Peer Revenue Impact factor: volatile

Papers

Fresh from arXiv, typeset properly. Highlighted words trade on the market.

Physics Quantum Physics

quant-ph Watch Quantum Physics

Wednesday 20 May

Quantum Physics

Optimal Quantum Feshbach Engines

Aaron Wandhammer, Vincent Hardel, Paul-Antoine Hervieux et al. (+1 more)

We develop an optimization framework for high-efficiency quantum cycles implemented with a trapped Bose-Einstein condensate, whose control parameters are the trap stiffness and the interaction strength tuned via a Feshbach resonance. Optimal driving protocols for each stroke of the cycle are obtained from a variational description of the condensate dynamics combined with Nelson's stochastic quantization, which maps the quantum evolution onto an effective Ornstein-Uhlenbeck process. The optimal protocol is obtained by minimizing a user-defined cost functional that selects the best trade-off between protocol duration and arbitrary physical constraints (such as the expended work or the proximity to an adiabatic evolution), and exhibits remarkable stability over repeated cycles. The method also provides a systematic route to optimal control for generic nonlinear Schrödinger equations, paving the way to optimal control strategies in fields as diverse as nonlinear optics, quantum fluids, and quantum plasmas.
Read the full abstract

All authors Aaron Wandhammer, Vincent Hardel, Paul-Antoine Hervieux, Giovanni Manfredi

Quantum Physics

An Exponential Sample-Complexity Advantage for Coherent Quantum Inference

Zhaoyi Li, Elias Theil, Aram W. Harrow et al. (+1 more)

Standard quantum inference converts quantum data into classical outputs. We study an alternative inference setting in which the desired output is quantum. Such settings include quantum purity amplification (QPA), random purification and approximate cloning, and density matrix exponentiation. We show that such protocols can achieve exponentially lower sample complexity than incoherent, measurement-mediated protocols, establishing, to the best of our knowledge, the first nontrivial separations between dimension-independent coherent and dimension-dependent incoherent sample complexity. For QPA with principal-eigenstate targets and $d$-dimensional inputs, at fixed output number and spectral gap, coherent processing achieves error $\varepsilon$ using $O(1/\varepsilon)$ copies, whereas any incoherent protocol requires $\Omega(d/\varepsilon)$ copies. Together, these separations motivate a unified picture of coherent quantum inference, in which an entanglement-breaking limit identifies the optimal incoherent counterpart.
Read the full abstract

All authors Zhaoyi Li, Elias Theil, Aram W. Harrow, Isaac Chuang

Quantum Physics

Combining non-parametric quantum states and MERA tensor networks for ground-state optimization

Julian Schuhmacher, Alberto Baiardi, Francesco Tacchino et al. (+1 more)

Hybrid tensor networks offer a promising route to enhance the expressivity of classical tensor network methods by incorporating quantum states prepared on a quantum computer. Existing approaches are limited by the variational optimization of the quantum component of the tensor network. In this work, we introduce an alternative strategy that combines a non-parametric quantum state prepared through quantum annealing and a classical isometric tensor network. The latter is variationally optimized while the former is used as a fixed, boundary tensor resource in the form of classical shadows. We demonstrate the feasibility of this approach through extensive numerical simulations on the transverse-field Ising model, showing that the optimization procedure remains robust under statistical and hardware noise. Moreover, our results indicate that our newly proposed setup improves the accuracy of the obtained ground state approximation compared to the original quantum simulation, without increasing the depth of the applied quantum circuits. Therefore, this setup offers a practical route to scale variational quantum algorithms towards the quantum utility scale.
Read the full abstract

All authors Julian Schuhmacher, Alberto Baiardi, Francesco Tacchino, Ivano Tavernelli

Quantum Physics

Modeling and Resource Optimization for Quantum Oracles

Zhihang Li, Bo Zhao, Chuanbing Han et al. (+6 more)

Quantum oracles are fundamental building blocks of many quantum algorithms, and their resource consumption directly affects performance, yet structured description and complexity analysis for their composition are still lacking. In this paper, we introduce the Framework for Oracle Recursion Modeling (FORM), a unified formal abstraction of multi-function composition in quantum oracles: it provides a structured description of the composition layer, makes its gate complexity exactly computable, and turns oracle design into an optimizable tree-construction problem. Based on this model, we propose the ShallowGrow algorithm, which constructs an oracle structure under a given ancilla budget and provably minimizes the number of function evaluations. On Boolean quadratic equation systems, ShallowGrow reduces Qiskit-measured circuit depth by 54.1% on average relative to the state-of-the-art W-cycle construction, with consistent reductions on the EPFL and ISCAS85 combinational logic networks under scarce ancilla budgets. Furthermore, pebbling-based syntheses trade space against time within the logic network of a function; ShallowGrow extends this trade-off across functions, and integrated with their published circuits it reduces the ancillary qubits of a complete oracle from one per constraint function to logarithmically many. With half as many ancillas as constraint functions, the T-count falls by a factor of 7.9 to 32 relative to the W-cycle-based construction.
Read the full abstract

