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Tuesday 4 November

Quantum Physics · Cross-listed from hep-ph

Testing Electromagnetic Memory via Acceleration-Induced Phase Imprints in Superconductors

Jie Sheng, Tsutomu T. Yanagida, Bo Gao et al. (+1 more)

Electromagnetic memory is an infrared observable of gauge theory associated with soft photons and large gauge transformations. Despite its fundamental theoretical importance, it has not yet been experimentally verified. From a phenomenological perspective, a transient electromagnetic configuration can leave a persistent gauge-invariant phase imprint on charged coherent states after the local field has vanished. We point out that the electric field and associated gauge potential induced inside a normal conductor by gravitational acceleration can provide a clean source for imprinting this phase, and it can then be read out through a superconducting protocol. For representative parameters, the predicted signal can lie within the range of present sensitivities, providing a possible tabletop route toward testing electromagnetic memory.
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All authors Jie Sheng, Tsutomu T. Yanagida, Bo Gao, Hong Ding

Quantum Physics

Decay of transmon qubit in a broadband one-dimensional cavity

Ya. S. Greenberg, A. A. Shtygashev and O. V. Kibis

We investigate the decay dynamics of a three-level artificial atom, a superconducting transmon qubit, weakly coupled to a continuum of modes in a broadband, one-dimensional cavity. Using the resolvent formalism, we derive analytical expressions for the resonance frequency shifts and widths, which are then evaluated numerically for a Gaussian density of states. We identify two distinct dynamical regimes, differentiated by the ratio of the qubit's coupling strength to the continuum bandwidth. When this ratio is much less than one, the system exhibits a Markovian regime in which the resonance width is practically independent of energy within the continuum band. As the ratio increases, the system transitions to a non-Markovian regime where the resonance width becomes strongly energy-dependent. In this regime, the qubit interacts with the continuum faster than the continuum can erase the information from the qubit's past. Furthermore, we demonstrate that the coupling between the transmon's second level and its ground state significantly influences the decay dynamics of the third level. The interaction between these two levels opens a fast two-photon decay channel, which broadens the transmon's second level.
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Quantum Physics · Cross-listed from cond-mat.quant-gas

Multi-Particle Quantum Walks in a Dipole-Conserving Bose-Hubbard Model

Sooshin Kim, Byungmin Kang, Perrin Segura et al. (+4 more)

When particles move through a crystal or optical lattice, their motion can sometimes become frozen by strong external forces – yet collective motion may still emerge through subtle many-body effects. In this work, we explore such constrained dynamics by realizing a dipole-conserving Bose-Hubbard model, where single atoms are immobile but pairs of particles can move cooperatively while preserving the system's center of mass, i.e. the overall dipole moment of the particle distribution. Starting from a one-dimensional chain of ultracold bosonic atoms in an optical lattice, we generate localized dipole excitations consisting of a hole and a doublon using site-resolved optical potentials and characterize their quantum walks and scattering dynamics. Our study provides a bottom-up investigation of a Hamiltonian with kinetic constraints, and paves the way for exploring low-energy phases of fractonic matter in existing experimental platforms.
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All authors Sooshin Kim, Byungmin Kang, Perrin Segura, Yanfei Li, Ethan Lake, Brice Bakkali-Hassani, Markus Greiner

Quantum Physics

One-step preparation of 3D Bell and 3D GHZ states with Rydberg atoms

Jiping Wang and Huapeng Liu

Three-dimensional Bell states and GHZ states serve as representative examples of high-dimensional entangled states. In this paper, we propose a scheme for generating three-dimensional Bell and GHZ entangled states using Rydberg atoms. By leveraging Rydberg-mediated interactions and introducing detuning, the system is effectively simplified into a chain-like configuration. To design effective couplings, we employ a centrosymmetric Gaussian distribution and optimize the relevant parameters. Furthermore, we take into account decoherence factors including atomic spontaneous emission, dephasing effects and random noise. Numerical simulations indicate that the proposed scheme can achieve high fidelity.
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Quantum Physics

Dirac delta-convergence of free-motion time-of-arrival eigenfunctions

John Jaykel P. Magadan and Eric A. Galapon

Previous numerical analyses on the Aharonov-Bohm (AB) operator representing the quantum time-of-arrival (TOA) observable for the free particle have indicated that its eigenfunctions represent quantum states with definite arrival time at the arrival point. In this paper, we give the mathematical proof that this is indeed the case. An essential element of this proof is the consideration of the eigenfunctions of the AB operator with complex eigenvalues. These eigenfunctions can be considered legitimate TOA eigenfunctions because they evolve unitarily to collapse at the arrival point at the time equal to the real part of their eigenvalue. We show that the time-evolved TOA position probability density distribution evaluated at the time equal to the real part of the eigenvalue forms a dirac delta sequence in the limit as the imaginary part of the eigenvalue approaches zero.
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Quantum Physics · Cross-listed from cs.LG

