Quantum Physics
Eve's forgery probability from her false acceptance probability: interactive authentication, Holevo information and the min-entropy
Pete Rigas
We obtain estimates for Eve's forgery probability, namely the probability that she is able to forge a message which Alice or Bob mistakenly accept over a noisy
Quantum channel for generating a shared
Quantum secret key. This probability is related to Eve's success probability obtained in a previous work due to Renner and Wolf, which was obtained from assumptions on the min-
entropy for characterizing asymmetric security. To demonstrate that protocols over noisy
Quantum channels are dependent upon a single, unified security threshold in comparison to multiple security parameters in the Renner-Wolf interactive authentication protocol framework we upper bound Eve's forgery probability with a Holevo-type quantity that can be made negligibly small. By leveraging estimates for Eve's false acceptance probability that have previously been obtained by the author, we obtain the desired security threshold by bounding the false acceptance probability with a suitably chosen two-universal function which serves as a counterpart to two-universal hashing functions that have previously been examined for cryptographic protocols in
Quantum key distribution. As a result the protocol is not only $\epsilon$-secure, for some $\epsilon>0$, but also composable against forgery and key leakage.
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Quantum Physics
Quantum Deep Learning: A Comprehensive Review
Yanjun Ji, Zhao-Yun Chen, Marco Roth et al. (+10 more)
Quantum deep learning (QDL) explores the use of both
quantum and
quantum-inspired resources to determine when deep learning's core capabilities, such as expressivity, generalization, and scalability, can be enhanced based on specific resource constraints. Distinct from broader
quantum machine learning, QDL emphasizes compositional depth at the pipeline level and the integration of
quantum or
quantum-inspired components within end-to-end workflows. This review provides an operational definition of QDL and introduces a taxonomy comprising four primary paradigms: hybrid
quantum-classical models,
quantum deep neural networks,
quantum algorithms for deep learning primitives, and
quantum-inspired classical
algorithms. Theoretical principles are connected to advanced architectures, software toolchains, and experimental demonstrations across superconducting, trapped-ion, photonic,
semiconductor spin, and neutral-atom systems, as well as
quantum annealers. Claims of
quantum advantage are critically assessed by distinguishing provable complexity-theoretic separations from empirical observations. The analysis characterizes trade-offs between model expressivity, trainability, and classical simulability, while systematically detailing the bottlenecks imposed by optimization landscapes, input-output access models, and hardware constraints. Applications are surveyed in domains encompassing image classification, natural
language processing, scientific discovery,
quantum data processing, and
quantum optimal control, underscoring fair benchmarking against optimized classical counterparts and a comprehensive assessment of resource requirements. This review serves as a tutorial entry point for graduate students while guiding readers to specialized literature. It concludes with a verification-aware roadmap to transition QDL from near-term demonstrations to scalable and fault-tolerant implementations.
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All authors Yanjun Ji, Zhao-Yun Chen, Marco Roth, David A. Kreplin, Christian Schiffer, Martin King, Oliver Anton, M. Sahnawaz Alam, Markus Krutzik, Dennis Willsch, Ludwig Mathey, Frank K. Wilhelm, Guo-Ping Guo
Quantum Physics
On some signatures of Lie-Hamilton System in Quantum Hamilton Jacobi Equation
Arindam Chakraborty
The general forms of
Quantum Hamilton Jacobi Equation for a particle of constant mass, position dependent effective mass and non-Hermitian Effective mass Swanson model have been considered. It has been found that the said equations can be recast in the form of Cayley-Klein Riccati equations which admit a Lie-Hamilton structure. The possible expressions of Lie
symmetry and Lie Integral have also been considered.
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Quantum Physics
High-Temporal-Resolution Measurements of the Impacts of Ionizing Radiation on Superconducting Qubits
Jihee Yang, Thomas J. Carroll, Philip Mason et al. (+19 more)
We measure the effect of ionizing radiation on superconducting
qubits with a timing resolution of 1 $\mu s$ using microwave kinetic inductance detectors (MKIDs) fabricated on the same substrate. We observe no correlation between two-level system (TLS) scrambling events and ionizing radiation events detected with the MKIDs, suggesting TLS scrambling events may not arise from ionizing radiation and instead the previously reported apparent correlation may be due to events without sufficient energy to trigger our MKIDs. We characterize the fast-time system recovery of transmons following a radiation event, where we observe the recovery of the enhanced
qubit relaxation and excitation to be well-described by an exponential recovery to the baseline quasiparticle density, with a characteristic time of $13\pm1\ \mu$s, and a peak quasiparticle density at the junction per deposited energy of $240/\mu m^3/MeV$. The fast recovery is consistent with literature reported values for Nb-based devices with direct injection of 2$\Delta_{\text{Al}}$
phonons, demonstrating the recovery is strongly dependent on the proximity of niobium to the junction.
