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Tuesday 14 July

Quantum Physics · Cross-listed from cond-mat.stat-mech

From stable periodic orbits to many-body chaos: doubly tunable prethermalization via engineering of an emergent band structure

Jianan Wang, Yang Hou, Andrea Pizzi et al. (+3 more)

We uncover a family of many-body periodic orbits in a periodically driven (Floquet) spin system away from the high-frequency limit. While linear stability analysis predicts that perturbed many-body trajectories remain close to stable periodic orbits, thermodynamic principles dictate that Floquet heating will ultimately set in. Our work aims to resolve the tension between these two expectations. In particular, we show that perturbations away from the stable periodic orbits feature a description akin to a quasiparticle band structure. A long-lived prethermal regime appears when modes around the gapless point are slowly populated. The dispersion determines the prethermal lifetime, and we show how band engineering leads to a "doubly tunable" parametric dependence of the prethermal lifetime $R^{-W}$, with $R$ the width in momentum space of the quasiparticle distribution and $W$ the exponent of the dispersion around the gapless point. Our results not only establish a powerful route toward stabilizing non-equilibrium phases of matter in driven many-body systems but also establish a conceptual bridge between periodic orbits in 'low-dimensional' nonlinear systems and many-body chaos.
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All authors Jianan Wang, Yang Hou, Andrea Pizzi, Johannes Knolle, Roderich Moessner, Hongzheng Zhao

Quantum Physics

Parallel Hadamard Test

Soichiro Imamura and Synge Todo

The Hadamard test is a fundamental building block widely used in many quantum computing algorithms. It estimates the real or imaginary part of $\langle \psi \vert U \vert \psi \rangle$, where $\vert \psi \rangle$ is a quantum state and $U$ is a unitary operator. In many algorithms, however, many such quantities must be estimated, leading to a large number of distinct circuit types, long computational times, and high financial costs. In this work, we propose the parallel Hadamard test, which combines multiple Hadamard tests into a single circuit type. We demonstrate how the parallel Hadamard test applies to three structural classes of workloads: arbitrary sets of unitary operators, prefix-product arrays, and Gram-matrix elements. For each class, we compare the cost of the parallel Hadamard test with that of the conventional one. Our unified approach significantly reduces the number of distinct circuit types, and can lower both computational time and financial costs in regimes where fixed per-circuit overheads dominate the total cost. In Gram-matrix workloads, it can also reduce the total number of shots when typical off-diagonal overlaps are small.
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Quantum Physics

Demonstration of tripartite cat states in two distinct classes of entanglement

May Chee Loke, Jonathan Schwinger, Kehui Yu et al. (+4 more)

Entanglement is a cornerstone of quantum mechanics and an essential resource for quantum computation, communication, and metrology. While bipartite entanglement is extensively studied, genuine multipartite entangled states remain largely inaccessible in the continuous-variable domain. Here, we experimentally realize continuous-variable tripartite entangled cat states of the two distinct classes with fundamentally inequivalent properties, namely GHZ-cat and W-cat states, encoded across three microwave resonators coupled to an ancillary superconducting transmon. We develop one-to-all conditional controls on the three oscillators by fully utilizing three energy levels of the transmon to generate these states in a single piece of hardware and employ an efficient subspace tomography protocol for state reconstruction. We obtain measurement-corrected state fidelities of $0.83\pm0.02$ for GHZ-cat and $0.70\pm0.02$ for W-cat, both exceeding the thresholds for genuine multipartite entanglement, with the W-state certification further supported by an optimized fully decomposable witness directly computed with the reconstructed density matrix. We validate the distinct entanglement classes by experimentally demonstrating that pairwise entanglement is present in the W-cat state but absent in the GHZ-cat state. Our technique for the on-demand generation and characterization of inequivalent classes of tripartite continuous-variable entanglement provides a testbed for studying multipartite bosonic entanglement and its robustness to loss, and a building block for future quantum information processing with multipartite bosonic codes.
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All authors May Chee Loke, Jonathan Schwinger, Kehui Yu, Yingshan Zhang, Amon M. Kasper, Tanjung Krisnanda, Yvonne Y. Gao

Quantum Physics

Heisenberg Scaling in Many-Body Kinetic Uncertainty Relation via Quantum Feedback

Hayato Yunoki and Yoshihiko Hasegawa

Kinetic uncertainty relations impose a fundamental constraint on counting precision by requiring large dynamical activity to suppress fluctuations. For $N$ independent constituents, the activity is additive and scales as $O(N)$, so the corresponding precision bound improves only as $O(1/N)$. A central question is whether many-body quantum systems can instead achieve Heisenberg scaling, with relative variance suppressed as $O(1/N^{2})$. Such scaling could enable substantially enhanced precision in quantum devices, including quantum clocks. Superradiance appears to provide a natural route because its collective emission activity can scale as $O(N^2)$. We show, however, that superradiance alone fails to achieve Heisenberg scaling because this enhanced activity is transient. We then demonstrate that jump-conditioned quantum feedback stabilizes the superradiant high-activity regime and converts this collective enhancement into Heisenberg scaling. We establish this mechanism analytically using a many-body kinetic uncertainty relation and mean-field analysis, and demonstrate through quantum-trajectory simulations that the actual relative counting fluctuation scales as $O(1/N^{2})$.
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Quantum Physics

