading blocks. We validate our approach through a comprehensive demonstration of QESNs functioning as quantum observers, applied in both high-fidelity simulations and hardware experiments utilizing data from a prototypical chaotic Lorenz system. Our results show that the QESN can predict long time-series with persistent memory, running over 100 times longer than the median T1 and T2 of the IBM Marrakesh QPU, achieving state-of-the-art time-series performance on superconducting hardware.
Authors: Adithi Udupa, Timo Hillmann, Rabsan Galib Ahmed, Andrea Smirne, Giulia Ferrini
Decoherence in quantum devices, such as qubits and resonators, is often caused by bistable fluctuators modeled as random telegraph noise (RTN), leading to significant dephasing. We analyze the impact of individual and multiple fluctuators on a bosonic mode in continuous variable systems, identifying non-Markovian behavior governed by two timescales: the fluctuator switching rate ($\xi$) and coupling strength ($\nu$). Using the Breuer-Piilo-Laine (BLP) measure, we show that for Gaussian states, squeezing and thermal fluctuations do not enhance non-Markovianity. In contrast, for non-Gaussian states, the measure becomes unbounded. For rotation-symmetric bosonic (RSB) codes, known for their error correction advantages, non-Markovianity grows linearly with code symmetry. We evaluate the performance of RSB codes under simultaneous loss and RTN dephasing. For a teleportation-based Knill error-correction circuit, the codes perform robustly in the Markovian limit. In the non-Markovian regime, the performance depends on the time the error correction is performed for a given codeword. The average gate fidelity of the error-corrected state in this case exhibits oscillations as a function of time due to the oscillatory nature of the dephasing function of the RTN noise; however, for most of the parameter ranges, the values stay above the break-even point. Extending to multiple fluctuators that produce $1/f$ noise, we observe that non-Markovianity decays with increasing fluctuator count, while the performance of RSB codes remains effective with increasing number of fluctuators.
Authors: Naga Dileep Varikuti, Soumik Bandyopadhyay, Philipp Hauke
Non-stabilizerness, alongside entanglement, is a crucial ingredient for fault-tolerant quantum computation and achieving a genuine quantum advantage. Despite recent progress, a complete understanding of the generation and thermalization of non-stabilizerness in circuits that mix Clifford and non-Clifford operations remains elusive. While Clifford operations do not generate non-stabilizerness, their interplay with non-Clifford gates can strongly impact the overall non-stabilizing dynamics of generic quantum circuits. In this work, we establish a direct relationship between the final non-stabilizing power and the individual powers of the non-Clifford gates, in circuits where these gates are interspersed with random Clifford operations. By leveraging this result, we unveil the thermalization of non-stabilizing power to its Haar-averaged value in generic circuits. As a precursor, we analyze two-qubit gates and illustrate this thermalization in analytically tractable systems. Extending this, we explore the operator-space non-stabilizing power and demonstrate its behavior in physical models. Finally, we examine the role of non-stabilizing power in the emergence of quantum chaos in brick-wall quantum circuits. Our work elucidates how non-stabilizing dynamics evolve and thermalize in quantum circuits and thus contributes to a better understanding of quantum computational resources and of their role in quantum chaos.
Authors: Tianqi Zheng, Yi Li, Yu Xiang, Qiongyi He
Certifying maximal quantum randomness without assumptions about system dimension remains a pivotal challenge for secure communication and foundational studies. Here, we introduce a generalized framework to directly certify maximal randomness from observed probability distributions across systems with arbitrary user numbers, without relying on the Bell-inequality violations. By analyzing probability distributions directly, we identify a class of quantum states and projective measurements that achieve maximal randomness in bipartite and tripartite scenarios, ensuring practical feasibility. Further analysis reveals a counterintuitive trade-off governing measurement incompatibility among users: sufficient incompatibility for one user permits arbitrarily small incompatibility for others, defying conventional symmetry assumptions in the Bell test. This asymmetry provides a pathway to optimize device-independent protocols by strategically distributing quantum resources. Our results establish a versatile and experimentally accessible route to scalable randomness certification, with implications for quantum cryptography and the physics of nonlocal correlations.
