Quantum Physics
End-to-End Molecular Dynamics with a Langevin Thermostat on Quantum Circuits
Masari Watanabe, Hirofumi Nishi, Taichi Kosugi et al. (+3 more)
We construct a
quantum-
circuit framework for finite-temperature molecular dynamics in the canonical ensemble (NVT) with a Langevin thermostat, connecting canonical state preparation to subsequent physical-property readouts. The classical
nuclear phase-space distribution is encoded as a Koopman–von Neumann (KvN) wave function, and canonical state preparation is formulated as Langevin-type Fokker–Planck relaxation. The
Hamiltonian Liouville flow, momentum friction, and momentum
diffusion are decomposed into separate
circuit blocks. The friction block is represented by a symmetrized momentum-space dilation, whereas the
diffusion block is implemented as a cosine filter realized by probabilistic imaginary-time
evolution (PITE). We analytically quantify the leading-order temperature bias caused by replacing the Gaussian
diffusion kernel with this PITE-realized cosine filter. This analysis yields an internal-temperature correction that targets the desired physical
equilibrium distribution. As a proof-of-concept demonstration connecting
quantum chemistry to KvN
nuclear dynamics, we study the H$_2$ molecule. Numerical simulations show relaxation from a nonequilibrium phase-space distribution to a canonical KvN state. From this canonical state, we demonstrate two complementary readouts: a dynamical
quantum-phase-estimation readout of the vibrational density of states associated with the H–H stretch coordinate and a static canonical evaluation of the transition-state-theory (TST) rate constant. This work demonstrates, in a minimal molecular system, a
circuit-level protocol that connects Langevin canonical state preparation to physical-property calculations, providing a concrete step toward
quantum–classical hybrid molecular dynamics on
quantum computers.
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All authors Masari Watanabe, Hirofumi Nishi, Taichi Kosugi, Shigekazu Hidaka, Ryo Sakurai, Yu-ichiro Matsushita
Quantum Physics
Koopman–von Neumann Molecular Dynamics for Green–Kubo Transport Coefficients
Masari Watanabe, Hirofumi Nishi, Taichi Kosugi et al. (+3 more)
We formulate the Green–Kubo transport coefficients of classical molecular dynamics as a readout problem for
quantum algorithms using the Koopman–von Neumann (KvN) representation. Both NVE and Nosé–Hoover-type NVT dynamics are derived as
unitary evolutions on
Hilbert spaces associated with the corresponding classical phase spaces. Numerical
benchmarks on finite grids show that the discretization error in the correlation function decreases as a power law in the number of grid points $N_z$. Equivalently, with $N_z=2^{n_z}$, the error decreases exponentially in the register size $n_z$, so a target accuracy $\epsilon$ requires $n_z=\mathcal{O}(\log(1/\epsilon))$
qubits. To read out a transport coefficient, we input a flux-excited state to
quantum phase estimation (QPE). The probability $P_0$ of measuring the QPE ancilla register in the all-zero state corresponds to a Bartlett-windowed Green–Kubo integral. With maximum-likelihood amplitude estimation, the statistical estimation of $P_0$ defined by this QPE oracle improves from the $N_{\rm queries}^{-1/2}$ scaling of direct shot sampling to scaling close to $N_{\rm queries}^{-1}$. Our
circuit-resource analysis shows that one step of the NVE propagator can be built with $\mathcal{O}(n^2)$ CX gates, where $n=n_x+n_p$ is the total number of position and momentum
qubits. For the NVT propagator, the centered-difference Pauli-decomposition implementation of the Nosé–Hoover friction term scales as $\mathcal{O}(n_\xi n_p\,2^{n_p})$, where $n_p$ and $n_\xi$ are the numbers of momentum and thermostat
qubits, respectively. The proposed framework is a concrete step toward translating the principles of
quantum algorithms into the transport-coefficient calculations required in practical molecular simulation.