Experimental demonstration of scalable quantum blockchain with exponentially superior quantum communication complexity

Feng Xie, Ming-Yang Li, Yongqiang Du et al. (+11 more)

To secure modern distributed digital infrastructures, quantum blockchains exploit quantum resources to achieve information-theoretic security and surpass the classical one-third fault-tolerance bound. However, existing high-fault-tolerant protocols face a fundamental scalability challenge: the blockchain trilemma imposes either exponential communication complexity or experimentally demanding multipartite entanglement. Here, we experimentally demonstrate a scalable quantum blockchain protocol based on weak coherent states that achieves an exponential reduction in quantum communication complexity. The protocol employs a circular quantum Byzantine agreement mechanism that preserves information-theoretic security while avoiding multipartite entanglement. We implement this protocol on a photonic integrated circuit platform, realizing a six-node network over commercially available telecommunication infrastructure. Compared with previous schemes, the protocol requires less than 4% of the quantum communication resources. Leveraging this advantage, we further demonstrate a quantum-secured token exchange application achieving a throughput of 805.3 transactions per second with zero failures. These results establish a practical pathway toward scalable quantum blockchain.
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All authors Feng Xie, Ming-Yang Li, Yongqiang Du, Chen-Xun Weng, Mingxuan Zhang, Xin Hua, Xiang Guan, Xin An, Jingzhe He, Xin Liu, Zhenrong Zhang, Xi Xiao, Hua-Lei Yin, Kejin Wei

Quantum Physics

A fault-tolerant quantum blockchain deployed on commercial telecommunications network

Yongqiang Du, Chen-Xun Weng, Feng Xie et al. (+10 more)

Popularized by the Bitcoin cryptocurrency, blockchain technology establishes a decentralized digital framework that utilizes cryptographic and consensus protocols to secure data against unauthorized modification. Consequently, blockchain has found broad adoption across diverse fields, including finance, data management, healthcare, and digital asset governance. In the quantum computing era, a paramount objective for blockchain is to preserve its foundational advantages of cryptographic integrity and decentralized fault-tolerant resilience. In principle, quantum digital signatures and quantum Byzantine agreement protocols offer foundational security guarantees and tolerate up to one-half of malicious nodes for blockchain. However, the practical realization of such a quantum-enhanced blockchain remains a significant and multifaceted challenge. Here, we propose and experimentally demonstrate a fully operational hybrid quantum blockchain architecture built on photonic integrated circuits and deployed over commercially available classical telecommunications infrastructure. The system achieves a fault tolerance of nearly one-half, surpassing the classical limit, while reaching consensus on a timescale of seconds. A deployed food traceability application validates the practicality of the proposed architecture, achieving a throughput of approximately 500 transactions per second. This work establishes a foundation for practical quantum blockchains, enabling secure, scalable, and decentralized information processing in the emerging quantum era.
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All authors Yongqiang Du, Chen-Xun Weng, Feng Xie, Ming-Yang Li, Mingxuan Zhang, Xin Hua, Xin An, Xiang Guan, Xin Liu, Zhenrong Zhang, Xi Xiao, Hua-Lei Yin, Kejin Wei

Quantum Physics · Cross-listed from cs.IT

Hermitian Hulls of Cyclic Codes and Exact Dual Distances of Extended Linear Codes

Liangdong Lu, Ruipan Yang, Yang Liu et al. (+2 more)

Let $\mathcal{C}$ be an $[n,k,d]$ linear code over $\F_{q^2}$ with parity-check matrix $H$. The rank $r=\Rank(HH^{\dagger})$ measures how far the Hermitian dual $\mathcal{C}^{\perp_h}$ is from being Hermitian self-orthogonal, and it is the least number of coordinates that must be adjoined to $\mathcal{C}^{\perp_h}$ to make it Hermitian self-orthogonal; equivalently, the Hermitian hull $\mathcal{C}\cap\mathcal{C}^{\perp_h}$ has dimension $(n-k)-r$. In this paper, we develop a structure theory of the Hermitian hull of a cyclic code and apply it to the construction of quantum stabilizer codes. For a cyclic code with defining set $T\subseteq\mathbb{Z}_n$, we show that $r$ is additive over the orbits of the involution $\tau(\nu)=-q\nu$ on the $q^2$-cyclotomic cosets and satisfies a parity constraint. A parameter dictionary exhibits $r$ as a gauge trading one-for-one against the code length, and a complementation duality shows that the complement of $T$ realises the logical-qubit count of the quantum code. The extreme values of $r$ are settled by closed subset-sum criteria, and the $\tau$-invariant defining sets are shown to be the unique locus on which the Hermitian dual is again cyclic. On the distance side, we determine the Hermitian dual distance of an $r$-fold extension of an arbitrary linear code exactly, and we prove that the admissible extension vectors lie in the image of $HH^{\dagger}$, which reduces the search space from $q^{2(n-k)}$ to $q^{2r}$. Applied to cyclic, quasi-cyclic and BCH codes, the construction improves $23$ best-known parameters in Grassl's code tables, one of them provably optimal.
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All authors Liangdong Lu, Ruipan Yang, Yang Liu, Qiang Fu, Guanmin Guo