All authors Zhihang Li, Bo Zhao, Chuanbing Han, Jie Zhao, Jinchen Xu, Guoqiang Shu, Yimin Gao, Woji He, Zheng Shan

2605.21380 · 20 May · v2: substantial revision. Renamed HRSE model to FORM (Framework for Oracle Recursion Modeling) and ASDT algorithm to ShallowGrow. Expanded experiments to EPFL and ISCAS85 combinational benchmarks and to complete oracle construction with pebbling-based leaf syntheses (LHRS, Zhang et al.). Rewrote Section V with new tables and figures. 14 pages, 6 figures, 5 tables

Abstract PDF $QNTM ×4$ALGO ×2$CRCT ×2$QUBT ×1

Quantum Physics · Cross-listed from math-ph

Essentially singular limits of Jacobi operators and applications to higher-order squeezing

Felix Fischer, Daniel Burgarth and Davide Lonigro

We study a family of Jacobi operators in which the diagonal entries are multiplied by a coupling parameter $\lambda\geq0$. Under suitable conditions, the operator is self-adjoint for every $\lambda>0$, while the formal limit at $\lambda=0$ is a symmetric Jacobi operator admitting a one-parameter family of self-adjoint extensions. A central ingredient of our analysis is the derivation of uniform bounds for square-summable generalized eigenvectors in the small-$\lambda$ regime, which combines discrete WKB methods with Airy-function asymptotics. Using these estimates, we analyze the limiting behavior $\lambda\to0$ in the strong resolvent sense, proving that for every sequence $\lambda_j\to0$ one can extract a subsequence along which the corresponding Jacobi operators converge to some self-adjoint extension of the limiting operator; conversely, every such extension can be obtained in this way. We call this behavior an essentially singular limit, by analogy with essential singularities in complex analysis. As an application, we study higher-order squeezing operators arising in quantum optics. Using the connection with Jacobi operators, we show that when the relative strength of the free-field term tends to zero, different self-adjoint extensions of the squeezing operator are selected along different sequences. In particular, this limit does not single out a physically distinguished self-adjoint extension, but instead identifies a distinguished subclass of extensions compatible with the underlying symmetry.
Read the full abstract

Quantum Physics

Evidence of Quantum Machine Learning Advantage with Tens of Noisy Qubits

Onur Danaci, Yash J. Patel, Riccardo Molteni et al. (+3 more)

Learning problems involving quantum data are natural candidates for demonstrating an advantage in quantum machine learning. Recent results indicate that, for certain tasks and under noiseless conditions, coherent processing of quantum data outperforms fixed-measurement schemes followed by classical processing. It remained uncertain whether this performance gap persists at a finite scale, and in the presence of noise that is unavoidable with current quantum devices. In this work, we present simulations and analysis of the performance of existing hardware on a learning problem known to exhibit asymptotic advantage, now subjected to noisy quantum data. Comparing coherent quantum processing directly against fixed-measurement schemes, our results demonstrate a clear performance separation at a scale of just 30 to 40 noisy qubits. Already at this scale, the fundamental bottleneck is no longer classical computation but data acquisition; matching the noisy coherent protocol with measure-first strategies would still require months or even years of measurements. By systematically evaluating hardware constraints such as state preparation, gate errors, readout errors, connectivity, and coherence times, we provide evidence that a demonstration of such a strong learning advantage is accessible on near-term devices.
Read the full abstract

All authors Onur Danaci, Yash J. Patel, Riccardo Molteni, Evert van Nieuwenburg, Vedran Dunjko, Jan A. Krzywda

Quantum Physics

Quantum Nonlocality and Device-Independent Randomness are Robust to Noisy Signaling Channels

Kuntal Sengupta and Lewis Wooltorton

Given a pair of isolated devices that accept random binary inputs and return binary outputs, a user can deduce from the observed data alone if the underlying mechanism can be explained classically. Bell's theorem further states that a classical explanation can be ruled out if the devices perform certain measurements on an entangled quantum state, underpinning the security of cryptographic protocols that are device-independent (DI). For certain protocols, such as those performed in a tight space, it might be difficult to perfectly enforce the non-signaling assumption required in Bell's theorem. This prompts the question: is quantum nonlocality robust to such imperfections? We show that if a binary channel sends a noisy copy of one party's input to the other before any measurements take place, the answer is yes. We completely characterize the vertices and facets of the local polytope in this scenario, and identify Bell inequalities that certify non-signaling quantum correlations. This is possible even when a near perfect copy of the input is sent. We go on to show that the identified inequalities are more robust to depolarizing noise than the Clauser-Horne-Shimony-Holt inequality when certifying DI randomness in this scenario. In addition, we characterize the local polytope when both parties receive a noisy copy of each other's input and make similar conclusions, leaving many new potential Bell inequalities to be explored.
Read the full abstract