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All authors Masari Watanabe, Hirofumi Nishi, Taichi Kosugi, Shigekazu Hidaka, Ryo Sakurai, Yu-ichiro Matsushita
Quantum Physics
Overcoming the Matrix-Product-State Encoding Barrier via DMRG-Guided Probabilistic Imaginary-Time Evolution
Masari Watanabe, Hirofumi Nishi, Taichi Kosugi et al. (+2 more)
Ground-state preparation is a fundamental task in
quantum simulation, because the overlap of the prepared state with the true ground state significantly affects the overall cost of subsequent
quantum algorithms. We propose a three-stage framework in which a
matrix product state (MPS) of an $N$-site system obtained by the density-
matrix renormalization group (DMRG) is loaded onto an $N$-
qubit quantum register through an optimization-free
matrix product disentangler (MPD) encoding
circuit, and the residual error is then reduced by probabilistic imaginary-time
evolution (PITE). We demonstrate that the central-bond Schmidt rank of intermediate states during MPS encoding grows logistically with the number of layers. Its inflection point $L^{*}$ marks the boundary of the efficient encoding regime. Beyond this point, the gain in fidelity slows rapidly, and the number of additional MPD layers required to reach a target infidelity $\varepsilon$ empirically scales as $\mathcal{O}(N^5\log(N/\varepsilon))$. To avoid this encoding-only tail, we stop the encoder at $L^{*}$ and suppress the remaining excited-state components by PITE, with the linear PITE schedule fixed deterministically from the ground-state energy, the effective gap, and the reference overlap estimated by DMRG. Numerical experiments on the
spin-$1/2$ staggered-field Heisenberg chain show that the framework avoids very deep encoding
circuits and substantially suppresses the post-selection overhead intrinsic to PITE. Combining classical preprocessing by DMRG, optimization-free MPS encoding, and deterministically scheduled PITE, the present framework offers a practical hybrid route to ground-state preparation in
quantum simulation.
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All authors Masari Watanabe, Hirofumi Nishi, Taichi Kosugi, Shinji Tsuneyuki, Yu-ichiro Matsushita
Quantum Physics
Simulation of smooth models of potentials with singular point using Many-Interacting-Worlds Method
Wen Chen and An Min Wang
The deterministic many-interacting-worlds method proposed in 2014 showed potential among the numerous interpretation of
quantum mechanics. The successful application of this method in harmonic oscillator has been promoted for a long time. In this article we continue the idea about using this method to solve some bounded systems different from harmonic oscillator potential and extend to 2 dimension cases. We focus on the potential with singularity like coulomb potential and finite trap potential by some asymptotic smooth method. The numerical simulation mainly based on the dynamical
algorithm proposed in many-interacting-worlds method will be used to approach the stationary states of given systems. Our results shows the consistency to the
matrix Numerov method in standard
quantum mechanics in solving bounded systems and provides the possibility to solve more complex systems.
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Quantum Physics
Alternative adiabatic quantum dynamics with algorithmic applications
Joseph Cunningham and Jérémie Roland
In adiabatic
quantum computing the aim is to track an eigenstate as the
Hamiltonian changes. In the usual setup this is achieved using the natural time-dependent
Hamiltonian evolution of the system and the main technical tool is the adiabatic
theorem. We propose several alternative processes that achieve the same goal, but can easily be implemented on a gate-based
quantum computer without the overhead of simulating time-dependent
Hamiltonian evolution. We give a general framework for deriving `adiabatic'
theorems for these processes. As an application, we give various
algorithms for solving the
Quantum Linear Systems Problem (QLSP) with optimal scaling in the condition number. One of these
algorithms was previously developed in [Cunningham, Roland 2024] and another can be seen as a randomised version of the discrete adiabatic
algorithm of [Costa et al. 2022]. We also describe versions of Trotterisation in our framework, which allows several results from [An et al. 2025] to be reproduced in a randomised setting. In particular, bounds on the Trotter error in terms of the fidelity are obtained that are asymptotically better than the standard bounds.
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Quantum Physics
Asymptotic magic state distillation with almost linear rate
Koki Ehara and Ryuji Takagi
The overhead exponent – characterizing the scaling of the number of noisy magic states with respect to the target
distillation error – has been a central quantity to
benchmark magic state
distillation protocols. On the other hand, a related but less investigated quantity motivated by an information-theoretic viewpoint is the asymptotic
distillation rate, the largest ratio of output to input magic states such that error vanishes asymptotically. These two quantities are tightly related in the specific case – the overhead exponent is zero if and only if the asymptotic
distillation rate is linear. However, their relationship in other regimes has been unclear. Here, we show that their quantitative relation is generally not robust, by presenting a family of magic state
distillation protocols with an overhead exponent not close to zero – in fact, larger than one – that still achieves the asymptotic rate arbitrarily close to the linear rate. This implies that the
distillation rate is not constrained by the overhead exponent within the sublinear rate regime. Notably, our protocol is based on error checking by measurements of logical Clifford operators, which underlies the recent magic state cultivation protocol, suggesting the potential of this mechanism for asymptotic magic state
distillation.