Authors: Jiong Li, Qing-Hu Chen
We investigate the spectral properties and quantum criticality of the two-photon Rabi-Stark model. Using the exact solution of this model, we rigorously derive a condition for complete spectral collapse, where all bound states vanish. In this case, the energy gap closes at a critical coupling, signaling a continuous quantum phase transition. The corresponding gap exponent differs from those in both the one-photon Rabi-Stark model and the quantum Rabi model, suggesting a distinct universality class. While in the general case, an infinite number of discrete bound states exist when spectral collapse occur and the energy gap remains open. By mapping to an inverse square potential well, these bound levels approach the threshold energy exponentially. Our results offer new insights into novel spectral phenomena in nonlinear quantum Rabi models, with potential implications for experimental realizations in circuit QED and trapped ion systems.
Authors: Yahui Chai, Yibin Guo, Stefan Kühn
Scattering processes are fundamental for understanding the structure of matter, yet simulating their real-time dynamics remains challenging for classical computers. Quantum computing and quantum-inspired methods offer a promising avenue for efficiently simulating such phenomena. In this work, we investigate meson scattering in a (1+1)-dimensional Z2 lattice gauge theory with staggered fermions. We develop a quantum subspace expansion technique to construct high-fidelity meson creation operators across a broad range of masses and momenta. Using Tensor Networks simulations, we study both elastic and inelastic scattering and provide a detailed analysis of energy transfer, entanglement entropy, and new particle production during the dynamics. In addition, we design an efficient quantum circuit for meson wave packet preparation using Givens rotations, significantly reducing the circuit depth compared to existing methods. Our work provides a non-variational and scalable framework for simulating meson scattering on near-term quantum devices, and provides a concrete strategy for quantum simulation to analyze non-perturbative dynamical processes in confining gauge theories.
Authors: Kausthubh Chandramouli, Kelly Mae Allen, Christopher Mori, Dror Baron, Mário A. T. Figueiredo
In the noisy intermediate-scale quantum (NISQ) era, quantum error mitigation (QEM) is essential for producing reliable outputs from quantum circuits. We present a statistical signal processing approach to QEM that estimates the most likely noiseless outputs from noisy quantum measurements. Our model assumes that circuit depth is sufficient for depolarizing noise, producing corrupted observations that resemble a uniform distribution alongside classical bit-flip errors from readout. Our method consists of two steps: a filtering stage that discards uninformative depolarizing noise and an expectation-maximization (EM) algorithm that computes a maximum likelihood (ML) estimate over the remaining data. We demonstrate the effectiveness of this approach on small-qubit systems using IBM circuit simulations in Qiskit and compare its performance to contemporary statistical QEM techniques. We also show that our method scales to larger qubit counts using synthetically generated data consistent with our noise model. These results suggest that principled statistical methods can offer scalable and interpretable solutions for quantum error mitigation in realistic NISQ settings.
Authors: Beatrice Magni, Xhek Turkeshi
We investigate the emergence of quantum complexity and chaos in doped Clifford circuits acting on qudits of odd prime dimension $d$. Using doped Clifford Weingarten calculus and a replica tensor network formalism, we derive exact results and perform large-scale simulations in regimes challenging for tensor network and Pauli-based methods. We begin by analyzing generalized stabilizer entropies, computable magic monotones in many-qudit systems, and identify a dynamical phase transition in the doping rate, marking the breakdown of classical simulability and the onset of Haar-random behavior. The critical behavior is governed by the qudit dimension and the magic content of the non-Clifford gate. Using the qudit $T$-gate as a benchmark, we show that higher-dimensional qudits converge faster to Haar-typical stabilizer entropies. For qutrits ($d=3$), analytical predictions match numerics on brickwork circuits, showing that locality plays a limited role in magic spreading. We also examine anticoncentration and entanglement growth, showing that $O(\log N)$ non-Clifford gates suffice for approximating Haar expectation values to precision $\varepsilon$, and relate antiflatness measures to stabilizer entropies in qutrit systems. Finally, we analyze out-of-time-order correlators and show that a finite density of non-Clifford gates is needed to induce chaos, with a sharp transition fixed by the local dimension, twice that of the magic transition. Altogether, these results establish a unified framework for diagnosing complexity in doped Clifford circuits and deepen our understanding of resource theories in multiqudit systems.