Quantum Physics

Software Between Quantum and Machine Learning – And Down to Pulses

Maja Franz, Melvin Strobl, Jonathan Hunz et al. (+5 more)

Contemporary quantum computing platforms remain, in essence, programmable physical systems whose control is typically mediated through unitary gate abstractions. While such abstractions provide a uniform interface, they obscure important aspects of the underlying hardware and may limit the exploitation of its full capabilities. Direct operation at the control-pulse level offers a more expressive and physically faithful paradigm, enabling, for instance, the implementation of tailored error-mitigation and optimisation strategies. However, this increased expressivity comes at the cost of greater quantum software development complexity, necessitating structured and accessible tooling. We present a software framework, integrated within the QML-Essentials package, that extends quantum machine learning (QML) methodologies to encompass pulse-level modelling. By embedding quantum optimal control techniques within a QML setting, our approach enables the seamless combination of gate-based and pulse-level representations. The framework provides a comprehensive suite of modelling and analytical capabilities. In particular, we introduce composable ansatz constructions based on interchangeable building blocks, and support for end-to-end optimisation of pulse parameters. Motivated by the central role of quantum Fourier models, we further incorporate a range of Fourier-analytic diagnostics, complemented by extended measures of entanglement. All performance-critical components are implemented in a high-performance environment using JAX and supported by a dedicated quantum simulator. Taken together, the framework facilitates reproducible and systematic investigations, while bridging the conceptual and practical divide between abstract circuit models and hardware-aware optimisation. It provides a robust foundation for future developments at the intersection of QML and quantum control.
Read the full abstract

All authors Maja Franz, Melvin Strobl, Jonathan Hunz, Lukas Scheller, Lucas van der Horst, Eileen Kuehn, Achim Streit, Wolfgang Mauerer

Quantum Physics

Benchmarking a machine-learning differential equations solver on a neutral-atom logical processor

Pauline Mathiot, Elio Garnaoui, Axel-Ugo Leriche et al. (+37 more)

We report on a performance comparison between physical and logical computations on a prototypical machine-learning application: solving differential equations using quantum kernel methods. The algorithm is implemented on an atom-based logical quantum processor, both at the physical and logical levels. We show that the kernel estimated from the logical implementation performs better than its physical counterpart on relevant metrics. We observe how such performance improvement can be traced back to specific noise-induced errors detected by the chosen encoding. We apply the computed quantum kernel to the task of solving differential equations, confirming how the superior performance of a logical quantum kernel is retained also at an end-to-end applicative level. Our findings show that experimental validation of end-to-end protocols can already highlight the positive impact of fault-tolerant implementations despite their higher quantum resource count, and guide application-informed architectural choices.
Read the full abstract

All authors Pauline Mathiot, Elio Garnaoui, Axel-Ugo Leriche, Evan Philip, Boris Albrecht, Clémence Briosne-Fréjaville, Lorenzo Cardarelli, Antoine Cornillot, Gwennolé Cournez, Luc Couturier, Julius De Hond, Rebecca El Koussaifi, Thomas Eritzpokoff, Florian Fasola, Antonio Andrea Gentile, Casper Gyurik, Clotilde Hamot, Loïc Henriet, Gaétan Hercé, Michael Kaicher, Lucas Lassablière, François-Marie Le Régent, Edgar Leroux, Yohann Machu, Hadriel Mamann, Luis Ortiz, Annie Paine, Thomas Pansiot, Arnaud Peloquin, Francisco Ponciano, Julien Ripoll, Raja Selvarajan, Adrien Signoles, Henrique Silvério, Siddhy Tan, Marie Taouzinet, Selim Touati, Louis Vignoli, Antoine Browaeys, Pascal Scholl

Quantum Physics

Semidefinite Programming for Optimal Quantum Cloning: A Computational Framework

Jörg Hettel

While algebraic derivations establish theoretical limits for quantum cloning, practical implementations require explicit operator representations that are often unavailable analytically. We present a computational framework that reformulates cloning optimization as a search over completely positive trace-preserving maps using the Choi-Jamiolkowski isomorphism and Semidefinite Programming. The framework (i) numerically certifies global optimality through primal-dual strong duality and (ii) automatically extracts operational Kraus operators from the optimal Choi matrix via spectral decomposition. We systematically treat universal, phase-covariant, asymmetric, and entanglement cloning scenarios, providing -for the first time - a unified computational catalogue of explicit, implementable Kraus representations across all major cloning families, including higher-order processes and arbitrary input state distributions. As an application, we analyse optimal cloning attacks on BB84 under depolarizing noise, demonstrating how the extracted operators enable quantitative security analysis in realistic noisy quantum channels. An open-source implementation enables community validation and extension.
Read the full abstract