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Quantum Physics
Quantum Mechanics: Problems and Paradoxes
L. V. Prokhorov, Alexander Ushakov S. and Yury Berdinsky
This book examines a number of problems of
quantum mechanics, most of which are not usually discussed. What is the origin of probabilities in the mechanics of the microworld? What is the nature of Planck's constant h? What is the nature of probability amplitudes? What is the wave function? A system of axioms for
quantum theory is formulated. A model is studied according to which a classical oscillator in a thermostat can be interpreted as a
quantum one. The measurement problem is discussed in detail. For advanced undergraduate students, graduate students, and specialists interested in the foundations of
quantum theory.
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Quantum Physics · Cross-listed from physics.optics
Observation of Electrically Tunable Chirality Inversion in a Slow-Light Waveguide
Xuchao Chen, Savvas Germanis, Nicholas J. Martin et al. (+8 more)
We identify
chiral inversion points in slow-light, glide-plane-symmetric, photonic-
crystal waveguides, defined as fixed locations where the local optical chirality changes sign over a narrow wavelength range. We experimentally access this behaviour using a waveguide-embedded InAs/InGaAs
quantum dot. The slow-light spectral region is determined from time-integrated and time-resolved photoluminescence, and the dot
exciton is electrically tuned across the slow-light bandwidth via the
quantum-confined Stark effect. As the emission wavelength is swept through the slow-light region, the directional emission contrast shows a strong wavelength dependence and a sign reversal, consistent with the identified
chiral inversion point. Numerical simulations attribute the switching primarily to the pronounced spectral variation of the local optical chirality for emitters displaced from the waveguide center. These results demonstrate on-demand electrical switching of
chiral light-matter coupling in nanophotonic waveguides and enable tunable
chiral interfaces for integrated
quantum photonic devices.
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All authors Xuchao Chen, Savvas Germanis, Nicholas J. Martin, Hamidreza Siampour, René Dost, Dominic J. Hallett, Ian Farrer, Akshay Kumar Verma, Maurice S. Skolnick, Luke R. Wilson, A. Mark Fox
Quantum Physics
A comparison of different master equations for driven-dissipative dynamics in composite quantum systems: Dispersive readout in structured electromagnetic environments
Prakritish Gogoi, Angela Riva, Émile Cochin et al. (+1 more)
Driven-dissipative
qubit-resonator dynamics, which are the basis of most dispersive superconducting
qubit measurement schemes, are often modeled with Lindblad master equations built from subsystem local jump operators, even when the
qubit and resonator are appreciably hybridized. In this work we revisit this setting using a microscopic Bloch-Redfield approach, where dissipation is constructed in the eigenbasis of the coupled
qubit-resonator
Hamiltonian with a complete, frequency dependent, open system description of the transmission line environment. Here, we show that the Lindblad and Bloch-Redfield decay rates can be quantitatively different in the absence of driving, while in the driven case we demonstrate that the time-independent Redfield dissipator and its time-dependent generalization can show qualitatively different behaviors as a function of driving strength. Finally, we investigate the effects of driving in structured spectral densities, recovering the suppression of measurement-induced relaxation in the presence of a so called Purcell filter.
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All authors Prakritish Gogoi, Angela Riva, Émile Cochin, Alex Chin
Quantum Physics
On the question of noise as a resource in quantum computing
J. Montes, F. Borondo and Gabriel G. Carlo
Noise is usually regarded as the main obstacle to achieving a scalable
quantum advantage, but recent evidence in
quantum reservoir computing [L. Domingo, F. Borondo, and G. G. Carlo. Taking advantage of noise in
quantum reservoir computing, Scientific Reports, 13:8790, 2023] suggests that certain channels can, in appropriate regimes, improve performance by enriching the reservoir's effective dynamics. Motivated by this idea we propose a
geometric mechanism to explain how non-unital noise applied together with a universal gate set leads to a faster approach to Haar-like distributions of the final states. We find that noise of this kind induces an effective volume expansion on the
manifold of pure states. In order to intuitively understand this we use a minimal 1
qubit model where we take the amplitude damping channel and combine it with a renormalization rule that associates to each resulting mixed state a representative pure state. This composition defines a globally expanding nonlinear map on the space of pure states. We analytically derive the local area expansion factor and identify the global expansion threshold. Finally, we combine amplitude damping with the G3 = {H, T, CNOT} universal gate set to show how the approach to Haar-like behavior is faster in an appropriate parameter region. This leads us to propose noise as a possible resource in future
quantum algorithms.