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All authors Jihee Yang, Thomas J. Carroll, Philip Mason, Robert Schwartz, Kenneth M. O'Hara, Jennifer Lund, Michael Gottschalk, Timothy Stephenson, Lawrence H. Friedman, Francisco Yumiceva, Justin Hackley, Aurelius L. Graninger, Chris Rotella, Pat Warner, Jonathan M. Cochran, Adam V. Bruce, Melody Wagner, James Wenner, Stan Steers, Christopher Moore, Alex Marakov, Bradley G. Christensen
Quantum Physics
Continuous variable quantum key distribution channel emulator for the SPOQC mission
Emma Tien Hwai Medlock, Vinod N. Rao, Ry Render et al. (+2 more)
In a free space optical (FSO) communication link from satellite to ground, the losses in the channel will be dynamic. Thus, the characterization of the FSO channel is of great importance and this can be emulated in the lab to evaluate the realistic performance of a satellite payload. In this work, we introduce a novel optical channel emulator capable of replicating these dynamics, especially for Low Earth Orbit based CubeSats. We demonstrate its ability to accurately emulate a satellite-to-ground optical communications channel under various atmospheric
turbulence strengths, satellite trajectories, and optical ground station parameters at a given optical wavelength of interest. Our satellite channel emulator was designed to test and
benchmark the performance of the continuous variable
quantum key distribution payload for the Satellite Platform for Optical
Quantum Communications mission - an in-orbit demonstrator for the UK's
Quantum Communication Hub, to be launched in early 2026.
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All authors Emma Tien Hwai Medlock, Vinod N. Rao, Ry Render, Timothy Spiller, Rupesh Kumar
Quantum Physics
Machine learning of quantum data using optimal similarity measurements
Zhenghao Li, Hao Zhan, Shana H. Winston et al. (+11 more)
Quantum machine learning seeks a computational advantage in data processing by evaluating functions of
quantum states, such as their similarity, that can be classically intractable to compute. For
quantum advantage to be possible, however, it is essential to bypass costly characterisation of individual data instances in favour of efficient, direct similarity evaluation. Here we demonstrate a sample-optimal, hardware-efficient protocol for estimating
quantum similarity – the state overlap – using bosonic
quantum interference. The sample complexity of this approach is independent of the system dimension and is information-theoretically optimal up to a constant factor. Experimentally, we implement the scheme on Prakash-1, a
quantum computing platform based on a fully programmable integrated photonic processor. By preparing and interfering qudit states on the chip to directly extract their overlap, we demonstrate classification and online learning of
quantum data with high accuracy in realistic noisy experiments. Our results establish joint overlap measurements as a scalable pathway to efficient
quantum data analysis and a practical building block for network-integrated
quantum machine learning.
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All authors Zhenghao Li, Hao Zhan, Shana H. Winston, Ewan Mer, Zhenghao Yin, Shang Yu, Yazeed K. Alwehaibi, Gerard J. Machado, Dayne Marcus Lopena, Lijian Zhang, M. S. Kim, Aonan Zhang, Ian A. Walmsley, Raj B. Patel
Quantum Physics
Trajectory of Probabilities, Probability on Trajectories, and the Stochastic-Quantum Correspondence
Győző Egri, Marton Gomori, Balazs Gyenis et al. (+1 more)
The probabilistic description of the time
evolution of a physical system can take two conceptually distinct forms: a trajectory of probabilities, which specifies how probabilities evolve over time, and a probability on trajectories, which assigns probabilities to possible histories. A lack of a clear distinction between these two probabilistic descriptions has given rise to a number of conceptual difficulties, particularly in recent analyses of
stochastic-
quantum correspondence. This paper provides a systematic account of their relationship. We define probability dynamics and
stochastic process families together with a precise notion of implementation that connects the two descriptions. We show that implementations are generically non-unique, that every probability dynamics admits a Markovian implementation, and characterize when non-Markovian implementations are possible. We expose fallacies in common arguments for the linearity of probability dynamics based on the law of total probability and clarify the proper interpretation of “transition
matrices” by distinguishing dynamics-level maps from the conditional probability
matrices of implementing processes. We further introduce decomposability as the appropriate general notion of stepwise
evolution for (possibly nonlinear) probability dynamics, relate it to divisibility in the linear case – showing that the two can come apart – and disentangle both notions from Markovianity and time-homogeneity. Finally, we connect these results to what we call statistical dynamics, in which linearity is indeed physically motivated, and contrast the framework with
quantum mechanics.