Federated Quantum Kernel Learning for Anomaly Detection in Multivariate IoT Time-Series

Kuan-Cheng Chen, Samuel Yen-Chi Chen, Chen-Yu Liu et al. (+1 more)

The rapid growth of industrial Internet of Things (IIoT) systems has created new challenges for anomaly detection in high-dimensional, multivariate time-series, where privacy, scalability, and communication efficiency are critical. Classical federated learning approaches mitigate privacy concerns by enabling decentralized training, but they often struggle with highly non-linear decision boundaries and imbalanced anomaly distributions. To address this gap, we propose a Federated Quantum Kernel Learning (FQKL) framework that integrates quantum feature maps with federated aggregation to enable distributed, privacy-preserving anomaly detection across heterogeneous IoT networks. In our design, quantum edge nodes locally compute compressed kernel statistics using parameterized quantum circuits and share only these summaries with a central server, which constructs a global Gram matrix and trains a decision function (e.g., Fed-QSVM). Experimental results on synthetic IIoT benchmarks demonstrate that FQKL achieves superior generalization in capturing complex temporal correlations compared to classical federated baselines, while significantly reducing communication overhead. This work highlights the promise of quantum kernels in federated settings, advancing the path toward scalable, robust, and quantum-enhanced intelligence for next-generation IoT infrastructures.
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All authors Kuan-Cheng Chen, Samuel Yen-Chi Chen, Chen-Yu Liu, Kin K. Leung

Quantum Physics

Localized to delocalized spatial quantum correlation evolution in structured bright twin beams

Jerin A Thachil, Chirang R Patel, U. Ashwin et al. (+1 more)

Quantum correlations in the spatial domain hold great promise for applications in quantum imaging, quantum cryptography and quantum information processing, owing to the infinite dimensionality of the associated Hilbert space. Here, we present a theoretical investigation, complemented by experimental measurements, of the propagation dynamics of the spatial quantum correlations in bright structured twin beams generated via a four-wave mixing process in a double-$\Lambda$ configuration in atomic vapor. We derive an analytical expression describing the evolution of the spatial quantum correlation distribution from the near field to the far field. To qualitatively support the theoretical predictions, we perform experiments measuring intensity-difference noise between different spatial subregions of the twin beams as they propagate from the near field to the far field. The presence of quantum correlations is manifested as squeezing in the intensity difference noise measurement. With a Gaussian pump, we observe localized correlations in the near field and localized anti-correlations in the far field. In contrast, with a structured Laguerre-Gaussian pump, there is a transition from localized correlations in the near field to delocalized correlations in the far field. The present results offer valuable insights into the fundamental behavior of spatial quantum correlations and open possibilities for potential applications in quantum information, quantum imaging and sensing.
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All authors Jerin A Thachil, Chirang R Patel, U. Ashwin, Ashok Kumar

Quantum Physics · Cross-listed from cs.CC

Spectral Certificates and Non-commutative Sum-of-Squares Lower Bounds for Hamiltonians

Nicholas Kocurek and Chinmay Nirkhe

A central question in quantum many-body physics is estimating the ground energy of a $k$-local Hamiltonian system. In this work, we present a spectral technique for certifying a lower bound on the ground energy of a random $n$-qubit Hamiltonian system defined as the sum of signed $k$-local Pauli operators. In particular, we prove that for any constant $\ell$, there exists an efficiently computable length $n^{O(\ell)}$ certificate that is always a lower bound on the ground energy with the promise that, with high probability over the random Hamiltonian distribution, the certificate value is an $\varepsilon$-good approximation of the true ground energy when the number of terms is sufficiently large. Second, we show by construction that this technique, while successful on average over random Hamiltonian systems, can fail to produce good certificates on worst-case instances. Our spectral technique for producing these certificates comes from extending classical results on $k$-XOR refutations to $k$-local Pauli Hamiltonians by crafting a quantum variant of the Kikuchi matrix for CSP refutations. To show the limitations of this technique, we prove non-commutative Sum-of-Squares lower bounds for worst-case signed $k$-local Pauli operators. More generally, we explore how the non-commutative Sum-of-Squares relaxation can be understood as augmenting the standard Sum-of-Squares relaxation with the commutation relations between the Pauli operators. We instantiate the resulting framework with a modification to prior quantum code-based NLTS Hamiltonians that yields stronger complexity guarantees for the low-energy space; our Hamiltonian family satisfies simultaneously (1) $\Omega(\log n)$-circuit depth lower bounds for all low-energy states, (2) constant-gap NP-hardness to approximate the ground energy, and (3) a constant-gap non-commutative Sum-of-Squares integrality gap up to $\Omega(n)$-levels.
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Quantum Physics