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Quantum Physics
Elfs, transducers and quantum walks
Simon Apers, Jérémie Roland and Yuxin Zhang
Electric flow sampling (elfs) is a new tool in the
quantum walk toolbox and a useful primitive for solving search, sampling and optimization problems on graphs. We refine this tool by showing that there exists a zero-error transducer for implementing elfs. More broadly, we establish a zero-error transducer for reflecting about the intersection of two subspaces, yielding an errorfree transducer version of the effective gap lemma. Building on this result, we obtain improved
quantum walk
algorithms for estimating effective resistances and span program witness sizes with an optimal error scaling, and for sampling from the random walk arrival distribution, via the composition of many elfs. Using this last
algorithm, we obtain an up-to-quadratic
quantum speedup for semi-supervised learning on expander graphs.
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Quantum Physics · Cross-listed from cond-mat.stat-mech
Hidden Ising models from the generalized Yang-Baxter equation
Akash Sinha, Somnath Maity, Pramod Padmanabhan et al. (+1 more)
We introduce a one dimensional
spin $\frac{1}{2}$
Hamiltonian with multi-site interactions, but still local. The
algebra of its
Hamiltonian densities resembles that of the transverse field Ising model. Using this fact we show that its spectrum is free-fermionic but with a huge degeneracy for each level. The source of the degeneracy is a set of local conserved quantities that act like a classical background field for the
quantum system. The thermodynamics of this system is contrasted with the standard Ising model. At the gapless points in the energy spectrum, we show that this system can be derived from the
quantum inverse scattering method adapted to a multi-site generalization of the Yang-Baxter equation as introduced by E. Rowell and Z. Wang. The $R$-
matrix is constructed using generators of extraspecial 2-groups. This helps us extract all the conserved charges and lay the framework for a general mechanism to generate such multi-site interaction
spin systems that are transverse field Ising models under the hood. A remark on how to obtain P. Fendley's free-
fermion in disguise models in this formalism is also included.
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All authors Akash Sinha, Somnath Maity, Pramod Padmanabhan, Vladimir Korepin
Quantum Physics
Quantum Networks Using Color Defects in Diamond: Principles, Progress, and Perspectives
Ayan Majumder, Cem Güney Torun, Tim Schröder et al. (+3 more)
Large-scale
quantum networks will enable entirely new applications of
quantum information science in fields such as
quantum communication, distributed
quantum computing, sensing, and metrology. To build nodes of such networks, diamond color defects are one of the promising candidates. Their excellent optical properties, fast
spin-
qubit control, and long
spin coherence times make them well-suited for
quantum information processing and
quantum memory applications. Additionally, recent advances in the heterogeneous integration of diamond nanophotonic structures with photonic integrated
circuits have made these systems more efficient and well-suited for scalable
quantum processor architectures. In this comprehensive review, we discuss the optical and
spin properties of these systems, recent progress in the building blocks of
quantum networks, and demonstrations of metropolitan-scale
quantum networks, as well as the challenges associated with these systems at both the fundamental and experimental levels, along with potential solutions.
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All authors Ayan Majumder, Cem Güney Torun, Tim Schröder, Gregor Pieplow, Prem Kumar, Kasturi Saha
Quantum Physics
A Neutral-Atom Quantum Compiler with Application-Specific Layout and Hub-Assisted Shuttling
Takahiko Satoh and Takaharu Yoshida
Compiling arbitrary-connectivity NISQ
circuits onto monolithic single-zone neutral-atom devices is constrained by a finite interaction range and a minimum separation between simultaneously addressable sites. Under the minimum-separation constraint, the SWAP-only configuration of our pipeline does not return a schedule within a practical time budget on a range of
circuits, including
circuits as small as nine
qubits. We address this with hub traps, a small number of dynamically placed empty traps that serve as transit waypoints, together with a per-gate rule that chooses between SWAP-based routing and hub-mediated shuttling. We evaluate the compiler on seventeen
benchmarks using analytic estimates of execution time and a per-layer fidelity proxy, comparing against a placement-matched baseline and against ablations of our own pipeline. Hub traps make these otherwise-unsolved
circuits compile in seconds to minutes and remove SWAP gates entirely on every completed
circuit, so their role is to enable routing rather than only to optimize fidelity. The benefit is concentrated on routing-dominated
circuits and is absent on routing-free ones, which we separate by the structure of the interaction graph. On the most routing-dominated
circuit the fidelity proxy improves by up to three orders of magnitude over the placement-matched baseline. The gain comes primarily from eliminating SWAP overhead, as the absolute fidelities there remain low.