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All authors Győző Egri, Marton Gomori, Balazs Gyenis, Gábor Hofer-Szabó
Quantum Physics
Ground state and persistent oscillations in the quantum East model
Adway Kumar Das and Achilleas Lazarides
For the 1D
quantum East model with open boundaries, we show that in the limit $s \to -\infty$, the ground state is accurately captured by a simple
spin-coherent product state. We further identify a low-
entanglement excited eigenstate that differs from the ground state only by a $\pi$-rotation of the boundary
spin, remaining well approximated by a
spin-coherent state. For a range of $-\infty<s<0$, the edge-coherent product state overlaps with two eigenstates separated by a size-independent energy gap, leading to persistent coherent oscillations of both global and local observables in the thermodynamic limit. These oscillations originate from boundary physics and are distinct from
quantum many-body scars or hypercube-like Fock-space mechanisms.
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Quantum Physics · Cross-listed from hep-th
Integrability breaking in semiclassical strings in Koopman-Krylov space
Rathindra Nath Das and Saskia Demulder
While very powerful, integrability in semiclassical string solutions is known to be a rare property. Motivated by the need to understand and characterise the large landscape of non-integrable string dynamics, we extend Krylov methods for probing chaos to classical systems. We introduce a Koopman-Krylov framework, formulated in the Koopman-von Neumann description of classical mechanics and implemented via a generator extended dynamic mode decomposition (gEDMD) approximation of the Koopman generator acting on observables. Using this framework, we study how integrability-breaking deformations of integrable string dynamics induce characteristic redistributions of spectral weight, leading to observable-dependent delocalisation and spreading in Krylov space. We illustrate the Koopman-Krylov diagnostics across three classes of non-integrable semiclassical string solutions.
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Quantum Physics · Cross-listed from cond-mat.str-el
From QED$_3$ to Self-Dual Multicriticality in the Fradkin-Shenker Model
Thomas T. Dumitrescu, Pierluigi Niro and Ryan Thorngren
We consider the Fradkin-Shenker ${\mathbb Z}_2$ gauge-
Higgs lattice model in 2+1 dimensions, i.e. the toric code deformed by an in-plane
magnetic field. Its phase diagram contains a multicritical CFT with gapless, mutually non-local electric and
magnetic particles, exchanged by a ${\mathbb Z}_2^{\mathsf{D}}$ self-duality
symmetry. We introduce a staggered generalization of the model in which these particles carry global $U(1)_e$ and $U(1)_m$ charges, respectively, and we propose a continuum QFT description in terms of QED$_3$ with $N_f = 2$ Dirac
fermion flavors and a charge-two
Higgs field with Yukawa couplings. The conjectured phase diagram harbors a multicritical CFT with $(O(2)_e \times O(2)_m)\rtimes\mathbb{Z}_2^\mathsf{D}$
symmetry, some of which is emergent in the QFT description. We compute the scaling dimensions of some operators using a large-$N_f$ expansion and find agreement with the emergent selection rules. The staggered model admits a deformation to the original Fradkin-Shenker model, which maps to unit-charge monopole operators in
Higgs-Yukawa-QED$_3$ that break the $U(1)_e \times U(1)_m$
symmetry. We show explicitly that this deformation reproduces all features of the Fradkin-Shenker phase diagram. Finally, we propose a multicritical duality between
Higgs-Yukawa-QED$_3$ and the easy-plane $\mathbb{ CP}^1$ model (i.e. two-
flavor scalar QED$_3$ with a suitable potential), which describes
spin-1/2 anti-ferromagnets on a square
lattice. This duality implies a first-order line of Néel-VBS transitions ending in a deconfined
quantum multicritical point, described by the same $O(2)_e \times O(2)_m$ symmetric CFT that arises in the staggered Fradkin-Shenker model, which separates it from a gapped ${\mathbb Z}_2$
spin liquid phase.