Multiplexed double-transmon coupler scheme in scalable superconducting quantum processor

Tianqi Cai, Chitong Chen, Kunliang Bu et al. (+7 more)

Precise control of superconducting qubits is essential for advancing both quantum simulation and quantum error correction. Recently, transmon qubit systems employing the single-transmon coupler (STC) scheme have demonstrated high-fidelity single- and two-qubit gate operations by dynamically tuning the effective coupling between qubits. However, the integration of STCs increases the number of control lines, thereby posing a significant bottleneck for chip routing and scalability. To address this challenge, we propose a robust control line multiplexing scheme based on a double-transmon coupler (DTC) architecture, which enables shared coupler control lines to substantially reduce wiring complexity. Moreover, we experimentally verify that this multiplexed configuration efficiently suppresses undesirable static $ZZ$ coupling while maintaining accurate control over two-qubit gate operations. We further demonstrate the feasibility of the architecture through two distinct gate implementations: a fast coupler $Z$-control-based CZ gate and a parametric iSWAP gate. To validate the practical applicability of this multiplexing approach in quantum circuits, we prepare Bell and three-qubit GHZ states using the proposed scheme with fidelity exceeding 99% and 96%, respectively. This multiplexed DTC architecture offers significant potential to minimize wiring overhead in two-dimensional qubit arrays, thereby greatly enhancing the scalability of superconducting quantum processors.
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All authors Tianqi Cai, Chitong Chen, Kunliang Bu, Sainan Huai, Xiaopei Yang, Zhiwen Zong, Yuan Li, Zhenxing Zhang, Yi-Cong Zheng, Shengyu Zhang

Quantum Physics · Cross-listed from cond-mat.quant-gas

Interaction-Induced Quasicrystalline Order: Emergence of Quasi-Solid and Quasi-Supersolid Phases

Chao Zhang

Deterministic quasiperiodicity in quantum systems has long been associated with localization, criticality, or glassy behavior, and has therefore been believed to suppress long-range order rather than stabilize it. Here we demonstrate the opposite: quasiperiodicity in interactions–without any quasiperiodic potential, disorder, or geometric modulation–can generate coherent, ordered quantum phases. We study hard-core bosons in one dimension with quasiperiodic long-range interactions, V_{ij}=V_0 \cos(\pi \alpha i)\cos(\pi \alpha j), where n=\alpha=(\sqrt{5}-1)/2 is the inverse golden ratio. Using large-scale path-integral quantum Monte Carlo simulations, we uncover thermodynamically stable incompressible plateaus at irrational densities tied to Fibonacci ratios. These plateaus exhibit sharp incommensurate Bragg peaks, signaling an emergent quasi-solid with long-range quasicrystalline density order. More strikingly, at nearby fillings and interaction strengths, we identify a quasi-supersolid phase that supports both Fibonacci density ordering and finite superfluid density–demonstrating that interaction-induced quasiperiodicity can stabilize supersolid coherence. Our results establish a new mechanism for realizing ordered quasicrystalline quantum matter, and provide realistic guidance for implementation in Rydberg atom arrays, multimode cavity-QED systems, and trapped-ion quantum simulators.
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Quantum Physics

Information recycling in coherent state discrimination

L. F. Melo, János A. Bergou and Alexandre B. Tacla

The discrimination of coherent states is a crucial component in quantum communication with continuous variables, especially in quantum key distribution protocols (CV-QKD), which rely on the ability to distinguish among different coherent states to establish a shared secret key between two parties. Here, we propose and analyze a strategy for distinguishing among N phase-symmetric coherent states, which optimally takes unambiguous discrimination (UD) to the deterministic regime, at the inevitable cost of having non-zero probability of error. Despite the disturbance introduced by the separation map used in the UD process, we show that for N > 2, the "failure" states of UD retain residual information about the original input states, which can be further used for discrimination. Rather than discarding inconclusive outcomes as in conventional UD, we show that the "failure" states of UD can be optimally recycled by performing a sequential minimum-error discrimination (MED). This strategy, which we call information recycling (IR), combines the benefits of both MED and optimal UD: It always provides conclusive results while allowing for a subset of those results to be error-free, which are identifiable by an ancillary system. We characterize the disturbance introduced by the state separation map by the infidelity between input and failure states, demonstrating that it lower bounds the error probability in the recycling stage. Furthermore, in the low-amplitude regime-relevant for long-distance CV-QKD applications-we show that the state separation achieves significant success while introducing relatively low disturbance to the input states after failed events. Our results open up new possibilities for adaptive and sequential discrimination protocols in continuous-variable settings, and could potentially be used in the design of next-generation receivers in quantum communication.
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Quantum Physics