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Quantum Physics
Quantitative semidefinite certificates for ground-state energies of Pauli Hamiltonians
Igor Klep, Nando Leijenhorst and Victor Magron
The $k$-local
Hamiltonian problem is a central model for
quantum many-body systems and
Hamiltonian complexity. Semidefinite programming and noncommutative sum-of-squares hierarchies provide systematic certificates for ground-state energies, but existing finite-convergence results give no quantitative guarantee on the accuracy of the low hierarchy levels accessible in computation. We prove explicit finite-level convergence rates for these hierarchies in the Pauli setting. For $k$-local
Hamiltonians whose Pauli expansion contains only even-weight terms, we show that both the NPA-type lower-bound hierarchy and the upper-bound hierarchy on the spectral minimum have error at most $C(k)\xi^{n,4}_{d+1}/n$, where $\xi^{n,4}_{d+1}$ is the smallest root of a Krawtchouk
polynomial and $C(k)$ is independent of the number of
qubits $n$ and the hierarchy level $d$. General $k$-local
Hamiltonians reduce to this even-weight case by adding one ancilla
qubit while preserving the spectrum. The proof constructs almost-reproducing kernels for the Pauli
algebra and relates their spectra to Krawtchouk
polynomials, giving a noncommutative analogue of recent kernel-based convergence analyses for commutative
polynomial optimization. These results provide the first quantitative finite-level accuracy guarantees for noncommutative semidefinite relaxations of Pauli
Hamiltonians.
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Quantum Physics
Quadratic Sums-of-Powers for Fixed-Parameter Tractable Quantum-Circuit Simulation
Alexis de Colnet, Floris Geerts, Rihan Hai et al. (+3 more)
Strongly simulating a
quantum circuit, that is, computing an output amplitude, can be done by summing the
circuit's Feynman paths: a weighted count over assignments to Boolean path variables. The
circuit's gates induce correlations among these variables, forming a graph whose structure controls several exact simulation routes. This sum-of-powers (SOP) viewpoint underlies recent simulators built on binary decision diagrams and weighted model counting. For a quadratic SOP with $n$ variables, even modulus $r$, and a rank-decomposition of its variable graph of width $k$, our dynamic program (DP) computes an amplitude using only $O(4^kpoly(n))$ arithmetic operations. For Clifford$+T$
circuits, the amplitude is given by an SOP with modulus $8$. Rank-width never exceeds linear rank-width, which governs some decision-diagram approaches, and is at most one greater than the Markov–Shi contraction complexity of the
circuit tensor network. Moreover, there are non-Clifford families of bounded rank-width where both competing parameters diverge. We also present a stabilizer-rank optimization, exploiting that the DP tables are stabilizer-type Gauss sums. Each subtree runs at the width price of its cut-ranks or at a magic price that discharges the non-Clifford phases below it. The resulting best total cost never exceeds $O(4^kpoly(n))$, yet is
polynomial on mixed families where the pure rank-width and pure $T$-count guarantees are both exponential. Clifford amplitudes take
polynomial time on any graph, the exact-amplitude consequence of Gottesman–Knill. A prototype evaluation on standard
circuit benchmarks finds treewidth bucket elimination the strongest baseline, with the new rank-width DP complementary: it wins on structured families where treewidth blows up.
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All authors Alexis de Colnet, Floris Geerts, Rihan Hai, Alfons Laarman, Joon Hyung Lee, Guillermo A. Pérez
Quantum Physics · Cross-listed from physics.atom-ph
Restoring Velocity Immunity via Dynamic Mirror Compensation in a Large-Area Dual-Atom-Interferometer Gyroscope
Jie Gu, Yin-fei Mao, Zhan-Wei Yao et al. (+10 more)
We propose and demonstrate a dynamical mirror compensation scheme to restore velocity immunity in a large-area dual-atom-interferometer gyroscope. In an ideal Mach-Zehnder configuration, the phase shift is inherently immune to atomic velocity, but this property is broken by the Earth's rotation via the Coriolis effect. We overcome this by actively rotating the Raman mirrors during the pulse sequence to cancel the time-dependent angular offset. The implementation relies on a decouplable calibration-compensation chain to remove rotation-induced time-dependent terms. The scheme is validated on a dual-atom-interferometer gyroscope with an interference area of 21.1 cm^2. After compensation, the phase's dependence on atomic velocity is reduced 40-fold, and the velocity contribution to scale-factor stability is evaluated to be 0.13 ppm. The sensor achieves a rotation sensitivity of 1.3\times10^{-8} rad/s/Hz^{1/2} and a stability of 1.9\times10^{-10} rad/s at 4500 s integration, together with a common-mode noise rejection ratio of up to 459, demonstrated in a seismic event. This work removes a key obstacle to scale-factor stabilization in atom-interferometer gyroscopes and paves the way for their applications in inertial
navigation and geophysics.