Quantum Physics · Cross-listed from physics.atom-ph

Non-equilibrium exciton dynamics in tailored molecular potentials of Rydberg ion crystals

Simon Euchner, Mathias B. M. Svendsen and Igor Lesanovsky

Trapped ions excited to high-lying electronic states combine strongly coupled collective vibrational and electronic degrees of freedom with long-ranged interparticle interactions. These ingredients enable the quantum simulation of biochemical processes, associated with the dynamics of excitons in non-perturbative parameter regimes. The key feature of such a quantum simulator are electronic-state-dependent molecular potential surfaces which can be strongly coupled. This allows to shed light on a variety of mechanisms underlying exciton transport. We illustrate this in a system of three trapped ions, which is amenable to an ab initio treatment. Given that ion traps can be routinely prepared with hundreds of ions, these quantum simulators can immediately realise scenarios which are inaccessible by current numerical methods.
Read the full abstract

Quantum Physics

Boundary Geometry Turns Entanglement into Steering

Yu-Xuan Zhang and Jing-Ling Chen

Quantum entanglement does not necessarily imply Einstein-Podolsky-Rosen steering. We identify a boundary mechanism that closes this gap when an entangled state meets the boundary of the trusted state space in a nondegenerate way. The mechanism is local: a projective assemblage that approaches a boundary contact with first-order tangential coherence but only second-order inward defect cannot be reproduced by any finite-measure local-hidden-state model. In finite dimensions this yields a support-kernel criterion for an arbitrary fixed steering cut: if a trusted conditional state is rank-deficient and its first-order block couples its support to its kernel, then the state is both NPT (negative partial transpose) and projectively steerable. The two-qubit product-null sector provides the minimal transparent realization. A product vector in the kernel gives the pure boundary contact, and a single coherence entry simultaneously serves as the tangential displacement, the NPT minor, and the steering obstruction. Hence, on this boundary, entanglement collapses to two-way projective steering – including the genuinely mixed rank-three branch and the rank-two case as a low-dimensional instance. We also provide a filtered standard form, an explicit rank-three Cholesky parametrization, and a compact boundary certificate.
Read the full abstract

Quantum Physics

Measurement of quantum states and nonlocal correlations

Grgeory D. Scholes

One of the major remaining questions concerning foundations of the quantum mechanical theory is how superpositions are measured such that definite outcomes are obtained. Here a resolution to that issue is proposed. It is shown how to obtain state vectors associated with measurements on the separated subystems of an entangled state, revealing how a single wavefunction encodes a special kind of set of statistical measurement outcomes. The result explains why measurements on the subsystems give definite outcomes and why measurements on one subsystem are correlated with those on the other. It is therefore concluded that the theory of quantum mechanics, without nonlinearities or ad hoc assertions, can explain both the mechanism of state vector collapse and the reason for the paradoxical nonlocal correlations between separated subsystems. The theory also explains why nonlocal correlations do not require any kind of interaction between the subsystems.
Read the full abstract

Quantum Physics

High-order harmonic generation from an atom in a disordered environment

Simon His, Camille Lévêque, Jérémie Caillat et al. (+2 more)

Using one-dimensional simulations analyzed through the lens of open quantum systems, we study the photoelectron's strong-field dynamics from an atom surrounded by a scattering environment stochastically structured. We theoretically investigate high-order harmonic generation from this situation. We show that local dephasing of the photoelectron wavepacket induced by elastic scattering leads to global decoherence. This drives a transition from quantum to classical behavior, as witnessed by the photoelectron probability density localizing around specific trajectories of the classical analog system: unstable periodic orbits. This phenomenon mirrors quantum scars traditionally observed in the eigenfunctions of time-independent systems, such as quantum billiards. Here, it emerges in-situ within a time-dependent framework, manifesting directly in the real-time dynamics from the ground state rather than solely through spectral analysis.
Read the full abstract

All authors Simon His, Camille Lévêque, Jérémie Caillat, Richard Taïeb, Jonathan Dubois

Quantum Physics

Enhanced quantum metrology by criticality-assisted noncommutative preparation

Ningxin Kong, Matteo G. A. Paris and Qiongyi He

Quantum criticality is a resource for quantum-enhanced metrology, but existing schemes face intrinsic limitations. These arise because using criticality directly in the encoding dynamics restricts the accessible parameters to those explicitly supported by the critical Hamiltonian, and the requirement for critical conditions narrows the effective estimation range. To solve this, we introduce a general framework termed criticality-assisted noncommutative preparation (CANP). In this approach, critical evolution is employed as a state-preparation resource. We establish the underlying algebraic conditions and show that the intrinsic noncommutativity between the preparation and encoding operations leads to a genuine enhancement of the quantum Fisher information (QFI). Remarkably, this enhancement may be achieved at fixed total sensing time and energy cost. The effect is quantified by the Wigner-Yanase skew information, which measures noncommutativity and exhibits the same critical scaling as the QFI. We demonstrate effective use of CANP in the quantum Rabi and Lipkin-Meshkov-Glick models. Our results establish CANP as a robust technique to effectively implement criticality-enhanced quantum metrology.
Read the full abstract