Quantum Physics

Nonequilibrium thermodynamics of the acoustoelectric quantum vacuum

Ryan O. Behunin, Andrew Shepherd, Francesco Intravaia et al. (+1 more)

The quantum vacuum can assume thermal properties as a consequence of system kinematics, highlighting the nuance of our definition of particles in quantum field theory. Here, we explore this phenomenon in acoustoelectric systems, involving the interaction of phonons and plasmons, where the charge carriers drift at a constant velocity exceeding the speed of sound. Through an open quantum systems analysis, we show that the acoustoelectric quantum vacuum acquires a thermal character with a temperature defined by the drift velocity and the phonon wavevector. Realistic parameters yield effective temperatures of several Kelvin, establishing acoustoelectric systems as a promising platform for the investigation of quantum vacuum effects.
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All authors Ryan O. Behunin, Andrew Shepherd, Francesco Intravaia, Matt Eichenfield

Quantum Physics

Constant-Depth Multi-Product Formula for Trotter Error Mitigation in Near-Term Digital Quantum Simulation

Chan Bin Bark, Sangjin Lee, Kwang Jun Ahn et al. (+3 more)

Digital quantum simulation of many-body dynamics faces a tension between algorithmic Trotter error and physical noise that accumulates with circuit depth. Typical higher-order product formulas mitigate the algorithmic error at the cost of deeper circuits. Our benchmark shows that, within the limited physical error budget, the feasible advantage is confined to absolute errors well below the $10^{-2}$ scale, which vanishes under noise levels of current quantum hardware. In sharp contrast to the previous Trotter error mitigation methods, we introduce a constant-depth multi-product formula (cd-MPF) that suppresses the Trotter error by combining multiple circuits at fixed circuit depth. We identify an auxiliary parameter $\alpha$, which reshapes the Trotter error terms while leaving the target evolution invariant. The classical linear combination of the measured expectation values cancels the leading $(\Delta t)^{2}$ algorithmic contribution and steepens the Trotter-error scaling with the two-qubit circuit depth $d$ from $d^{-2}$ to $d^{-4}$. Combined with physical-noise mitigation, our method can serve as a key ingredient for realizing long-time quantum dynamics simulation on near-term hardware.
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All authors Chan Bin Bark, Sangjin Lee, Kwang Jun Ahn, Sangmo Cheon, Moon Jip Park, Youngseok Kim

Monday 13 July

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

Synthetic twist geometry in mesoscopic quantum circuits: geometric capacitance and chiral mode splitting

Edilberto O. Silva

We propose a route for encoding a helicoidally twisted synthetic geometry in mesoscopic quantum circuits. A torsionless spatial metric with twist parameter $\Om$ defines an electrostatic capacitance kernel whose angular–axial sector contains the chiral coupling $-2\Om mk$. We show that this kernel can be implemented by a finite, quantizable circuit graph built only from positive two-node capacitors: diagonal bridges generate the cross term, and the dimensionless twist is the ratio of diagonal to total angular capacitance. The resulting capacitance matrix enters the Hamiltonian through $C^{-1}(\etaT)$ and splits counter-rotating synthetic modes. Exact finite-graph diagonalization confirms the analytic spectrum and shows that closed longitudinal circulation is required; with open boundaries the bridge phase is gauge removable and the doublets remain degenerate. Simulated spectroscopy resolves the chiral fan, while disorder simulations indicate that calibrated percent-level reactive disorder does not obscure the deterministic splitting in the parameter range considered. Twist textures produce interface modes, Josephson nonlinearities make the Kerr sector chirality dependent, and the same doublet provides a detuning knob for non-Hermitian exceptional-point control. For representative circuit-QED parameters the splitting is in the tens-of-MHz range, well above typical resonator linewidths.
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Quantum Physics · Cross-listed from physics.bio-ph

Influence of Oscillating Magnetic Fields on the Electric Dipole Moment of Radical Pairs in Cryptochrome Based Magnetoreception

Ali Soltanmanesh, Mahboobe Sehati, Sareh Rostami et al. (+2 more)

Radical pairs induced by light-driven reduction of cryptochrome protein constitute a spin dependent mechanism that is accompanied by an electric dipole moment and is found to be sensitive to external magnetic fields. In this research, to investigate for the further proof of such model, the simultaneous effect of the Earth's static magnetic field and the time-dependent magnetic field noise on the electric dipole moment of the radical pair has been studied within the quantum mechanical framework. The effect of the external magnetic field discussed in different angles regarding the Earth magnetic field within various frequencies and magnitudes. The sensitivity of the system behavior to the external magnetic field frequencies and magnitudes, vastly differs among the changes in the magnetic field angle to the Earth's static field. Furthermore, the sensitivity studied under the effect of the environmental noise. The relative spatial orientation of the two magnetic field components plays an important role in the time evolution of the electric dipole moment. Also, deeper discussions on specific relative orientations of the external magnetic fields, such as 24 degree, shows that the quantum model of radical pairs which is based on dipole moment, is in agreement with the results of the birds behavorial studies. These findings provide new insights into the sensitivity of the radical pair model to the combination of magnetic fields and may contribute to a comprehensive understanding of the phenomenon of magnetoreception and the advancement of bioinspired magnetic sensors.
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All authors Ali Soltanmanesh, Mahboobe Sehati, Sareh Rostami, Abolfazl Bahrampour, Alireza Bahrampour