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All authors Jie Gu, Yin-fei Mao, Zhan-Wei Yao, An-qing Zhang, Si-Bin Lu, Shao-kang Li, Min Jiang, Xiao-Li Chen, Min Ke, Xi Chen, Run-Bing Li, Jin Wang, Ming-Sheng Zhan
Quantum Physics
Evaluating Parameter Transfer in FALQON Across Graph Families
Alisson dos Passos Fumaco, Marcos Vinicius Reballo, Fernando Augusto Caletti de Barros et al. (+2 more)
We evaluate FALQON parameter transfer for Max-Cut, transferring sequences from small donors ($n \in \{8,10,12\}$) to 14-node recipients. Using 3-regular and Erdős-Rényi families, we show that transfer success is dictated by the recipient graph, not the donor. Transfer excels for dense recipients – achieving high approximation ratios regardless of the donor – but remains challenging in sparse cross-family cases. Crucially, performance is highly resilient to donor size, with 8-node donors matching larger instances. Thus, cheap small graphs can provide robust parameters for larger targets, significantly reducing the measurement overhead of the feedback loop.
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All authors Alisson dos Passos Fumaco, Marcos Vinicius Reballo, Fernando Augusto Caletti de Barros, Gabriel Fernandes Thomaz, Eduardo I. Duzzioni
Quantum Physics
Dynamical Casimir photons from rotation of a nonspherical particle
Guilherme C. Matos, Lucas Bianchi, Jeremy N. Munday et al. (+3 more)
We consider a non-spherical neutral particle spinning in free space and interacting with the electromagnetic
quantum vacuum. When the rotation axis is orthogonal to the particle
symmetry axis, the scattered field develops frequency sidebands that induce the parametric emission of dynamical Casimir
photon pairs. Under the structural constraint of a maximum tip velocity, the emission rate is maximized for a nearly spherical geometry and is further enhanced near a polaritonic resonance. For realistic material parameters, even these optimized upper bounds remain exceedingly small, setting stringent quantitative limits on free-space rotational dynamical Casimir emission with a single nanoparticle.
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All authors Guilherme C. Matos, Lucas Bianchi, Jeremy N. Munday, François Impens, Reinaldo de Melo e Souza, Paulo A. Maia Neto
Quantum Physics
Verifying Adversarial Robustness in Quantum Machine Learning: from theory to physical validation via a software tool
Ji Guan and Mingsheng Ying
As with classical neural networks,
quantum machine learning (QML) models are vulnerable to small input perturbations that can significantly alter output predictions. Certifying the robustness of QML models, particularly on NISQ hardware, is therefore a fundamental step toward trustworthy
quantum AI. This chapter reviews our recently developed comprehensive formal framework for verifying
adversarial robustness in QML. The core of this framework is a fidelity-based robustness lower bound computable directly from the measurement outcome distribution, which enables both formal verification and empirical estimation on real
quantum devices. Additionally, the optimal bound can be computed via semidefinite programming (SDP) with full knowledge of the
quantum machine learning models. We incorporate these results into: (1) an efficient formal verification framework; (2) VeriQR, the first dedicated QML robustness verification tool; and (3) the first experimental
benchmark of
quantum adversarial robustness on a 20-
qubit superconducting processor. Together, these systematic advances enable scalable, physically grounded robustness evaluation of QML models.
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Quantum Physics
Claim against Measurement: Statistical Artefacts in Quantum Error Mitigation Benchmarks
Dominik Köster and Wolfgang Mauerer
QEM is widely regarded as a plausible bridge from NISQ devices to FTQC. Yet the empirical studies used to assess the effectiveness of QEM techniques on concrete problems have received comparatively little scrutiny with respect to the validity of their conclusions. We systematically review 81 recent QEM papers using an eight-criterion framework covering statistical rigour, reproducibility, and reporting quality. Among the applicable papers, only 15 (25%) use inferential methods, while 25 (42%) report uncertainty only descriptively, without testing whether the claimed effects are statistically supported. To demonstrate the consequences of these omissions, we use ZNE as a representative and widely used case study and identify two compounding sources of artefacts in current QEM
benchmarks. First, we observe parameter sensitivity: in a 132-configuration sweep, implicitly assumed choices such as scale factors, extrapolation method, and hardware calibration are not merely incidental but active, with variations changing conclusions from statistically significant improvement to statistically significant degradation. Second, we identify a drift-induced effectiveness illusion: in a 72-hour longitudinal study on real hardware, temporal drift alone can make the same ZNE configuration exhibit an effect size more than three times as large, depending solely on when it is executed, and also drastically reduces the effective number of independent observations. These findings do not imply that QEM methods are intrinsically unsound; rather, they show that current evaluation practice can make mitigation performance appear more robust than the evidence warrants. We therefore propose minimum reporting standards for QEM evaluations, including explicit parameter documentation, robustness checks, longitudinal drift assessment, and inferential statistical testing with effect-size reporting.