Quantum Physics

Enhanced Reinforcement Learning-based Process Synthesis via Quantum Computing

Austin Braniff, Fengqi You and Yuhe Tian

In this work, we present quantum reinforcement learning (RL) as a solution strategy for process synthesis problems. Building on our prior work, we develop a generalized framework that formally poses process synthesis as a Markov decision process and introduces quantum-enhanced RL algorithms to solve it with improved scalability. Earlier implementations of quantum-based RL for process synthesis were limited by qubit requirements, which scaled poorly with problem complexity. This work overcomes this challenge by introducing state encoding algorithms to decouple qubit requirements from problem size. A classical RL-based solution strategy is used as a baseline to benchmark the quantum algorithms under identical training conditions. All algorithms are evaluated across a flowsheet synthesis problem of increasing unit counts to analyze their performance and scalability. Results show that all approaches are capable of identifying the optimal flowsheet designs in small design spaces. For moderate-scale unit counts, quantum approaches demonstrate competitive performance on a per-episode basis and improved efficiency on a per-parameter basis versus the classical RL benchmark. This work provides a foundation for future quantum computing applications within process systems engineering, establishes a controlled benchmark for comparing classical and quantum algorithms, and shows that the proposed quantum variants remain competitive for the process synthesis problem examined in this work.
Read the full abstract

Quantum Physics

Velocity-Controlled Directional Readout of Single Photons

Mohamed Hatifi

Photodetection is usually treated in the frame in which the detector is at rest relative to the optical apparatus. We show that uniform motion of an electric Glauber detector changes the single-click POVM realized on two counterpropagating single-photon modes. Motion Doppler-shifts the alternatives in the detector frame; finite bandwidth then converts propagation direction into a detection bias without decohering the photon. For a Lorentzian response near one Doppler branch, the readout crosses from phase-sensitive to direction-sensitive with a quality-factor-enhanced onset. Finite-time integration adds Doppler-beat visibility loss, separating passive covariance from measurement change.
Read the full abstract

Quantum Physics

PIQC: Scalable Distributed Quantum Computing via Photonic Integration of Designed Molecular Quantum Nodes

Anna Aubele, Gregor Bayer, Tim R. Eichhorn et al. (+16 more)

There is a growing consensus that large-scale, fault-tolerant quantum computing (FTQC) necessitates high-fidelity photonic interconnects to overcome the scaling limits of monolithic architectures. However, most current platforms were not originally designed for native photonic connectivity and require significant engineering overhead. To overcome these fundamental hardware limitations, we recently introduced a rationally designed organic molecule that serves as an ideal quantum node, featuring a robust qubit-photon interface (QPI) and a long-lived nuclear-spin register. In this work, we present PIQC (Photonic Integrated Quantum Circuits), a distributed architecture designed to scale these molecular nodes into a functional quantum computer. The PIQC framework integrates five mutually reinforcing innovations: (i) Designer molecular qubits, i.e. carbene molecules in an isosteric host that provide millisecond-coherence electron spins with high spectral stability and spin-dependent optical emission, (ii) deterministic nuclear registers made of synthetically placed $^{13}$C or $^{14}$N labels that enable fast ($\sim 1~\mu$s), high-fidelity electron-nuclear gates, (iii) hybrid photonic integration, which allows molecular films to seamlessly integrate with existing mature fabrication technologies, e.g. thin-film lithium niobate (TFLN), (iv) heralded entanglement protocols that can tolerate up to 70% photon loss, and (v) stairway Floquetification, i.e. high-rate quantum low-density parity-check (qLDPC) codes that are converted into Floquet codes, reducing syndrome extraction to weight-two Bell-pair measurements that match PIQC's networked hardware. PIQC offers a hardware-efficient, commercially viable pathway toward a utility-scale quantum computer based on distributed FTQC.
Read the full abstract

All authors Anna Aubele, Gregor Bayer, Tim R. Eichhorn, Tobias Hahn, Fedor Jelezko, Paul Mentzel, Philipp Neumann, Matthias Pfender, Martin B. Plenio, Alex Retzker, Simon Roggors, Alon Salhov, Jochen Scharpf, Tobias A. Schaub, Nico Striegler, Thomas Unden, Julia Zolg, Sella Brosh, Ilai Schwartz