Preserving fermionic statistics for single-particle approximations in microscopic quantum master equations

Mikayla Z. Fahrenbruch, Anthony W. Schlimgen and Kade Head-Marsden

Microscopic master equations have gained traction for the dissipative treatment of molecular spin and solid-state systems for quantum technologies. Single particle approximations are often invoked to treat these systems, which can lead to unphysical evolution when combined with master equation approaches. We present a mathematical constraint on the system-environment parameters to ensure that microscopically-derived Markovian master equations preserve fermionic, $N$-representable statistics when applied to reduced systems. We demonstrate these constraints for the recently derived unified master equation and universal Lindblad equation, along with the Redfield master equation for cases when positivity issues are not present. For operators that break the constraint, we explore the addition of Pauli factors to recover $N$-representability. This work promotes feasible applications of novel microscopic master equations for realistic chemical systems.
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Quantum Physics · Cross-listed from cs.LG

QuPCG: Quantum Convolutional Neural Network for Detecting Abnormal Patterns in PCG Signals

Yasaman Torabi, Shahram Shirani and James P. Reilly

Early identification of abnormal physiological patterns is essential for the timely detection of cardiac disease. This work introduces a hybrid quantum-classical convolutional neural network (QCNN) designed to classify S3 and murmur abnormalities in heart sound signals. The approach transforms one-dimensional phonocardiogram (PCG) signals into compact two-dimensional images through a combination of wavelet feature extraction and adaptive threshold compression methods. We compress the cardiac-sound patterns into an 8-pixel image so that only 8 qubits are needed for the quantum stage. Preliminary results on the HLS-CMDS dataset demonstrate 93.33% classification accuracy on the test set and 97.14% on the train set, suggesting that quantum models can efficiently capture temporal-spectral correlations in biomedical signals. To our knowledge, this is the first application of a QCNN algorithm for bioacoustic signal processing. The proposed method represents an early step toward quantum-enhanced diagnostic systems for resource-constrained healthcare environments.
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Monday 3 November

Quantum Physics · Cross-listed from q-bio.NC

A Stochastic Quantum Neural Network Model for Ai

Gautier-Edouard Filardo and Thibaut Heckmann

Artificial intelligence (AI) has drawn significant inspiration from neuroscience to develop artificial neural network (ANN) models. However, these models remain constrained by the Von Neumann architecture and struggle to capture the complexity of the biological brain. Quantum computing, with its foundational principles of superposition, entanglement, and unitary evolution, offers a promising alternative approach to modeling neural dynamics. This paper explores the possibility of a neuro-quantum model of the brain by introducing a stochastic quantum approach that incorporates random fluctuations of neuronal processing within a quantum framework. We propose a mathematical formalization of stochastic quantum neural networks (QNNS), where qubits evolve according to stochastic differential equations inspired by biological neuronal processes. We also discuss challenges related to decoherence, qubit stability, and implications for AI and computational neuroscience.
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Quantum Physics

Software for Creating Scalable Benchmarks from Quantum Algorithms

Noah Siekierski, Stefan Seritan, Neer Patel et al. (+3 more)

Creating scalable, reliable, and well-motivated benchmarks for quantum computers is challenging: straightforward approaches to benchmarking suffer from exponential scaling, are insensitive to important errors, or use poorly-motivated performance metrics. Furthermore, curated benchmarking suites cannot include every interesting quantum circuit or algorithm, which necessitates a tool that enables the easy creation of new benchmarks. In this work, we introduce a software tool for creating scalable and reliable benchmarks that measure a well-motivated performance metric (process fidelity) from user-chosen quantum circuits and algorithms. Our software, called $\texttt{scarab}$, enables the creation of efficient and robust benchmarks even from circuits containing thousands or millions of qubits, by employing efficient fidelity estimation techniques, including mirror circuit fidelity estimation and subcircuit volumetric benchmarking. $\texttt{scarab}$ provides a simple interface that enables the creation of reliable benchmarks by users who are not experts in the theory of quantum computer benchmarking or noise. We demonstrate the flexibility and power of $\texttt{scarab}$ by using it to turn existing inefficient benchmarks into efficient benchmarks, to create benchmarks that interrogate hardware and algorithmic trade-offs in Hamiltonian simulation, to quantify the in-situ efficacy of approximate circuit compilation, and to create benchmarks that use subcircuits to measure progress towards executing a circuit of interest.
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All authors Noah Siekierski, Stefan Seritan, Neer Patel, Siyuan Niu, Thomas Lubinski, Timothy Proctor