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Quantum Physics · Cross-listed from cond-mat.dis-nn
Enhanced Density Fluctuations Near a Disordered Chiral Topological Transition
Hai-Tao Ding, Sen Mu, Leong-Chuan Kwek et al. (+2 more)
The universal statistics of density fluctuations of localized
quantum states may offer unprecedented opportunities to probe and understand
quantum transport in connection with dimensionality, coherence,
symmetry and disorder. To date, the possible role of topological phase transitions in the fluctuation statistics is not studied yet. Using a Su-Schrieffer-Heeger chain subject to off-diagonal disorder (so that
chiral symmetry is preserved), this work investigates how a disorder driven topological phase transition impacts on the spatial fluctuations of the logarithmic wave-packet density $\ln P(r)$ at distance $r$ from the initial excitation. Away from the transition, in both topological and trivial localized phases, the standard deviation follows the conventional one-dimensional scaling $\sigma[\ln P(r)]\sim r^{\theta}$ with $\theta\simeq 1/2$. Near the transition, however, the fluctuation growth is enhanced: the fitted exponent $\theta$ increases above $1/2$ in a nonmonotonic manner before returning close to $1/2$ at criticality. We interpret this behavior from the energy-resolved density of states and localization length. Near the transition, several energy sectors carry appreciable spectral weight and exhibit competitive decay rates, preventing a single localization scale from dominating the accessible wave-packet tail and thereby enhancing the fluctuations of $\ln P(r)$. Our results establish wave-packet fluctuation statistics as a dynamical diagnostic of disordered
chiral topological transitions and motivate broader studies of fluctuation phenomena in disordered topological
quantum systems.
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All authors Hai-Tao Ding, Sen Mu, Leong-Chuan Kwek, Gabriel Lemarié, Jiangbin Gong
Quantum Physics
Toward Practical Two-Way Covert Communication
Paul N. Fessatidis, Wyatt Wallis, Tae E. Cooper et al. (+6 more)
We study two-way covert communication schemes, where information is transmitted by passively modulating a reflected signal back to the source. We consider optical systems, described by
quantum bosonic channels. While broadband classical and
quantum light sources offer high covert throughput in theory, the associated mode-matching and phase-synchronization requirements make them impractical. Therefore, we employ a narrowband
laser source to experimentally demonstrate a proof-of-concept two-way covert communication system, where the adversary is assumed to be
quantum-capable. Furthermore, we propose a correlator-based receiver that attains the broadband gain offered by a
quantum light source without the need for precise mode matching.
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All authors Paul N. Fessatidis, Wyatt Wallis, Tae E. Cooper, Mark J. Meisner, Jaim Bucay, Saikat Guha, Shelbi L. Jenkins, Michael S. Bullock, Boulat A. Bash
Quantum Physics
Gate Parameter Lee-Yang Zeros and Dynamical Phases in Quantum Circuits
Yunfeng Jiang, Chang Liu, Yu Wu et al. (+1 more)
We propose gate-parameter Lee-Yang zeros of Loschmidt amplitudes as probes of dynamical phases in finite
quantum circuits. We study Floquet
circuits constructed from two-
qubit fSim gates with identical parameters, for which the Loschmidt amplitude becomes a rational function of the gate parameters after a suitable change of variables. At fixed system size and large
circuit depth, the zeros in one complexified gate parameter, with the other held fixed, condense onto limiting curves. In contrast to conventional Loschmidt or Fisher zeros in complex time, these zeros live directly in the complex plane of a tunable gate parameter. We show that the limiting set has two origins: a state-dependent component controlled by overlaps with Floquet eigenstates, and a universal component fixed by the Floquet spectrum. As the remaining gate parameter is varied, the universal zero set reorganizes abruptly, providing a finite-
qubit diagnostic of a dynamical phase transition. We demonstrate this behavior in a Bethe ansatz solvable brickwork
circuit and in longer-range fSim
circuits outside this solvable structure. The mechanism follows from the Beraha-Kahane-Weiss
theorem together with local unitarity, and is therefore spectral rather than a special consequence of integrability.