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Quantum Physics · Cross-listed from cs.LG
Long Range Frequency Tuning for QML
Michael Poppel, Markus Baumann, Sebastian Wölckert et al. (+2 more)
Angle-encoded variational
quantum circuits admit a truncated
Fourier series representation of their output, but approximating functions with maximum frequency $\omega_{\max}$ using fixed unary encoding requires $\mathcal{O}(\omega_{\max})$ encoding gates. Trainable-frequency (TF)
circuits promise a reduction by learning the data-encoding prefactors alongside the ansatz parameters, adapting the accessible frequency spectrum to the target during training. We identify a practical barrier that prevents this promise from being realized: the prefactor gradient is suppressed by the spectral gap between the
circuit's accessible frequencies and the target spectrum, independently of the ansatz parameters, confining gradient-driven prefactor movement to a narrow neighborhood of initialization. We propose ternary grid initialization – setting prefactors to $\{1, 3, 9, \ldots, 3^{k-1}\}$ – which ensures every target frequency within $[-\omega_{\max}, \omega_{\max}]$ lies within $\tfrac{1}{2}$ unit of the accessible spectrum at initialization, so that the spectral-gap bound no longer constrains the target-driven gradient to be small. This is a necessary condition for reliable convergence, whose sufficiency we establish empirically. On a synthetic
benchmark with target frequencies shifted well beyond the standard initialization range, ternary initialization achieves median $R^2 = 0.997$ versus $0.18$ for unary initialization, with $100\%$ of runs achieving $R^2 > 0.95$ against $0\%$. CMA-ES with $20\times$ the evaluation budget reaches only $25\%$ success, confirming the limitation is a property of the optimization landscape rather than of gradient-based optimization specifically. Real-world validation on two
benchmark datasets demonstrates consistent advantages over both fixed and trainable unary baselines.
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All authors Michael Poppel, Markus Baumann, Sebastian Wölckert, Claudia Linnhoff-Popien, Jonas Stein
Quantum Physics
Butterfly Echo Protocol for Axis-Agnostic Heisenberg-Limited Metrology
Jacob Bringewatt, Leon Zaporski, Matthew Radzihovsky et al. (+4 more)
The extreme sensitivity of chaotic systems to external perturbations makes them natural candidates for sensing applications. We propose a single-shot echo-based protocol for estimating small rotations about an unknown axis that leverages random symmetric probe states prepared via chaotic dynamics. In contrast to previous protocols for this axis-agnostic rotation sensing problem that depend on difficult-to-prepare anticoherent states, the random probe states used in our protocol can be prepared via constant-depth chaotic
circuits composed of random one-axis twisting pulses. Further, the signal of interest can be extracted simply by measuring the total
spin polarization. We demonstrate analytically that our protocol achieves Heisenberg scaling relative to an arbitrary rotation axis that need not be a priori known. We also investigate the effects of collective and single-particle dephasing in our protocol using analytical and numerical tools. While the requirements on dephasing rates to maintain Heisenberg sensitivity are strict, they are achievable in near-term experiments, for instance, in magnetometric rotosensing with high-
spin lanthanide atoms such as dysprosium-164.
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All authors Jacob Bringewatt, Leon Zaporski, Matthew Radzihovsky, Jasmine Albert, Alexey V. Gorshkov, Vladan Vuletic, Gregory Bentsen
Quantum Physics
Many-body gravitating quantum systems with Bose-Einstein condensates and dipolar analogue
Youssef Trifa, Dario Cafasso, Marco Fattori et al. (+1 more)
Quantum probes of
gravity in the Newtonian regime, based on mass-energy equivalence in clocks or spatial superpositions in interferometers, share a common description in terms of an effective
qubit-
qubit coupling. Here we extend this framework to atomic ensembles, regarded as interacting collective qudits. The many-body enhancement boosts the signal-to-noise and increases the effective interaction rate, facilitating the observation of gravitationally-induced
entanglement and decoherence, certified by metrological witnesses based on local and collective
spin squeezing. We further identify trapped bimodal Bose-Einstein condensates with long-range interactions, including dipolar couplings, as a programmable analogue platform for simulating gravitating
quantum dynamics at accessible time and energy scales. Extending the protocol to a sensor network broadens the
entanglement-detection window.