Quantum Physics

Superconducting pairing correlations on a trapped-ion quantum computer

Etienne Granet, Sheng-Hsuan Lin, Kevin Hémery et al. (+31 more)

The Fermi-Hubbard model is the starting point for the simulation of many strongly correlated materials, including high-temperature superconductors, whose modelling is a key motivation for the construction of quantum simulation and computing devices. However, the detection of superconducting pairing correlations has so far remained out of reach, both because of their off-diagonal character - which makes them inaccessible to local density measurements - and because of the difficulty of preparing superconducting states. Here, we report measurement of significant pairing correlations in three different regimes of Fermi-Hubbard models simulated on Quantinuum's Helios trapped-ion quantum computer. Specifically, we measure non-equilibrium pairing induced by an electromagnetic field in the half-filled square lattice model, d-wave pairing in an approximate ground state of the checkerboard Hubbard model at $1/6$-doping, and s-wave pairing in a bilayer model relevant to nickelate superconductors. These results show that a quantum computer can reliably create and probe physically relevant states with superconducting pairing correlations, opening a path to the exploration of superconductivity with quantum computers.
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All authors Etienne Granet, Sheng-Hsuan Lin, Kevin Hémery, Reza Haghshenas, Pablo Andres-Martinez, David T. Stephen, Anthony Ransford, Jake Arkinstall, M.S. Allman, Pete Campora, Samuel F. Cooper, Robert D. Delaney, Joan M. Dreiling, Brian Estey, Caroline Figgatt, Cameron Foltz, John P. Gaebler, Alex Hall, Ali Husain, Akhil Isanaka, Colin J. Kennedy, Nikhil Kotibhaskar, Ivaylo S. Madjarov, Michael Mills, Alistair R. Milne, Annie J. Park, Adam P. Reed, Brian Neyenhuis, Justin G. Bohnet, Michael Foss-Feig, Andrew C. Potter, Ramil Nigmatullin, Mohsin Iqbal, Henrik Dreyer

Quantum Physics

ZZ-Free Two-Transmon CZ Gate Mediated by a Fluxonium Coupler

Junyoung An, Helin Zhang, Qi Ding et al. (+19 more)

Eliminating residual ZZ interactions in a two-qubit system is essential for reducing coherent errors during quantum operations. In a superconducting circuit platform, coupling two transmon qubits via a transmon coupler has been shown to effectively suppress residual ZZ interactions. However, in such systems, perfect cancellation usually requires the qubit-qubit detuning to be smaller than the individual qubit anharmonicities, which exacerbates frequency crowding and microwave crosstalk. To address this limitation, we introduce TFT (Transmon-Fluxonium-Transmon) architecture, wherein two transmon qubits are coupled via a fluxonium qubit. The coupling mediated by the fluxonium eliminates residual ZZ interactions even for transmons detuned larger than their anharmonicities. We experimentally identified zero-ZZ interaction points at qubit-qubit detunings of 409 MHz and 616 MHz from two distinct TFT devices. We then implemented an adiabatic, coupler-flux-biased controlled-Z gate on both devices, achieving CZ gate fidelities of 99.64(6)% and 99.68(8)%.
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All authors Junyoung An, Helin Zhang, Qi Ding, Leon Ding, Youngkyu Sung, Roni Winik, Junghyun Kim, Ilan T. Rosen, Kate Azar, Renee DePencier Piñero, Jeffrey M. Gertler, Michael Gingras, Bethany M. Niedzielski, Hannah Stickler, Mollie E. Schwartz, Joel Î-j. Wang, Terry P. Orlando, Simon Gustavsson, Max Hays, Jeffrey A. Grover, Kyle Serniak, William D. Oliver

Quantum Physics · Cross-listed from cond-mat.mes-hall

Current cross-correlation spectroscopy of Majorana bound states

Michael Ridley, Eliahu Cohen, Christian Flindt et al. (+1 more)

The clock speed of topological quantum computers based on Majorana zero mode (MZM)-supporting nanoscale devices is limited by the time taken for electrons to traverse the device. We employ the time-dependent Landauer-Büttiker transport theory for current cross-lead correlations in a superconducting nanowire junction hosting MZMs. From the time-dependent quantum noise, we are able to extract traversal times for electrons crossing the system. After demonstrating a linear scaling of traversal times with nanowire length, we present a heuristic formula for the traversal times which accurately captures their behaviour. We then connect our framework to a proposed experimental verification of this discriminant between spurious and genuine MZMs utilizing time-resolved transport measurements.
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All authors Michael Ridley, Eliahu Cohen, Christian Flindt, Riku Tuovinen