Quantum Physics

Loophole-Robust Certification of Quantum Advantage

Prosanta Pal, Gargee Sharma and Ramakrishna Podila

Claims of quantum advantage should remain robust even when classical strategies have access to side information correlated with the benchmark under evaluation, just as Bell certification must account for measurement dependence. We formalize such correlations as benchmark dependence, a task-level generalization of measurement dependence. For every bounded-reward task, we show that the optimal benchmark-dependent classical score obeys $S_\eta\leq\min\{1,S_{\mathrm{cl}}+\eta\}$, and construct a family of tasks that saturates this bound, showing that the linear dependence on $\eta$ is tight without further assumptions. For repeated product tasks with roundwise dependence, we obtain the stronger multiplicative bound $S_\eta^{(n)}\leq(\omega_{\mathrm c}+\eta)^n$, and extend the framework to finite-sample data, mutual-information constraints, multipartite tasks, and correlations distributed along a causal path. Applying these results to aggregated IBM hardware data, we obtain positive raw-count cycle-product certificates of 0.0812 for CHSH and 0.2178 for Mermin–GHZ, while the nine-context magic-square construction remains uncertified; readout-mitigated values are reported separately as sensitivity estimates. We also analyze a non-Bell quantum-kernel benchmark, where a label-construction variable has measured conditional dependence $\widehat{\eta}_{\lambda}^{(Y)}=0.5$, above the threshold $\eta_{\mathrm{req}}=0.375$, required to close the reported score gap, and yields perfect classical classification. The framework therefore converts a quantum–classical score separation into a quantitative lower bound on the benchmark-correlated classical information required to explain the score separation.
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Quantum Physics

Nearly optimal polynomial approximations for the quantum singular value transform

Evan Rule

We introduce polynomial approximations of the even and odd step functions on the interval $[-1,1]$ with simple Chebyshev coefficients, making their numerical implementation straightforward. We derive rigorous error bounds and demonstrate that these polynomials are nearly optimal in the sense that their error deviates from the theoretically optimal error by a multiplicative factor that grows logarithmically with the polynomial order. From these polynomials, we derive related nearly optimal polynomial approximations that can be used to perform quantum phase estimation, linear amplitude amplification, eigenvalue thresholding, and other quantum algorithms using the quantum singular value transform.
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Quantum Physics · Cross-listed from physics.med-ph

Roadmap Towards Quantum Entanglement Positron Emission Tomography (QE-PET)

Paweł Moskal, Shiva Abbaszadeh, Harmanjeet Singh Bilkhu et al. (+19 more)

Annihilation photons are quantum entangled in their polarization, a property that is not accessible in state-of-the-art clinical Positron Emission Tomography (PET). This roadmap describes the current status of research in the emerging field of quantum entanglement applications involving these photons for medical diagnosis. It outlines the underlying physics phenomena and the development of detector systems that can serve as a foundation for future Quantum Entanglement PET (QE-PET) scanners. These scanners will be capable of utilizing the entanglement between annihilation photons by measuring their Compton scattering on electrons. This roadmap comprises up-to-date experimental results on the study of quantum entanglement and the decoherence of annihilation photons, alongside the current theoretical understanding of these phenomena. The methods and detector technologies described herein are being developed (i) in order to enhance standard PET imaging by suppressing random coincidences in the reconstruction of annihilation site density distributions, (ii) in order to establish the degree of quantum entanglement as a diagnostic biomarker for tissue pathology and oxygenation, and (iii) in order to elaborate a method for pH imaging. Whether entanglement-based imaging and pH mapping can be successfully translated into clinical practice remains an open question and a subject of exciting ongoing research. This roadmap serves as an invitation to the scientific community to join this burgeoning field.
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All authors Paweł Moskal, Shiva Abbaszadeh, Harmanjeet Singh Bilkhu, Peter Caradonna, Pragya Das, Praveen Gurunath Bharathi, Ana Marija Kožuljević, Zdenka Kuncic, Deepak Kumar, Mihael Makek, Marek Nowakowski, Siddharth Parashari, Kaustav Prasad, Gregory R. Romanchek, Sushil Sharma, Kenji Shimazoe, Ewa Stępień, Sodai Takyu, Miwako Takahashi, Mizuki Uenomachi, Ritesh Verma, Taiga Yamaya

Quantum Physics

Anticipating Decoder Side-channel Attacks in Fault-tolerant Quantum Computers

Shashvat Shukla, Dan E. Browne and Shin Nishio

As quantum computing emerges as an applied technology, there is a growing need to protect quantum computers against information security attacks. This work identifies a new class of side-channel attacks against fault-tolerant quantum computers, in which the syndrome data that is sent to the decoder system is used to infer which computation (logical circuit) is taking place on the quantum computer. Our work introduces the concept of gate fingerprints, which describes those patterns present in syndrome data that indicate which logical operation took place on the quantum computer. We show different effects by which logical operations produce gate fingerprints by focusing on Clifford+T computation in the surface code. We then explore how gate fingerprint information can be used to make inferences about the circuits or algorithms run on a quantum computer. Our findings indicate that decoder systems can be a vector for side-channel attacks and thus to prevent this, decoder systems should either be secured or built by a trusted party.
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Quantum Physics · Cross-listed from physics.optics