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All authors Youssef Trifa, Dario Cafasso, Marco Fattori, Luca Pezzè
Quantum Physics
Copy-cup Gates in Tensor Products of Group Algebra Codes
Ryan Tiew and Nikolas P. Breuckmann
We determine conditions on classical group
algebra codes so that they have pre-orientation for cup products and copy-cup gates. This defines
quantum codes that have constant-depth $\operatorname{CZ}$ and $\operatorname{CCZ}$ gates constructed via tensor products of classical group
algebra codes, including hypergraph and balanced products. We show that determining the conditions relies on solving the perfect matching problem in graph theory. Conditions are fully determined for the 2- and 3-copy-cup gates, for group
algebra codes up to weight 4, including for codes with odd check weight. These include the bivariate bicycle codes, which we show do not have the pre-orientation for either type of copy-cup gate. We show that abelian weight 4 group
algebra codes satisfying the non-associative 3-copy-cup gate necessarily have a code distance of 2, whereas codes that satisfy conditions for the symmetric 3-copy-cup gate can have higher distances, and in fact also satisfy conditions for the 2-copy-cup gate. Finally we find examples of
quantum codes from the product of abelian group
algebra codes that have inter-code constant-depth $\operatorname{CZ}$ and $\operatorname{CCZ}$ gates.
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Quantum Physics
Efficient evaluation of fundamental sensitivity limits and full counting statistics for continuously monitored Gaussian quantum systems
Francesco Albarelli and Marco G. Genoni
Generalized master equations (GMEs)—time-local but generally neither trace-preserving nor Hermiticity-preserving—are convenient tools to compute properties of the environment of an open or continuously monitored
quantum system. A two-sided master equation yields the fidelity and
quantum Fisher information (QFI) of environment states, thereby setting fundamental limits for hypothesis testing and parameter estimation under continuous monitoring. For unmonitored noise or inefficient detection, the QFI of the detectable part of the environment may be obtained from a recently derived GME acting on multiple system replicas. Tilted master equations provide the full counting statistics of
quantum jumps and diffusive measurements, enabling, e.g., studies of
quantum thermodynamics beyond average values. Here we focus on bosonic linear systems, governed by a quadratic
Hamiltonian and linear jump operators, whose dynamics preserves Gaussianity. For Gaussian initial operators, we recast a generic GME as a compact set of ordinary differential equations for the covariance
matrix (a Riccati-type equation), first moments, and normalization. These equations can be integrated efficiently without
Hilbert-space truncation, and admit analytical results in simple settings. We also provide specialized forms for fidelity/QFI and full counting statistics. We illustrate the formalism with a continuously monitored optical parametric oscillator, using it to determine sensitivity limits for frequency estimation and to
benchmark Hasegawa's thermodynamic uncertainty relations.
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Quantum Physics
Quantum Confocal Microscopy in Fock Space with a 19 dB Metrological Gain
Ziyue Hua, Chuanlong Ma, Yilong Zhou et al. (+12 more)
Quantum metrology promises measurement precision beyond classical limits by exploiting large-scale
quantum states, yet realizing this advantage faces two fundamental challenges: the deterministic preparation of non-trivial
quantum probes and the efficient extraction of metrological information in high-dimensional
Hilbert spaces. Here, we introduce
quantum confocal microscopy in Fock space that simultaneously resolves both challenges. Drawing a direct analogy between classical wave optics and
quantum state
evolution in a bosonic mode, we construct a confocal system with two Fock-space lenses. The first lens deterministically focuses a coherent state into a
quantum probe with a tightly concentrated
photon-number distribution, while the second lens maps the metrological information back to the
vacuum state for efficient readout. Using a superconducting
circuit QED platform, we prepare focused probe states with mean
photon numbers up to ${N} = 500$, achieving a 21.5$\pm$1.1 dB compression of the
photon-number uncertainty relative to a coherent state, with a scalable
quantum circuit of $\mathcal{O}(1)$ operational depth. We demonstrate a displacement sensitivity scaling as $N^{-0.416}$, approaching the Heisenberg scaling ($N^{-0.5}$), and achieve a record metrological gain of 19.06$\pm$0.13 dB beyond the standard
quantum limit. This work establishes
quantum confocal microscopy as a scalable and practical framework for
quantum-enhanced precision measurement, readily extendable to other bosonic platforms and high-dimensional
quantum many-body systems.