Quantum Physics · Cross-listed from cs.LG

Q-SYNTH: Hybrid Quantum-Classical Adversarial Augmentation for Imbalanced Fraud Detection

Adam Innan, Mansour El Alami, Nouhaila Innan et al. (+2 more)

Credit card fraud detection is fundamentally challenged by extreme class imbalance, where fraudulent transactions are rare yet operationally critical. This imbalance often biases supervised learners toward the legitimate class, leading to high overall accuracy but weaker fraud-class recall and F1-score. This paper introduces Q-SYNTH, a hybrid classical–quantum generative adversarial framework in which a parameterized quantum circuit serves as the generator and a classical neural network serves as the discriminator. Q-SYNTH is designed for minority-class fraud synthesis in tabular data and is evaluated along two dimensions: statistical fidelity to real fraud samples and downstream performance for fraud detection. To this end, generated samples are assessed using distributional similarity measures based on Kolmogorov-Smirnov statistics and Wasserstein distances, real-vs-synthetic detectability measured by AUC-ROC, and downstream classification performance across both quantum and classical classifiers. Under the reported protocol, Q-SYNTH reduces marginal distribution mismatch relative to a classical GAN baseline while maintaining competitive downstream fraud-detection performance. Although SMOTE achieves the strongest feature-wise similarity and the classical GAN attains the highest downstream performance in several settings, Q-SYNTH offers a favorable compromise between distributional fidelity and downstream performance, supporting the feasibility of hybrid quantum augmentation for imbalanced fraud detection.
Read the full abstract

All authors Adam Innan, Mansour El Alami, Nouhaila Innan, Muhammad Shafique, Mohamed Bennai

Quantum Physics

Optimization of Secret Key Rate for BB84 under Collective Rotation Noise

Wajiha Masood and Afshan Irshad

Practical quantum key distribution (QKD) systems operate under noise, but security of most protocols have been analyzed under ideal noiseless scenarios. In this work, we investigated security performance of BB84 protocol under effect of collective rotation noise. Using theoretical quantum information frameworks, we analyzed key security parameters including quantum bit error rate (QBER), mutual information and secret key rate (SKR). Security of protocol is studied under various eavesdropping scenarios based on intercept and resend attacks. Our results show that collective rotation noise has a significant impact on the information shared between the two parties. Particularly, we extended prior treatments by suggesting a noise engineering strategy where we identified a non-zero noise range where information accessed by Eve is minimized while corresponding SKR degradation remains relatively small. This analysis provide insights into robustness of BB84 protocol under realistic noisy channels and may contribute towards development of more resilient QKD systems.
Read the full abstract

Quantum Physics

Towards transistor-based quantum computing

Y.-D. Liu, X. Xu, Q.-R. Wang et al. (+1 more)

In this work, we propose and study in depth a universal quantum computing architecture based on a quantum construction of transistors. Our teleportation-based quantum transistors, called “telesistors”, are ground states of systems with symmetry-protected topological order, hence suppress certain noises and provide high-fidelity Clifford gates without the need for active error correction. This physical protection, quantified by the string order parameters, serves as a low-overhead foundation upon which conventional fault-tolerant encoding (e.g., with stabilizer codes) can be built to achieve universal quantum computation. This architecture shows rich connections with current known architectures, and some desirable merits especially compared with the qubit-based circuits regarding modularity, integration, and program storage. Our study shows that it is plausible to realize it with current technology in the near future.
Read the full abstract

All authors Y.-D. Liu, X. Xu, Q.-R. Wang, D.-S. Wang

Quantum Physics

Quantum theory of a three-photon Kerr parametric oscillator

Alessandro Bruno, Patrick P. Potts, Alexander Grimm et al. (+1 more)

We investigate the quantum properties of a nonlinear Kerr oscillator driven by a three-photon pump. We derive both exact and approximate analytical expressions for the ground state of this interacting model. The exact solution arises at an exact spectral degeneracy, while the approximate solution describes regimes of quasi-degeneracy of the energy spectrum. In both cases, the threefold (quasi)degenerate ground-state manifold consists of quantum superpositions of three macroscopically distinct states. These states differ qualitatively from conventional three-component Schrödinger's cat states due to the presence of squeezing with a distinctive parametric dependence. By varying the detuning between the oscillator and the three-photon pump, we show that the squeezing can be enhanced, suppressed, or even reversed, leading to a squeezing-to-anti-squeezing transition. We analyze the generation and stabilization of these superposition states, their robustness against perturbations and analytically quantify the leakage to excited states. Our analysis elucidates how the three-photon Kerr parametric oscillator can be used to encode a Kerr-cat qutrit protected against phase-flip errors.
Read the full abstract