Quantum Physics

Geometric characterization of non-Gaussian entanglement for finite stellar rank states

Carlos E. Lopetegui-Gonzalez, Massimo Frigerio and Mattia Walschaers

We introduce a general framework for the analysis of non-Gaussian entanglement in bosonic states of finite stellar rank. The central result is the full characterization of their entanglement structure through the atomic decomposition of their stellar polynomial and its associated structural graph, whose connected components determine the mode-intrinsic entanglement content of the state and all partitions compatible with passive separability. An essential ingredient in this construction is the concept of essential variables, which identify the minimal number of effective modes involved in a core state, in direct correspondence with the symplectic rank. This reduction provides the foundation for decomposing stellar polynomials into atomic factors and for revealing the underlying entanglement structure. Building on this, we derive complete separability criteria for two-mode states, expressed through hyperplane decompositions of zero sets, and for stellar-rank-2 states across an arbitrary number of modes. Applications to several example states illustrate how the method isolates genuinely non-Gaussian resources and quantifies preparation complexity.
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Quantum Physics

All-optical turbulence mitigation for free-space quantum key distribution using stimulated parametric down-conversion

Aaron A. Aguilar-Cardoso, Cheng Li, Tobey J.B. Luck et al. (+4 more)

In this work, we propose and demonstrate a turbulence-resilient scheme for free-space quantum communication. By leveraging the phase conjugation property of stimulated parametric down-conversion, our scheme enables all-optical dynamic correction of spatial-mode distortion induced by atmospheric turbulence, thereby enhancing the secure key rate in high-dimensional quantum key distribution. We develop a theoretical model that provides detailed guidelines for selecting the optimal basis and spatial properties needed to maximize the efficiency of the proposed scheme. Both numerical simulations and experimental results show that, even under strong turbulence, our scheme can reduce the quantum error rates well below the security threshold. These results highlight the potential of nonlinear optical approaches as powerful tools for robust quantum communication in realistic free-space environments. Our work could have important implications for the practical implementation of secure quantum channels over long free-space distances.
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All authors Aaron A. Aguilar-Cardoso, Cheng Li, Tobey J.B. Luck, Manuel F. Ferrer-Garcia, Jeremy Upham, Jeff S. Lundeen, Robert W. Boyd

Quantum Physics

Towards Continuous-variable Quantum Neural Networks for Biomedical Imaging

Daniel Alejandro Lopez, Oscar Montiel, Oscar Castillo et al. (+1 more)

Continuous-variable (CV) quantum computing offers a promising framework for scalable quantum machine learning, leveraging optical systems with infinite-dimensional Hilbert spaces. While discrete-variable (DV) quantum neural networks have shown remarkable progress in various computer vision tasks, CV quantum models remain comparatively underexplored. In this work, we present a feasibility study of continuous-variable quantum neural networks (CV-QCNNs) applied to biomedical image classification. Utilizing photonic circuit simulation frameworks, we construct CV quantum circuits composed of Gaussian gates, such as displacement, squeezing, rotation, and beamsplitters to emulate convolutional behavior. Our experiments are conducted on the MedMNIST dataset collection, a set of annotated medical image benchmarks for multiple diagnostic tasks. We evaluate CV-QCNN's performance in terms of classification accuracy, model expressiveness, and resilience to Gaussian noise, comparing against classical CNNs and equivalent DV quantum circuits. This study aims to identify trade-offs between DV and CV paradigms for quantum-enhanced medical imaging. Our results highlight the potential of continuous-variable models and their viability for future computer-aided diagnosis systems.
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All authors Daniel Alejandro Lopez, Oscar Montiel, Oscar Castillo, Miguel Lopez-Montiel

Quantum Physics

Spread and circuit complexity as a measure of particle content and phase space fluctuations

Satyaki Chowdhury

In this work, we investigate the relation between different notions of quantum complexity, namely, circuit and spread complexity and physically meaningful quantities such as the particle content of the quantum state and the variances of position and momentum operators. Using a harmonic oscillator with time-dependent mass and frequency as a toy model, we show that both circuit and spread complexity at any instant is determined by the mean number of quanta and its rate of change. Furthermore, both complexity and its growth are directly linked to the variances of the position and momentum operators, providing a clear physical interpretation of complexity in terms of the state's excitation and phase-space fluctuation. Although the analysis is carried out for a single time-dependent oscillator, the results have direct relevance for quantum field theory in curved backgrounds, where individual field modes effectively behave as time-dependent oscillators. This offers new insights into how quantum complexity encodes particle production and phase space fluctuations in non-holographic systems. Finally, we establish a precise and potentially universal relation between spread and circuit complexity for the time evolved state suggesting deeper connections between different complexity measures in the context of field theories on curved backgrounds.
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Quantum Physics