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All authors Ziyue Hua, Chuanlong Ma, Yilong Zhou, Yifang Xu, Zi-Jie Chen, Weizhou Cai, Jiajun Chen, Lintao Xiao, Hongwei Huang, Weiting Wang, Hekang Li, Haohua Wang, Ming Li, Chang-Ling Zou, Luyan Sun
Quantum Physics
Gaussian mode coupling of spectrally broadband photons from bulk spontaneous parametric down-conversion: A spatial-spectral mode analysis of fiber coupling
Carlos Sevilla-Gutiérrez, Varun Raj Kaipalath and Fabian Steinlechner
Photon sources based on spontaneous parametric down-conversion (SPDC) are central to experimental
quantum optics and
quantum technologies. Their performance is commonly quantified by three metrics: pair-collection probability, heralding efficiency, and spectral purity. In bulk-
crystal SPDC, these metrics are known to be mutually constrained, yet the physical origin of the resulting trade-offs is often obscured. We show that these trade-offs originate from the frequency-dependent population of discrete spatial modes in the SPDC emission. By performing a Laguerre-Gauss mode decomposition at each frequency component, we show how spectral-spatial non-separability impacts collection probability, heralding efficiency, and purity. We apply this framework to two widely used quasi-phase-matching configurations: collinear degenerate type-0 and type-II SPDC in periodically poled bulk
crystals, and quantify how different phase-matching functions shape the spectral-spatial mode structure. In particular, for type-II SPDC we compare standard periodically poled and aperiodically poled Gaussian phase matching. We experimentally validate some of our theoretical results using spatial- and spectral-projection measurements. This spectral-spatial mode analysis provides a quantitative and predictive framework for understanding and engineering bulk-
crystal photon sources, enabling systematic multi-parameter optimization beyond qualitative design guidelines.
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Quantum Physics
Polarization-selective quantum cooperative response in dual-species atom arrays
Huan Wang, Shangguo Zhu, Yun Long et al. (+4 more)
Atom arrays have emerged as a powerful platform for
quantum light-matter interfaces, yet single-species arrays are constrained by in-plane
symmetry, restricting polarization control. Here we investigate the cooperative optical response of dual-species subwavelength atom arrays, in which intrinsic polarizability difference breaks in-plane
symmetry. By engineering the
lattice constants and detunings, the arrays exhibit polarization-dependent subradiant modes, enabling complete reflection of a specific polarization component. Leveraging this mechanism, we assemble array units as functional pixels and demonstrate a scalable polarization-selective
quantum light modulator. Our work establishes a dynamically reconfigurable atomic-photonic platform for versatile subwavelength
quantum optical elements.
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All authors Huan Wang, Shangguo Zhu, Yun Long, Fei Zhang, Yinghui Guo, Mingbo Pu, Xiangang Luo
Quantum Physics
Quantum diffusion for a quantum particle with a correlated Gaussian noise
Yun Jeong Kang, Sung Kyu Seo and Kyungsik Kim
We investigate the diffusive behavior of a
quantum particle driven by a correlated Gaussian noise. We derive the analytical solution of the joint probability density function and obtain explicit expressions for the mean square momentum and the mean square displacement.
Quantum Physics
Connecting Quantum Contextuality and Nonlocality
Jianqi Sheng, Dongkai Zhang and Lixiang Chen
Quantum theory departs from classical physics in its treatment of correlations, most prominently through the phenomena of contextuality and nonlocality. Once regarded primarily as foundational curiosities, these effects are now understood as key operational resources for
quantum computation, communication, and simulation. Although traditionally investigated in distinct settings, recent theoretical and experimental advances have revealed deep conceptual, mathematical, and operational connections between them. This review presents a unified perspective on these developments based on sheaf-theoretic and graph-theoretic frameworks, which provide theory-independent characterizations of statistical correlations. These approaches clarify the structural relationship between contextuality and nonlocality, facilitate the formulation of experimentally testable inequalities, and guide implementations in realistic physical platforms, with particular emphasis on photonic systems. By bridging abstract theoretical structures and concrete experimental realizations, this review sheds light on the nonclassical foundations of
quantum correlations and their emerging role in
quantum technologies.
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Quantum Physics · Cross-listed from hep-lat
Scaling and Luescher Term in a non-Abelian (2+1)d SU$(2)$ Quantum Link Model
Paul Ludwig, Timo Jakobs and Carsten Urbach
We investigate a non-Abelian SU$(2)$
quantum link model in $2+1$ dimensions on a hexagonal
lattice using tensor network methods. We determine the static
quark potential for a wide range of bare coupling values and find that the theory is confining. We also probe the existence of a Lüscher term and find a clear signal with a $g^2$ dependent coefficient, in qualitative agreement with a strong coupling expansion. Correspondingly, the width of the strings scales logarithmically with the string length again for all $g^2$-values, providing evidence for a rough string, with no indication for a roughening transition.