Long baseline optical interferometric imaging with active phase stabilization

Joshua J. Collier, Leuca Patmore, John S. Wallis et al. (+3 more)

Astronomical observations allow us to better our understanding of the universe. As we observe smaller and more distance features, we run into the diffraction limit of our observation system. This limit is a function of the wavelength observed and the size of the primary aperture used. We can synthesize a larger primary aperture by implementing interferometry. Long baseline optical interferometry would lead to significant improvements in astronomical imaging resolution. Building optical interferometers with free space baselines becomes significantly more difficult as the baseline increases. A promising alternative is to use optical fiber to connect telescopes. Fiber-based interferometers are much more susceptible to phase noise than their free-space counterparts due to inhomogeneities in the fiber medium. This leads to significant degradation of interferometric signals used for astronomical measurements. We implement phase stabilization techniques used in quantum communications to stabilize an optical interferometer with a 170km pseudo-baseline. We use a quantum optimal measurement technique with this interferometer to resolve the extent of a source four times smaller than the diffraction limit of the system within 1.5% error. These results bring the potential for a full-scale on-sky long baseline interferometer significantly closer. A 350km baseline optical interferometer at 1550nm would allow us to resolve sub microarcsecond features in the universe.
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All authors Joshua J. Collier, Leuca Patmore, John S. Wallis, Elrina Hartman, Benjamin P. Dix-Matthews, David R. Gozzard

Quantum Physics

Axion like particles multi-parameter sensing

Hassan Manshouri, Moslem Zarei and Mehdi Abdi

The search for the axion like particles (APLs)-one of the deepest puzzles in modern cosmology-may hold the key to understanding dark matter and dark energy. In this work, we introduce a setup taking advantage of the quantum metrology techniques to constrain the hypothetical mass and coupling constants of APLs by employing exotic pseudoscalar spin-spin interactions between fermions mediated by ALPs. A key advantage of our approach is the exploitation of position-dependent spin sensor results to the high sensitivity of the probe. To simultaneously investigate the axion mass and coupling constants, we invoke a multi-probe detection strategy. Through this strategy, we circumvent the singularity of the quantum Fisher information matrix as an ultimate upper bound on the sensitivity of any probe. For the axion masses in the range of $m_a {\le} 10^{-3} \text{eV}$, this setup can exclude values of axion coupling constants $g^e_pg^n_p$ down to $10^{-7}$.
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Quantum Physics

Witnessing the Quantum Mpemba Effect with a Single Observable

J. M. Z. Choquehuanca, P. A. C. Obando, M. S. Sarandy et al. (+1 more)

The Mpemba effect, in which a system farther from equilibrium relaxes faster than one closer to it, defies the intuition of nonequilibrium statistical mechanics. Here, we introduce a general framework to unambiguously witness this phenomenon based on a single observable defined for Markovian open quantum systems. The Mpemba observable bypasses full state tomography, offering utility for state preparation and thermal task optimization. We illustrate our method using local magnetization as a Mpemba signature, applying it to a qubit under generalized amplitude damping.
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All authors J. M. Z. Choquehuanca, P. A. C. Obando, M. S. Sarandy, F. M. de Paula

Quantum Physics · Cross-listed from math.NA

Splitting Analysis for Yukawa Potential

Di Fang and Jiaqi Zhang

Splitting methods are among the most classical and fundamental tools for the simulation of quantum dynamics, and their importance has grown further with the rise of quantum computing. In this work, we analyze the Schrödinger equation with Yukawa potential, a physically relevant and widely used model potential. It may be viewed as a Coulomb interaction with exponential decay at spatial infinity, preserving the Coulomb singularity at the origin while removing the long-range Coulomb tail. We prove that the operator splitting for this unbounded Hamiltonian achieves a global $1/4$-order convergence rate in the time step for many-body Yukawa interactions, with explicit polynomial dependence on the number of particles. The result holds for all initial wavefunctions in $H^2(\mathbb R^{3N})$, the natural domain of the Hamiltonian, and our numerical experiments are consistent with the theoretical estimates. To identify the sharp obstruction behind this rate, we prove a short-time lower bound in the one-body setting of order $t^{5/4}$ for the one-step error, which rules out any uniform global estimate of order better than $1/4$ in general. This agreement with the optimal $1/4$ rate in the Coulomb case is particularly interesting, as Yukawa potential is short-ranged compared to Coulomb potential. For the many-body upper bound, one of the new technical ingredients is the explicit polynomial-in-system-size Sobolev estimates of many-body Yukawa systems. These estimates are crucial for obtaining fully a priori bounds that depend only on the norms of the initial states, rather than on the solution at time $t$. For the one-body lower bound, we leverage a new analysis argument based on Fourier analysis and Kato smoothing.
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Quantum Physics