What is special about the Kirkwood-Dirac distributions? Only they produce natural conditional expectations

Matéo Spriet, Christopher Langrenez, Raymond Brummelhuis et al. (+1 more)

Among the many quasiprobability representations of quantum mechanics, the family of Kirkwood-Dirac (KD) representations has come to the foreground in recent years. Each such KD representation is determined by the choice of two complementary complete sets of commuting observables $\hat A$ and $\hat B$ with respect to which it is Born-compatible, meaning that it correctly reproduces their Born probabilities for every state. We identify in this paper what property uniquely characterizes the KD representations among all such $\hat A$ and $\hat B$ Born-compatible quasiprobability representations. For that purpose, we first define a natural notion of quantum conditional expectation of an observable $\hat X$, given an observable $\hat Y$, in a state $\hat \rho$, as a best estimator and we show that it has the basic properties generally expected of a conditional expectation. We then show that only the KD representations provide a notion of conditional expectation, given $\hat B$ (or given $\hat A$) that coincides with the above quantum conditional expectation. As a byproduct of our analysis, we show a state-dependent no-go theorem. We prove that, if the quantum conditional expectation of an observable $\hat X$, given an observable $\hat Y$ in a state $\hat \rho$ admits an anomalous value, then there cannot exist a Born-compatible joint probability distribution $\mu(x,y)$ for $\hat X$ and $\hat Y$ in the state $\hat \rho$ for which the associated conditional probability $\mu(x|y)$ yields a conditional expectation that coincides with the quantum conditional expectation. We further apply our findings to revisit a standard model for phase estimation in quantum metrology. We show in particular that, within the real sector of a given KD representation, the classical Fisher information of this phase estimation problem vanishes identically.
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All authors Matéo Spriet, Christopher Langrenez, Raymond Brummelhuis, Stephan De Bièvre

2511.01996 · 3 Nov · We have strengthened our characterization of the KD distributions. We have also considerably developped the physical interpretation and operational significance of the objects that we introduce. We furthermore apply our results to a metrology problem to give a novel interpretation of the KD real sector. The paper now also serves as an introductory review of the subject

Abstract PDF $QNTM ×6$ESTM ×1$THRM ×1

Quantum Physics

Entropy-based random quantum states

Harry J. D. Miller

In quantum information geometry, the curvature of von-Neumann entropy and relative entropy induce a natural metric on the space of mixed quantum states. Here we use this information metric to construct a random matrix ensemble for states and investigate its key statistical properties such as the asymptotic eigenvalue density and mean entropy. We present an algorithm for generating these entropy-based random density matrices, thus providing a new recipe for random state generation that differs from the well established Hilbert-Schmidt and Bures-Hall ensemble approaches. We also prove a duality between the entropy-based state ensemble and a random Hamiltonian model constructed from the thermodynamic length over the set of Gibbs states. This Hamiltonian model is found to display Wigner level repulsion, implying that the dual state ensemble can be realised as a random Gibbs state with respect to a class of chaotic Hamiltonians. As an application we use our model to compute the survival probability of a randomly evolved thermofield double state, predicting a ramp and plateau over time that is characteristic of quantum chaos. For other applications, the entropy-based ensemble can be used as an uninformative prior for Bayesian quantum state or Hamiltonian tomography.
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Quantum Physics · Cross-listed from cond-mat.mes-hall

Dynamical Phase Transitions Across Slow and Fast Regimes in a Two-Tone Driven Duffing Resonator

Soumya S. Kumar, Javier del Pino, Letizia Catalini et al. (+2 more)

The response of nonlinear resonators to multifrequency driving reveals rich dynamics beyond conventional single-tone theory. We study a Duffing resonator under bichromatic excitation and identify a competition between the two drives, governed by their detuning and relative amplitudes. In the slow-beating regime, where the tones are closely spaced, the secondary drive acts as a modulation that induces dynamical phase transitions between coexisting stationary states. We introduce the cycle-averaged amplitude as an order parameter and map the resulting phase diagram as a function of the drive detuning and amplitude ratio, capturing the pronounced asymmetry observed for blue versus red detuning in experiment. We devise a model to link the onset of these transitions to the resonance properties around the nonlinear stationary mode of the system. Our results provide a framework for controlling driven nonlinear systems, enabling state manipulation, and sensing in nanomechanical, optical, and superconducting circuit platforms.
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All authors Soumya S. Kumar, Javier del Pino, Letizia Catalini, Alexander Eichler, Oded Zilberberg