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Quantum Physics
Dequantization Barriers for Guided Stoquastic Hamiltonians
Yassine Hamoudi, Yvan Le Borgne and Shrinidhi Teganahally Sridhara
We construct a probability distribution, induced by the Perron–Frobenius eigenvector of an exponentially large graph, which cannot be efficiently sampled by any classical
algorithm, even when provided with the best-possible warm-start distribution. In the
quantum setting, this problem can be viewed as preparing the ground state of a stoquastic
Hamiltonian given a guiding state as input, and is known to be efficiently solvable on a
quantum computer. Our result suggests that no efficient classical
algorithm can solve a broad class of stoquastic ground-state problems. Our graph is constructed from a class of high-degree, high-girth spectral expanders to which self-similar trees are attached. This builds on and extends prior work of Gilyén, Hastings, and Vazirani [
Quantum 2021, STOC 2021], which ruled out dequantization for a specific stoquastic adiabatic path
algorithm. We strengthen their result by ruling out any classical
algorithm for guided ground-state preparation.
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Quantum Physics
Excited-state quantum phase transitions and chaos in a three-level Lipkin model
Alberto Mayorgas, Pedro Pérez-Fernández, Álvaro Sáiz et al. (+1 more)
Excited-state
quantum phase transitions (ESQPTs) have been extensively studied in two-level models, but their characterization remains challenging in systems displaying mixed regular and chaotic dynamics. In this work, we investigate ESQPTs within the three-level Lipkin-Meshkov-Glick model, where an enlarged
Hilbert space and multiple separatrices give rise to rich spectral structures strongly influenced by chaos. To investigate the different dynamical regions, we have calculated Poincaré sections and Peres
lattices. In addition, by combining chaos-sensitive measures with standard ESQPT diagnostics, we provide a static analysis of ESQPT signatures in this model and establish a robust framework for future studies of its dynamical behavior. The degree of chaos and the Kullback-Leibler divergence are found to be very effective chaos-sensitive measures, which are complementary to ESQPT diagnostics such as the mean field limit and the participation ratio. Hence we provide a standard framework to work with ESQPTs in chaotic three-level systems.
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All authors Alberto Mayorgas, Pedro Pérez-Fernández, Álvaro Sáiz, José Miguel Arias
Quantum Physics · Cross-listed from cond-mat.quant-gas
A Maxwell Fish-Eye Lens in a Bose-Einstein Condensate
Jelte Duchêne, Elinor Kath, Floriane Arrouas et al. (+7 more)
We experimentally realize an analogue of the optical Maxwell fish-eye lens (MFEL) using phononic excitations in a Bose-Einstein condensate (BEC). A MFEL is characterized by a radially symmetric, spatially varying refractive index with the remarkable property that rays emitted from any point within the lens are perfectly focused at their antipodal points. While the implementation of such gradient-index lenses is challenging in conventional optical systems, BECs offer a highly tunable platform in which the spatially varying speed of sound of collective excitations –
phonons, the acoustic analogues of
photons – can be engineered and their dynamics observed in real time. Time-resolved measurements of
phonon wavefronts reveal focusing behavior that shows good agreement with analytical theory and numerical simulations. This work provides both a
geometric and physical framework for engineering effective refractive indices using ultracold atoms, and simulating wave propagation on effective spherical geometries.
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All authors Jelte Duchêne, Elinor Kath, Floriane Arrouas, Hanyi Jang, Helmut Strobel, Markus K. Oberthaler, Jay Mehta, Liam M. Farrell, Wyatt Kirkby, Duncan H.J. O'Dell
Quantum Physics · Cross-listed from cond-mat.stat-mech
Thermodynamic uncertainty relation under continuous measurement and feedback with quantum-classical-transfer entropy
Kaito Tojo, Takahiro Sagawa and Ken Funo
We derive a thermodynamic uncertainty relation (TUR) under
quantum continuous measurement and feedback control. By incorporating the
quantum-classical-transfer
entropy, which quantifies the information gained by continuous measurement, we show that the precision of currents is constrained by information-thermodynamic costs such as the
entropy production and information gain. Our result shows that information gain has the potential to enhance the precision of currents beyond the bounds set by the conventional TUR. We illustrate the bound with a driven two-level system under continuous measurement and feedback, demonstrating that feedback achieves higher precision of currents while suppressing the
entropy production.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Coupling-energy driven pumping through quantum dots: Role of coherences
Lukas Litzba, Gernot Schaller, Jürgen König et al. (+1 more)
We study the impact of off-resonant tunneling and coherences on the electron pumping through
quantum dots. Thereby, we focus on two electron-pump setups where lowest-order tunneling processes are suppressed and the pump is exclusively driven by modulations of the coupling energy. The first setup is driven by switching on and off the couplings between the
quantum dot and the leads, while the second setup employs measurements of the dot occupation. We derive exact solutions for arbitrarily strong tunnel couplings in the absence of Coulomb interaction, identify parameter regimes with optimal pumping currents or optimal energy efficiency, and discuss similarities between both pumping mechanisms.