Categorical Tensor-Graph Semantics for Quantum Algorithms

Naihong Hu, Ruining Li and Futao Wang

This paper investigates foundational quantum computing protocols from the intuitive perspective of categorical tensor-graph semantics within the category FHilb. While conventional Hilbert-space formalisms often conceal the structural nature of quantum algorithms behind high-dimensional matrix operations, the topological framework directly encodes algorithmic functionalities into their graphical skeletons. We provide a comprehensive topological reinterpretation of the Bernstein–Vazirani and Simon algorithms, demonstrating how topological transformations distill their core mathematical essence and clarify the operational mechanisms of oracles. Going beyond the standard qubit model, we construct explicit representations for the qutrit-adapted topological Deutsch–Jozsa and single-shot Grover algorithms. In particular, we establish a necessary and sufficient condition for the single-shot Grover search. We further implement CNOT gates via complementary Frobenius structures and investigate a diagrammatic decomposition scheme for the W-state preparation protocol. By bridging tensor category theory with practical quantum algorithmic design, this work furnishes a composable, scalable diagrammatic toolkit essential for automated circuit optimization across the evolving quantum hardware ecosystem.
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Quantum Physics

Error correction on an array of superconducting qubits with defective components

Julien M. Drouet, Xanda C. Kolesnikow, Campbell K. McLauchlan et al. (+6 more)

A solid-state quantum-computing architecture will require the fabrication of arrays of many coupled qubits. It is inevitable that this process will produce qubits and couplers with varying performance, with some components underperforming due to imperfect fabrication. Quantum error-correction requires high-performing components and hence these defects must be dealt with, either by adapting the code to exclude the defects, or by informing the decoder to accommodate defects in post-processing. Here we implement and compare strategies to operate distance-5 surface codes on a quantum processor consisting of a square-lattice array of 120 superconducting qubits. We demonstrate a dramatic reduction in the probability of a logical error in a memory experiment by excluding underperforming components, compared with both a standard approach of ignoring defects, and a defect-aware decoding approach. We observe up to 2.8X improvement in logical errors per round when excluding defects compared with the standard defect-ignorant approach (1.62% compared to 4.49%). In contrast, defect-aware decoding gives only modest gains. Defects are also expected to be particularly harmful for measurement-based logical operations. Using a stability experiment we show that excluding defects resurrects measurement-based logic gate performance, observing a 6.3% per-round suppression of failure rate when excluding defects, compared to zero suppression otherwise. Furthermore, we show a further substantial decrease in logical errors when using leakage post-selection in combination with our defect exclusion strategies, resulting in a distance-5 code outperforming the best distance-3 in one basis. Our experiments therefore give a proof-of-principle demonstration of the essential utility of defect exclusion methods in the scale-up of solid-state quantum computing approaches.
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All authors Julien M. Drouet, Xanda C. Kolesnikow, Campbell K. McLauchlan, Georgia M. Nixon, Seok-Hyung Lee, Dominic J. Williamson, Stephen D. Bartlett, Benjamin J. Brown, Robin Harper

Quantum Physics

Defeating Barren Plateaus with Task-Aligned Symmetry

Ruipeng Xing, Yanan Li, Zhen Shang et al. (+3 more)

Barren plateaus – the exponential vanishing of gradients – are a fundamental obstacle to training scalable quantum neural networks. Whether they arise in quantum recurrent neural networks (QRNNs), a natural architecture for sequential data, remains a pressing question. Here we show that the decisive ingredient for trainability in QRNNs is not the recurrent circuit topology per se, but enforcing time-translation symmetry through parameter sharing across time steps. We prove that, without parameter sharing, QRNNs suffer from barren plateaus, with gradient variance decaying exponentially with sequence length. Imposing parameter sharing across time steps fundamentally alters this scaling, transforming it into a polynomial dependence and thereby suppressing the barren plateau. Numerical simulations corroborate these analytical predictions. By rigorously showing how time-translation symmetry suppresses barren plateaus and enhances learning capability in QRNNs, our work establishes task-aligned symmetry as a constructive resolution to the expressivity-trainability tension in quantum neural networks.
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All authors Ruipeng Xing, Yanan Li, Zhen Shang, Shengbin Wang, Yongjian Gu, Zhimin Wang

Quantum Physics

Molecular Docking with Quantum Circuit Evolution

Gleydson Fernandes de Jesus, Bruno Oziel Fernandez and Marcelo A. Moret

Molecular docking is an important step in drug discovery, enabling the evaluation of receptor-ligand affinity while reducing experimental costs and increasing the number of possible tests. However, the high computational cost associated with molecular docking remains a limiting factor that can restrict both the experimental precision and the scale of the problems being addressed. To improve the future applicability of molecular docking, recent works have proposed the use of quantum algorithms based on Gaussian Boson Sampling quantum computers and also gate-based quantum computers. In this work, we propose the use of Quantum Circuit Evolution (QCE) for solving the molecular docking problem, a gate based and gradient-free quantum evolutionary method whose evolution is driven by the random application of unitary operations to a quantum circuit. The proposed algorithm demonstrated the ability to find the best solution to the problem in fewer steps than the methods presented in previous studies, exhibiting fast and stable convergence.
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Quantum Physics