All authors Alessandro Bruno, Patrick P. Potts, Alexander Grimm, Matteo Brunelli

Quantum Physics

Impurity-induced loss bursts from anomalous scale-free localization in a non-Hermitian dissipative lattice

Hui Liu and Zhihao Xu

We identify anomalous scale-free localization and associated impurity-induced loss bursts in a non-Hermitian dissipative cross-stitch lattice. By a local basis rotation, the model is mapped onto an effective non-Hermitian Su-Schrieffer-Heeger lattice, where local impurities act as tunable effective boundaries. For the parameter choice considered here, the impurity-free periodic-boundary-condition point is reached at $\eta=1$. Tuning $\eta$ away from this point drives the system through generalized-boundary-condition regimes toward two effective open-boundary-condition-like limits at $\eta\to0$ and $\eta\to\infty$. For finite $\eta\notin\{0,1\}$, the loop-sector spectra remain separated from the real-energy axis, while the corresponding eigenstates exhibit scale-free localization pinned by the impurity. Unlike conventional impurity-induced scale-free localization, the Lyapunov exponent depends explicitly on the eigenenergy, making the localization strength eigenstate dependent. We further show that this anomalous eigenmode structure produces an impurity-induced loss burst: The long-time integrated dissipation probability develops a pronounced local enhancement relative to the surrounding background near an impurity-generated effective boundary, even when the initial wave packet is prepared far away. In the single-impurity case, the burst region consists of the impurity site and its adjacent effective-boundary site, and the effect occurs without imaginary-gap closing. For multiple impurities, local burst regions emerge around all impurities, while the dominant burst boundary is selected by the initial wave-packet position and the nonreciprocal drift direction. These results connect anomalous scale-free localization with controllable dissipation dynamics in non-Hermitian lattices.
Read the full abstract

Quantum Physics

Photon Efficiency of High-Dimensional Quantum Key Distribution

Vera Uzunova and Marcin Jarzyna

We investigate entanglement-based quantum key distribution protocols, with particular emphasis on their efficiency under realistic conditions of satellite quantum communications, where performance is limited by the low power of a received signal and background radiation. We focus on scenarios where each photon pair is used to encode multiple qubits in order to optimally utilize the weak signal. By optimizing over the source intensity and the number of encoded qubits we study the theoretical information limit for the QKD efficiency. We show that the optimal efficiency is attained for finite entangled photons pair production probability which is in contrast to conventional communication efficiency maximized in the limit of vanishing signal strength. The multiqubit encoding can enhance the secret key rate by up to an order of magnitude compared to single-qubit schemes.
Read the full abstract

Quantum Physics · Cross-listed from hep-th

Wasserstein Space of Quantum Chaos

Koji Hashimoto, Norihiro Tanahashi and Kentaroh Yoshida

We find that the effective dimension of the Wasserstein space of energy eigenstates decreases as a quantum system becomes more chaotic. To demonstrate this, we study a quantum coupled harmonic oscillator system using Husimi Q-representations, to which Sinkhorn-regularized optimal transport is applied to construct an embedding geometry via the Gram-spectrum method. We also demonstrate that exponential OTOC growth, referred to here as quantum scrambling even in the absence of chaos, induces a folding structure in the emergent Wasserstein space, which may underlie the chaotic reduction of the Wasserstein dimension. At the separatrix (the scrambling point) of the inverted harmonic oscillator, the Wasserstein distance correctly captures the Lyapunov exponent. Furthermore, we discover that a branching structure in the Wasserstein space signals quantum scar states within the chaotic sea of phase space. Our optimal transport approach thus provides a new diagnostic for quantum chaos, quantum scrambling, quantum scars, and quantum Lyapunov exponents. The observed chaotic dimensional reduction also supports the recent conjecture [arXiv:2604.17649] that the Wasserstein space serves as an emergent holographic space through the manifold hypothesis, since chaoticity is a characteristic signature of black holes in holography.
Read the full abstract

Quantum Physics

Ergotropy and Work Extraction in Quantum Heat Engines via Quantum Channels

Indrajith VS and Disha Verma

This paper explores quantum heat engines based on qubit and qutrit working media interacting with thermal environments through generalized amplitude damping (GAD) channels. We investigate how quantum channels can be employed to model heat absorption, dissipation, and work extraction in open quantum thermal machines, and derive the conditions required for positive work extraction. The effects of quantum correlations, emission probability, population redistribution, and system–environment interactions on the thermodynamic performance of the engine are systematically analyzed across different operational regimes. In addition, we examine the ergotropy of qubit and qutrit systems under dissipative dynamics to understand how environmental effects influence the maximum extractable work. Our results demonstrate that multilevel quantum systems exhibit enhanced work extraction capability and improved robustness against decoherence compared to two-level systems, providing further insight into the role of dissipative dynamics and quantum resources in realistic quantum thermodynamic devices.
Read the full abstract