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All authors Lukas Litzba, Gernot Schaller, Jürgen König, Nikodem Szpak
Quantum Physics
Q-Tag: Watermarking Quantum Circuit Generative Models
Yang Yang, Yuzhu Long, Han Fang et al. (+4 more)
Quantum cloud platforms have become the most widely adopted and mainstream approach for accessing
quantum computing resources, due to the scarcity and operational complexity of
quantum hardware. In this service-oriented paradigm,
quantum circuits, which constitute high-value intellectual property, are exposed to risks of unauthorized access, reuse, and misuse. Digital watermarking has been explored as a promising mechanism for protecting
quantum circuits by
embedding ownership information for tracing and verification. However, driven by recent advances in generative artificial intelligence, the paradigm of
quantum circuit design is shifting from individually and manually constructed
circuits to automated synthesis based on
quantum circuit generative models (QCGMs). In such generative settings, protecting only individual output
circuits is insufficient, and existing post hoc,
circuit-centric watermarking methods are not designed to integrate with the generative process, often failing to simultaneously ensure stealthiness, functional correctness, and robustness at scale. These limitations highlight the need for a new watermarking paradigm that is natively integrated with
quantum circuit generative models. In this work, we present the first watermarking framework for QCGMs, which embeds ownership signals into the generation process while preserving
circuit fidelity. We introduce a symmetric sampling strategy that aligns watermark encoding with the model's Gaussian prior, and a synchronization mechanism that counteracts
adversarial watermark attack through latent drift correction. Empirical results confirm that our method achieves high-fidelity
circuit generation and robust watermark detection across a range of perturbations, paving the way for scalable, secure copyright protection in AI-powered
quantum design.
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All authors Yang Yang, Yuzhu Long, Han Fang, Zhaoyun Chen, Zhonghui Li, Weiming Zhang, Guoping Guo
Quantum Physics
Geometric control of maximal entanglement via bound states in the continuum
Alexis R. Legón, Mario Miranda Rojas, Pedro Orellana et al. (+1 more)
Bound states in the continuum (BiCs) convert dissipative open systems into effectively closed
quantum subspaces through destructive interference. We show that two identical
giant atoms coupled to a one-dimensional waveguide support BICs that coincide with maximally entangled atomic states. Most importantly,
entanglement is predominantly determined by the
geometric design; the ratio of intra-atomic connection lengths fixes the concurrence, while the propagation phase between atoms selects a family of Bell-like states. We further analyze the dynamical stability of these maximally entangled BICs under exact time
evolution, revealing a clear hierarchy of robustness against parameter perturbations. Our results establish an analytical bridge between
symmetry, geometry,
entanglement, and BICs in
giant-atom waveguide platforms.
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All authors Alexis R. Legón, Mario Miranda Rojas, Pedro Orellana, Ariel Norambuena
Quantum Physics
Cryptographic Fragility of Standard Quantum Repeater Protocols
Abhishek Sadhu and Sharu Theresa Jose
The security of the proposed
quantum Internet relies on repeater protocols designed under the assumption of
stochastic, characterizable noise. We demonstrate that in
adversarial environments this assumption induces performance vulnerabilities for computationally bounded repeater nodes. We show that the standard BBPSSW
distillation protocol recursively purifies error syndromes rather than
entanglement. This leads to a state of low fidelity despite diagnostic metrics indicating perfect convergence. Moreover, we show that the verifier cannot check the
adversarial influence via the maximum likelihood estimation
algorithm since it is blind to computationally bounded observers. To address these vulnerabilities, we propose a Cryptographic Network Stack centered on a trapdoor verification protocol. The protocol exploits private randomness to restore operational stability without requiring channel characterization.
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