HarmQ: Harmonic Backdoor Attacks Against Quantum Neural Networks

Junrui Zhang, Zemin Chen, Chunsheng Xin et al. (+2 more)

Quantum Neural Networks (QNNs) have emerged as a promising paradigm for quantum machine learning in the Noisy Intermediate-Scale Quantum (NISQ) era, leveraging quantum phenomena such as superposition and entanglement to process information in exponentially large Hilbert spaces. However, QNNs inherit critical security vulnerabilities from classical neural networks, particularly susceptibility to backdoor attacks. Existing attack methods designed for classical systems fail against QNNs due to quantum-specific constraints: aggressive downsampling required by limited qubit resources destroys conventional triggers, while the spectral learning bias of parameterized quantum circuits (PQCs) restricts learnable patterns. To tackle this, we present HarmQ, a quantum-native backdoor attack that exploits PQCs' inherent Fourier decomposition bias through harmonic trigger patterns. Our approach employs sinusoidal perturbations on coarse grids with block-uniform structure, ensuring survival through downsampling while aligning with PQCs' preference for low-frequency components. This enables effective backdoor injection under realistic black-box conditions where attackers access only training data. Experiments on MNIST and Fashion-MNIST demonstrate that HarmQ achieves attack success rates exceeding 99% while maintaining over 90% clean accuracy, significantly outperforming existing methods including BadNets (2.77% ASR), Watermark (7.96% ASR), Q-FGSM (44.32% ASR) and QUAP (3.40% ASR). Parametric t-SNE visualizations of quantum state representations confirm that harmonic triggers create distinctly separated clusters, evidencing HarmQ as a fundamental security threat for QNNs.
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All authors Junrui Zhang, Zemin Chen, Chunsheng Xin, Hongyi Wu, Rui Ning

Quantum Physics

Entangling Power and Symmetries in the Quantum Rabi Model

Ian Low, Jens Koch and Sahel Ashhab

The quantum Rabi model is a standard effective Hamiltonian in studies of light-matter interaction, capturing the simplest nontrivial setting in which a qubit couples to a single harmonic oscillator. Within the broader Rabi family, we focus on two special cases: the Jaynes-Cummings (JC) model, which carries an explicit $U(1)$ symmetry, and the asymmetric quantum Rabi model (AQRM), which possesses a parameter-dependent "hidden” symmetry that appears only at integer bias, $\varepsilon/\omega\in\mathbb{Z}$, and is not manifest in the Hamiltonian. We use the time-averaged entangling power as an operator-level diagnostic of these symmetry structures. Since the oscillator Hilbert space is infinite-dimensional, we compare two finite input ensembles: a Haar average after Fock-space truncation and a coherent-state average at fixed mean occupation $\bar{n}$. Both diagnostics show peaks at the integer-bias points of the AQRM, where the hidden symmetry resides. In contrast, the manifest $U(1)$ symmetry at the JC point instead gives a weak dip. Thus, the time-averaged entangling power responds to both the hidden symmetry and the $U(1)$ symmetry in the Rabi family, with the sign of the response indicating how the symmetry reorganizes the spectral expansion. These results demonstrate that the entangling power can serve as an operator diagnostic to reveal the presence and properties of hidden and manifest symmetries in light-matter systems.
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Quantum Physics

Observation of gravity-like signatures in holographic codes on a quantum computer

Debopriyo Biswas, Gong Cheng, Krishnanand Karthikeyan et al. (+12 more)

The unification of quantum mechanics and general relativity remains one of the major open problems of theoretical physics. The Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence provides a valuable theoretical framework for this effort via a holographic duality between a theory of quantum gravity in asymptotically AdS spacetime and a conformal quantum field theory on the lower-dimensional boundary. Here, we implement a toy model of this duality called the HaPPY code, a quantum error-correcting code in the form of a tensor network with hyperbolic entanglement patterns, on a trapped-ion quantum computer. We present the first experimental confirmation of the Faulkner-Lewkowycz-Maldacena formula in this model - a key test of the holographic correspondence. We then enrich it with non-stabilizerness, or magic, and observe entropic precursors expected of emergent gravity. Finally, we present and measure a code construction whose entropic behavior is reminiscent of a highly quantum wormhole. Our experiments illustrate how quantum computers can serve as testbeds for modeling the emergence of spacetime.
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All authors Debopriyo Biswas, Gong Cheng, Krishnanand Karthikeyan, Diana Muñoz-Valencia, Vincent P. Su, Hrant Gharibyan, Daiwei Zhu, Grant Salton, Evgeny Epifanovsky, Martin Roetteler, Christopher Monroe, John Preskill, Norbert M. Linke, ChunJun Cao, Crystal Noel

Quantum Physics

Dissipative phase transitions and chaos in two-photon driven quantum optomechanics

Giovanni Bragadin, Filippo Ferrari, Lorenzo Fioroni et al. (+4 more)