Quantum Physics

High-Dimensional Carrier-Assisted Entanglement Purification Based on Mutually Unbiased Bases

Zihua Song, Lin Chen and Yongge Wang

Distilling high-dimensional entanglement under general asymmetric Pauli noise remains challenging. For the unprocessed qutrit single-carrier recurrence studied here, severe noise asymmetry can prevent the convergence condition from being satisfied. In this paper, we investigate carrier-assisted entanglement purification protocols, namely CAEPP and mCAEPP, first for two-qutrit systems, and show that the unprocessed single-carrier recurrence is bottlenecked by marginal $X$-error probabilities. To overcome this limitation, we introduce MUB-guided preprocessing that combines randomized inversion-pair symmetrization with deterministic maximum-line alignment. For any qutrit Pauli channel with initial fidelity $p_{00}>1/3$, we prove that, conditioned on successful syndrome outcomes, the MUB-adapted recurrence converges for every sufficiently large fixed carrier number and that its fixed-point fidelity approaches one in the subsequent large-carrier limit. We further extend the algebraic carrier-assisted framework and the asymmetric-noise bottleneck to arbitrary qudit dimensions, and show that in prime-power dimensions the finite-field preprocessing gives the sufficient threshold $p_{00}>1/d$. A certified equal-target comparison over a finite deterministic optimization domain further shows that neither the MUB-adapted strategy nor complete Pauli-label twirling uniformly dominates the other in finite-resource cost.
Read the full abstract

Quantum Physics

Symmetry-Induced Relaxation Comb and Strong Quantum Mpemba Effect in Long-Range XXZ Spin Chains

Zijun Wei, Mingdi Xu, Yefeng Song et al. (+3 more)

We uncover a symmetry-filtered mechanism for anomalous dissipative relaxation in a long-range XXZ spin chain subject to local dephasing. At the isotropic point, the coherent Hamiltonian has global $SU(2)$ symmetry, whereas the full Liouvillian retains only the $U(1)$ symmetry associated with total magnetization. This structure pins a family of spatially uniform zero-$U(1)$-charge left eigenoperators with exact eigenvalues $\lambda=-2q$, forming a Liouvillian relaxation comb. For the ferromagnetic Dicke ground state, the overlap envelope on this comb is known exactly at finite size and becomes Gaussian in the large-$S$ limit. Since higher-$q$ components decay rapidly, the $q=1$ comb tooth controls the long-time dynamics and yields universal $D(t)\sim e^{-2t}$ relaxation independent of system size and interaction range. This mode-accessibility filtering realizes a spectral strong quantum Mpemba effect: an initially farther state relaxes faster than closer thermal states because slow non-steady Liouvillian modes are inaccessible. Weak breaking of the Hamiltonian $SU(2)$ symmetry restores slow-mode overlap and suppresses this acceleration.
Read the full abstract

All authors Zijun Wei, Mingdi Xu, Yefeng Song, Xiang-Ping Jiang, Yangqian Yan, Lei Pan

Quantum Physics · Cross-listed from hep-th

Chaos-Integrability Transition in the BPS Subspace of the $\mathcal{N}=2$ SYK Model

Leon Miyahara and Shono Shibuya

We study chaos-integrability transition purely within a BPS subspace of a specific supersymmetric model that interpolates between the chaotic $\mathcal{N}=2$ SYK model and an integrable $\mathcal{N}=2$ "commuting" SYK model. Using the framework of BPS chaos, we analyze the spectrum of an operator projected onto the BPS subspace. We numerically find that its spectral statistics exhibit random-matrix behavior near the SYK limit and smoothly transitions to Poisson statistics near the integrable limit. Our results provide a direct example of a chaos-integrability crossover diagnosed solely from BPS states.
Read the full abstract

Quantum Physics

Symmetric dilations of Pauli channels and semigroups

Marco Cattaneo

We explore the symmetry properties of Stinespring dilations of single-qubit Pauli channels, addressing both the generic case and the specific examples of phase damping and depolarizing channels. For each scenario, we derive the representation of the Pauli group acting on the Hilbert space of the environment. We then focus on dilations that are continuous in time and driven by a time-independent Hamiltonian, and on collision models that generate a Pauli dynamical semigroup in the limit of fast collisions. First, we complement some recent general results on these types of dilations (M. Cattaneo, Phys. Rev. A 111, 022209 (2025)) with some additions and clarifications, including the case of covariant channels with strongly conserved quantities. Next, we show that the covariance property of Pauli channels impose strong constraints on both the dilation Hamiltonian and the initial state of the environment, and demonstrate how these constraints can be exploited to explicitly construct the time-dependent dilations in all considered cases. Our results are relevant for the quantum simulation of Pauli channels via unitary dilations and of Pauli semigroups via collision models, both in the laboratory and on quantum computers.
Read the full abstract