Authors: Konstantinos Papatryfonos, Louis Vervoort
We propose two quantum experiments - modified Bell tests - that could detect contextual hidden variables underlying quantum mechanics. The experiments are inspired by hydrodynamic pilot-wave systems that mimic a wide range of quantum effects and exhibit a classical analog of contextuality. To justify the experiments, we show that contextual hidden variables are 'physics as usual' if a unification between quantum mechanics and general relativity is possible. Accordingly, contextual theories can bypass Bell's theorem in a way that is both local and non-conspiratorial. We end with a note on the relevance of exploratory experiments in the foundations of quantum physics.
Authors: R. N. Clark, B. Puzio, O. M. Green, S. T. Pradyumna, O. Trojak, A. Politi, J. C. F. Matthews
Quantum technologies promise profound advances in communication security, sensing and computing. The underpinning hardware must be engineered to generate, manipulate and detect quantum phenomena with exceptional performance, whilst being mass-manufacturable for real-world applications. A leading approach is chip-scale quantum photonics. The continuous-variable regime for quantum optics has been exploited in a number of technologies, including the detection of gravitational waves, by operating below the standard quantum limit of the light's shot noise. The availability of room-temperature, deterministic sources and high efficiency detectors suitable for continuous-variable state generation and measurement is a compelling motivation for this particular paradigm. This review focusses on efforts to integrate sources and detectors of continuous-variable light states into chip-scale photonic integrated circuits.
Authors: Roman Sahakyan, Romik Sargsyan, Edgar Pogosyan, Karen Arzumanyan, Emil A. Gazazyan
A gradient-based optimization approach combined with automatic differentiation is employed to ensure high accuracy and scalability when working with high-dimensional parameter spaces. Numerical simulations confirm the effectiveness of the proposed method: the population is reliably transferred to the target state with minimal occupation of intermediate levels, while the control pulses remain smooth and physically implementable. The developed framework serves as a universal and experimentally applicable tool for automated control pulse design in quantum systems. It is particularly useful in scenarios where analytical methods or manual parameter tuning--such as standard schemes like STIRAP--prove to be inefficient or inapplicable.
Authors: Senrui Chen, Akel Hashim, Noah Goss, Alireza Seif, Irfan Siddiqi, Liang Jiang
Noise is a major challenge for building practical quantum computing systems. Precise characterization of quantum noise is crucial for developing effective error mitigation and correction schemes. However, state preparation and measurement (SPAM) errors on many current platforms can introduce large ambiguity into conventional noise characterization methods. In this work, we propose a scheme for enhancing quantum noise characterization using additional energy levels. We first develop a comprehensive theory on the identifiability of n-qudit SPAM noise given high-quality single-qudit control, showing the existence of gauge freedoms which can be completely described using subsystem depolarizing maps. We then show how to use these extra energy levels to reduce the gauge ambiguity in characterizing both SPAM and gate noise in the qubit subspace. We experimentally implement these ideas on a superconducting quantum computing device and demonstrate a qutrit-enabled enhancement in noise characterization precision.
Authors: Łukasz Grzelka, Łukasz Skowronek, Karol Życzkowski
We present an effective set of necessary and sufficient criteria for block-positivity of matrices of order $4$ over $\mathbb{C}$. The approach is based on Sturm sequences and quartic polynomial positivity conditions presented in recent literature. The procedure allows us to test whether a given $4\times 4$ complex matrix corresponds to an entanglement witness, and it is exact when the matrix coefficients belong to the rationals, extended by $\mathrm{i}$. The method can be generalized to $\mathcal{H}_2\otimes\mathcal{H}_d$ systems for $d>2$ to provide necessary but not sufficient criterion for block-positivity. We also outline an alternative approach to the problem relying on Gröbner bases.