Quantum Physics

Stability of mixed-state phases under weak decoherence

Yifan F. Zhang and Sarang Gopalakrishnan

We prove that the Gibbs states of classical, and commuting-Pauli, Hamiltonians are stable under weak local decoherence: i.e., we show that the effect of the decoherence can be locally reversed. In particular, our conclusions apply to finite-temperature equilibrium critical points and ordered low-temperature phases. In these systems the unconditional spatio-temporal correlations are long-range, and local (e.g., Metropolis) dynamics exhibits critical slowing down. Nevertheless, our results imply the existence of local "decoders" that undo the decoherence, when the decoherence strength is below a critical value. An implication of these results is that thermally stable quantum memories have a threshold against decoherence that remains nonzero as one approaches the critical temperature. Analogously, in diffusion models, stability of data distributions implies the existence of computationally-efficent local denoisers in the late-time generation dynamics.
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Quantum Physics · Cross-listed from hep-th

Entanglement asymmetry in gauge theories: chiral anomaly in the finite temperature massless Schwinger model

Adrien Florio and Sara Murciano

The entanglement asymmetry has emerged in recent years as a practical quantity to study phases of matter. We present the first study of entanglement asymmetry in gauge theories by considering the chiral anomaly of the analytically solvable massless Schwinger model at both zero and finite temperatures. At zero temperature, we find the asymmetry exhibits logarithmic growth with system size. At finite temperature, we show that it is parametrically more sensitive to chiral symmetry-breaking than the corresponding local order parameter: while the chiral condensate decays exponentially, the asymmetry decreases only logarithmically. This establishes the entanglement asymmetry as a promising tool to probe (finite-temperature) phase transitions in gauge theories.
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Quantum Physics

Generation and read-out of many-body Bell correlations with a probe qubit

Marcin Płodzień and Jan Chwedeńczuk

As demand for quantum technologies increases, so does the need to generate and classify non-classical correlations in complex many-body systems. We introduce a simple and versatile method for creating and certifying entanglement and many-body Bell correlations. This method relies on a single qubit interacting with an $N$-qubit system. We demonstrate that: (i) such pairwise interaction is sufficient to induce many-body quantum correlations, and (ii) the qubit can serve as a probe to extract all information about these correlations. Thus, single-qubit measurements reveal multi-partite entanglement and $N$-body Bell correlations, enabling the rapid and efficient certification of non-classicality in complex systems.
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Quantum Physics

The Born Ultimatum: Conditions for Classical Surrogation of Quantum Generative Models with Correlators

Mario Herrero-Gonzalez, Brian Coyle, Kieran McDowall et al. (+4 more)

Quantum Circuit Born Machines (QCBMs) are powerful quantum generative models that sample according to the Born rule, with complexity-theoretic evidence suggesting potential quantum advantages for generative tasks. Here, we identify QCBMs as a quantum Fourier model independently of the loss function. This allows us to apply known dequantization conditions when the optimal quantum distribution is available. However, realizing this distribution is hindered by trainability issues such as vanishing gradients on quantum hardware. Recent train-classical, deploy-quantum approaches propose training classical surrogates of QCBMs and using quantum devices only for inference. We analyze the limitations of these methods arising from deployment discrepancies between classically trained and quantumly deployed parameters. Using the Fourier decomposition of the Born rule in terms of correlators, we quantify this discrepancy analytically. Approximating the decomposition via distribution truncation and classical surrogation provides concrete examples of such discrepancies, which we demonstrate numerically. We study this effect using tensor-networks and Pauli-propagation-based classical surrogates. Our study examines the use of IQP circuits, matchcircuits, Heisenberg-chain circuits, and Haldane-chain circuits for the QCBM ansatz. In doing so, we derive closed-form expressions for Pauli propagation in IQP circuits and the dynamical Lie algebra of the Haldane chain, which may be of independent interest.
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All authors Mario Herrero-Gonzalez, Brian Coyle, Kieran McDowall, Ross Grassie, Sjoerd Beentjes, Ava Khamseh, Elham Kashefi

Quantum Physics · Cross-listed from math.PR

Preface to Contextuality in Random Variables: A Systematic Introduction, by E. N. Dzhafarov, J. V. Kujala, and V. H. Cervantes

Ehtibar N. Dzhafarov

This is the preface for the book by E. N. Dzhafarov, J. V. Kujala, and V. H. Cervantes, titled Contextuality in Random Variables: A Systematic Introduction. It is to be published by Cambridge University Press in 2026.