We investigate nonequilibrium criticality and chaos in a two-photon-driven optomechanical system. The parametric drive preserves a discrete $\mathbb{Z}_2$ symmetry of the optical field, while radiation-pressure coupling transfers the resulting nonlinear dynamics to a mechanical oscillator. Combining semiclassical stability analysis, exact Liouvillian spectra, and stochastic quantum trajectories, we show that this driven-dissipative optomechanical model supports both first- and second-order dissipative phase transitions. At negative detuning a second-order transition yields spontaneous breaking of the cavity-parity symmetry in the thermodynamic limit. At positive detuning the same symmetry breaking coexists with a first-order transition, signaled by metastability and by an additional symmetric Liouvillian mode. At stronger pump power the mean-field dynamics loses all stable fixed points and develops limit cycles and chaotic attractors with positive Lyapunov exponent. Quantum trajectories in this regime display chaotic-like motion, enhanced steady-state entropy, and delocalization over many entropic Liouvillian modes. These results establish two-photon-driven optomechanics as a platform where dissipative criticality, symmetry breaking, and quantum signatures of chaos emerge within the same experimentally accessible setting.
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All authors Giovanni Bragadin, Filippo Ferrari, Lorenzo Fioroni, Matteo Seclì, Vincenzo Macrì, Vincenzo Savona, Alberto Mercurio

Quantum Physics

Quantum magic and non-commutativity as computational resources in quantum reservoir computing

Wei Xia, Shuaifan Cao, Xingze Qiu et al. (+1 more)

Quantum reservoir computing (QRC) provides a hardware-efficient paradigm for temporal information processing on near-term quantum devices. Despite rapid experimental progress, a rigorous understanding of the structural conditions required for its scalable quantum-enhanced performance remains lacking. Here, we develop a theoretical framework in Pauli-Liouville space that provides a unified analytical treatment of the echo state property (ESP), nonlinear expressive power, and quantum resources. We first analyze the widely used qubit-resetting scheme and establish that quantum magic generated by reservoir dynamics is a necessary condition for effective computation, a requirement more fundamental than ESP. However, we prove that this architecture faces inherent expressivity limitations: all nonlinear processing originates exclusively from the classical encoding map, imposing an unavoidable trade-off between nonlinearity and memory capacity. To circumvent this structural bottleneck, we rigorously analyze Hamiltonian encoding, in which temporal inputs are embedded directly into the continuous dynamics generator. We show that the ESP is natively guaranteed by the Liouvillian spectral gap, decoupling it from quantum magic. Crucially, for any non-trivial drive Hamiltonian, the discrete-time update map exhibits a transcendental, infinite-order nonlinear dependence on the instantaneous input. Moreover, the intrinsic non-commutativity of the open-system generators governs the temporal coupling of these nonlinearities, producing highly non-separable processing of the input history. Our results establish a rigorous theoretical hierarchy of QRC architectures and provide prescriptive design principles for experiments targeting genuine quantum advantages in temporal processing.
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All authors Wei Xia, Shuaifan Cao, Xingze Qiu, Xiaopeng Li

Quantum Physics

Quantum incompatibility of Born probabilities

Esteban Castro-Ruiz, Nathan Cohen, Luis C. Barbado et al. (+1 more)

Quantum theory challenges the view that individual measurement outcomes are predefined and independent of the measurement context. Yet the quantum state itself – the catalogue of probabilities for all possible measurements – is usually assumed to be well defined. We argue that this assumption tacitly relies on measurements being performed relative to ideal, infinitely-resourceful reference frames. We show that, when measurements are made relative to non-ideal quantum reference frames, the probabilities themselves become indefinite: even in the limit of arbitrarily large number of runs, the relative frequencies may remain uncertain. The uncertainty is irreducible in a quantum-mechanical sense, as we show by proving a Bell-type theorem for relative frequencies. We further propose a quantum-optical implementation of these relational measurements based on pulsed homodyne detection. Our findings motivate an extension of the notion of the quantum state to regimes constrained by finite resources. We expect them to be especially relevant at the interface between quantum theory and general relativity, where the resources and information available in a bounded region of spacetime are fundamentally limited.
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All authors Esteban Castro-Ruiz, Nathan Cohen, Luis C. Barbado, Časlav Brukner

Quantum Physics · Cross-listed from cond-mat.str-el

Symmetry-twisted multi-entropies: Order parameters for 2D SPT phases

Ramanjit Sohal, Michael Levin and Ruben Verresen

Although symmetry-protected topological phases (SPTs) can be distinguished by their entanglement properties, it has been unclear how to extract this information directly from expectation values beyond the 1D case. Here, we close this gap and propose a pair of nonlocal order parameters that can detect and distinguish all bosonic SPTs in 2D protected by internal discrete, Abelian unitary symmetries. The desired topological invariants are extracted by these quantities by effectively simulating the SPT path integral on topologically non-trivial spacetime manifolds. Our order parameters are defined in terms of expectation values of partial symmetry and permutation operations acting on fixed numbers of replicas of the system in finite spatial regions. These expectation values correspond to symmetry-twisted versions of multipartite entanglement quantities known as multi-entropies. We show explicitly that our two order parameters detect symmetry-protected four-party and six-party entanglement, respectively, and we constrain possible "spurious" contributions. We analytically test our proposal in fixed-point lattice models. Our results suggest multipartite entanglement to be a defining feature of SPTs; indeed, we expect our methods to generalize to fermionic and higher-dimensional systems.
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