Authors: Carlo Marconi, Guillem Müller-Rigat, Jordi Romero-Pallejà, Jordi Tura, Anna Sanpera
Symmetric quantum states are fascinating objects. They correspond to multipartite systems that remain invariant under particle permutations. This symmetry is reflected in their compact mathematical characterisation but also in their unique physical properties: they exhibit genuine multipartite entanglement and notable robustness against noise and perturbations. These features make such states particularly well-suited for a wide range of quantum information tasks. Here, we provide a pedagogic analysis of the mathematical structure and relevant physical properties of this class of states. Beyond the theoretical framework, robust tools for certifying and verifying the properties of symmetric states in experimental settings are essential. In this regard, we explore how standard techniques -- such as quantum state tomography, Bell tests, and entanglement witnesses -- can be specifically adapted for symmetric systems. Next, we provide an up-to-date overview of the most relevant applications in which these states outperform other classes of states in specific tasks. Specifically, we address their central role in quantum metrology, highlight their use in quantum error correction codes, and examine their contribution in computation and communication tasks. Finally, we present the current state-of-the-art in their experimental generation, ranging from systems of cold atoms to implementations via quantum algorithms. We also review the most significant results obtained in the different experimental realizations. Despite the notable progress made in recent years with regard to the characterisation and application of symmetric quantum states, several intriguing questions remain unsolved. We conclude this review by discussing some of these open problems and outlining promising directions for future research.
Authors: Ole Sönnerborn
Quantum speed limits set fundamental lower bounds on the time required for a quantum system to evolve between states. Traditional bounds, such as those by Mandelstam-Tamm and Margolus-Levitin, rely on state distinguishability and become trivial for cyclic evolutions where the initial and final states coincide. In this work, we explore an alternative approach based on isoholonomic inequalities, which bound the length of closed state space trajectories in terms of their holonomy. Building on a gauge-theoretic framework for mixed state geometric phases, we extend the concept of isoholonomic inequalities to closed curves of isospectral and isodegenerate density operators. This allows us to derive new quantum speed limits that remain nontrivial for cyclic evolutions. Our results reveal deep connections between the temporal behavior of cyclic quantum systems and holonomy.
Authors: Kiki Dekkers, Laura Serino, Nicola DAlessandro, Abhinandan Bhattacharjee, Benjamin Brecht, Armin Tavakoli, Christine Silberhorn, Jonathan Leach
Violation of Bell inequalities is an essential requirement for many quantum information and communication protocols. In high-dimensional systems, Bell inequality tests face the challenge of implementing genuinely multi-outcome measurements, since the emulation of these with separate dichotomic projections opens a binarisation loophole that local hidden variable theories can exploit. Here we show that the joint spectral intensity of a two-photon entangled state contains access to the necessary multi-outcome measurements to overcome this obstacle and certify and violate a Bell inequality for high-dimensional states. This result is contrary to the belief that the joint spectral intensity is a phase-insensitive quantity and does not have sufficient information to certify entanglement or Bell-nonlocality. Using this approach, we violate the CGLMP Bell inequality up to dimension d = 8, all with negligible p-values, and for the first time close the binarisation loophole in high-dimensional Bell experiments. Guaranteeing Bell-nonlocal correlations using frequency-only measurements removes the technological hurdle of measurements in the temporal domain, thus greatly simplifying any practical implementation of future high-dimensional quantum information protocols.
Authors: Yuxiang Liu, Fanxu Meng, Lu Wang, Yi Hu, Sixuan Li, Xutao Yu, Zaichen Zhang
Designing effective quantum kernels is a central challenge in Quantum Machine Learning (QML), particularly under the limitations of Noisy Intermediate-Scale Quantum (NISQ) devices with a limited number of qubits, error-prone gate operations, and restricted qubit connectivity. To address this, we propose HaQGNN, a hardware-aware quantum kernel design method that integrates quantum device topology, noise characteristics, and Graph Neural Networks (GNNs) to evaluate and select task-relevant quantum circuits that define quantum kernels. First, each quantum circuit is represented as a directed acyclic graph that encodes hardware-specific features, including gate types, target qubits, and noise characteristics. Next, two GNNs are trained to predict surrogate metrics, Probability of Successful Trials (PST) and Kernel-Target Alignment (KTA), for fast and accurate fidelity and performance estimation. Additionally, feature selection is further incorporated to reduce input dimensionality and improve compatibility with limited-qubit devices. Finally, extensive experiments on three benchmark datasets, Credit Card (CC), MNIST-5, and FMNIST-4, demonstrate that HaQGNN outperforms existing baselines in terms of classification accuracy. Our results highlight the potential of learning-based and hardware-aware strategies for advancing practical quantum kernel design on near-term quantum hardware.
Authors: Xicheng Wang, Erich J Mueller
Motivated by experiments on Rydberg atom arrays, we explore the properties of uniform quantum superpositions of kagome dimer configurations and construct an efficient algorithm for experimentally producing them. We begin by considering the thin cylinder limit, where these states have simple descriptions. We then develop a matrix product representation of the states on arbitrary cylinders, which leads to a natural protocol to efficiently grow them. We explain how our approach can be adapted to other quantum computing hardware.
Authors: Hiu Yung Wong
After publishing my book "Quantum Computing Architecture and Hardware for Engineers: Step by Step" [1] (now I call it Volume I), in which spin qubit and superconducting qubit quantum computers were covered, I decided to continue to write the second volume to cover the trapped ion qubit quantum computer, which was also taught in my EE274 class. I follow the same structure as in Volume I by discussing the physics, mathematics, and their connection to laser pulses and electronics based on how they fulfill the five DiVincenzo's criteria. I also think it would be a good idea to share the second volume on arXiv so that more people can read it for free, and I can continue to update the contents. As of July 2025, I have finished the trapped ion quantum computer part. In the future, I plan to write more critical topics in a step-by-step manner to bridge engineers who did not receive rigorous training in Physics to the quantum computing world.
Authors: Jianlong Lin, Mari Cieszynski, William Christopherson, Darman Khan, Lintao Li, Elizabeth Goldschmidt, Brian DeMarco
We describe a 88Sr+ ion trap apparatus with the capability to produce high-quality 408 nm photons aimed at distributed quantum computing and networking applications. This instrument confines ion chains using a surface electrode trap with a two-dimensional magneto-optical trap as an atomic source. Several laser systems spanning 400-1100 nm are used to achieve high fidelity state preparation and readout. Photons are produced via the decay of an exited state, which is accessed using a custom 408 nm laser system that produces 150 ps optical pulses using non-linear photonics. We demonstrate single photon production through a Hanbury Brown-Twiss measurement for one to six ions.
Authors: Ian Tan
We prove that every 5-qubit absolutely maximally entangled (AME) state is equivalent by a local unitary transformation to a point in the unique ((5,2,3)) quantum error correcting code C. Furthermore, two points in C are equivalent if and only if they are related by a group of order 24 acting on C. There exists a set of 3 invariant polynomials that separates equivalence classes of 5-qubit AME states. We also show that every 4-qubit pure code is equivalent to a subspace of the unique ((4,4,2)) and construct an infinite family of 3-uniform n-qubit states for even $n\geq 6$. The proofs rely heavily on results from Vinberg and classical invariant theory.
Authors: Mahmoud Mahdian, Ali Babapour-Azar, Zahra Mousavi, Rashed Khanjani-Shiraz
Quantum measurements are inherently noisy, hindering reliable entanglement detection and limiting the scalability of quantum technologies. While error mitigation and correction strategies exist, they often impose prohibitive resource overheads. Here, we introduce a machine-learning-based approach to achieve noise-resilient entanglement classification even with imperfect measurements. Using support vector machines (SVMs) trained on features extracted from Pauli measurements, we develop a robust optimal entanglement witness (ROEW) that remains effective under unknown measurement noise. By optimizing SVM parameters against worst-case errors, our protocol significantly outperforms conventional methods in classification accuracy. Numerical experiments demonstrate that ROEW achieves high-fidelity entanglement detection with minimal measurements, even when measurement errors exceed 10\%. This work bridges machine learning and quantum information science, offering a practical tool for noise-robust quantum characterization and advancing the feasibility of entanglement-based technologies in real-world settings.
Authors: Kean Chen, Nengkun Yu, Zhicheng Zhang
Time-reversal of unitary evolution is fundamental in quantum information processing. Many scenarios, particularly those in quantum learning and metrology, assume free access to the time-reverse of an unknown unitary. In this paper, we settle the query complexity of the unitary time-reversal task: approximately implementing $U^{-1}$ given only black-box access to an unknown $d$-dimensional unitary $U$. We provide a tight query lower bound $\Omega((1-\epsilon)d^2)$ for the unitary time-reversal to within diamond norm error $\epsilon$. Notably, our lower bound applies to general coherent protocols with unbounded ancillas, and holds even when $\epsilon$ is an average-case distance error. Moreover, our result implies a query lower bound $\Omega(d^2)$ for approximately implementing control-$U$ up to an irrelevant phase, which is also tight with respect to the dimension.