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All authors Yunfeng Jiang, Chang Liu, Yu Wu, Yang Zhang
Quantum Physics
Indistinguishability of photonic qubits emitted from trapped $^{40}$Ca$^+$ ions via pulsed excitation
Pascal Baumgart, Max Bergerhoff, Jonas Meiers et al. (+2 more)
We investigate the indistinguishability of Raman
photons generated from two trapped $^{40}$Ca$^+$ ions using few-nanosecond excitation pulses. We elucidate how spontaneous scattering back to the initial state affects Hong-Ou-Mandel interference. We identify the mean number of back-decays as a measurable single-emitter quantity that correlates with achievable interference visibility of
photons from two identical emitters.
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All authors Pascal Baumgart, Max Bergerhoff, Jonas Meiers, Stephan Kucera, Jürgen Eschner
Quantum Physics
Adaptive Stabilizer State Fidelity Certification
Kun Wang
Certifying the fidelity of a prepared state to a target stabilizer state is a fundamental task in
quantum information processing. Ref. [Phys. Rev. A 99, 042337 (2019)] gave the optimal worst-case lower bound from one fixed stabilizer generator gauge, but gauge dependence can leave a large fidelity ambiguity. We develop an adaptive extension that reports the full certified fidelity interval. First, for a single gauge, we derive the complementary optimal worst-case upper bound. We then formulate gauge selection as an adaptive design problem in which each round solves exact endpoint linear programs and chooses a new gauge by a witness elimination policy. We prove monotonic tightening, exact recovery once all nontrivial stabilizers are covered, and the worst-case necessity of full coverage. Finally, we identify structured syndrome distributions for which adaptivity beats this exponential
benchmark, and we numerically confirm faster concentration.
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Quantum Physics
Chain rules for conditional entropies in quantum cryptography: limitations and improvements
Lewis Wooltorton, Peter Brown and Omar Fawzi
Security proofs in
quantum cryptography rely on conditional
entropies. In a many-round protocol, their estimation is a challenging task; one must account for the most general attacks by an eavesdropper, including those that are not independently and identically distributed (i.i.d.) across all rounds. Chain rules address this problem by relating the conditional
entropy of a structured, but non-i.i.d. process to a sum of
entropy contributions from each round. They are a key ingredient in
entropy accumulation
theorems (EATs), which provide a versatile security proof framework for many protocols in
quantum cryptography. Recently, chain rules in the setting of trusted devices have lead to tight i.i.d. reductions at a finite number of rounds, and whether analogous results can be recovered in the device-independent (DI) setting has not been addressed. Surprisingly, we show that a natural tightening of the chain rule of Dupuis et al. [Commun. Math. Phys. 379, 867-913, (2020)] that would answer this question affirmatively cannot hold, highlighting a limitation of the current DI security proof approach. Nonetheless, we show that an intermediate improvement is possible by proving a new chain rule in this setting. Following the framework of Arqand et al. [Phys. Rev. X 15, 041013 (2025)], we use our chain rule to provide a slightly tighter version of the Rényi EAT in certain contexts. In addition, we provide a self-contained framework that unifies existing chain rules and compares their applications, framing our results in a broader context.
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Quantum Physics
Resolving the phase space
Zdenek Hradil and Jaroslav Rehacek
We show that the effective resolution of
quantum inference is determined by a sampling Gram operator that defines the experimentally accessible degrees of freedom and the reconstruction bandwidth supported by the data. Acting analogously to a transfer function in classical imaging, this operator together with scaling of deviance provides an operational criterion for distinguishing genuinely resolved features from structures arising from incomplete sampling, statistical fluctuations, or reconstruction assumptions. Reconstruction in the Gram eigenbasis emerges naturally as a measurement-adapted compression of the inference problem, retaining only those modes supported by typical measurement outcomes. Within finite frame theory, the same structure appears as the frame operator governing stable reconstruction. The resulting framework connects
quantum tomography, inverse reconstruction, and measurement resolution, providing a practical tool for assessing whether fine phase-space structures inferred in contemporary experiments are genuinely resolved by the measurement or merely reconstructed by the model.
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Quantum Physics
Incompleteness is necessary for activation of nonlocality without entanglement
Atanu Bhunia, Saronath Halder, Preeti Parashar et al. (+1 more)
A set of orthogonal product states is said to exhibit "
quantum nonlocality without
entanglement" if it is locally indistinguishable, i.e. no sequence of local operations and classical communication (LOCC) can perfectly discriminate the states. Building on this foundational idea, recent studies have highlighted the phenomenon of "genuine activation of hidden nonlocality", where a set of initially distinguishable orthogonal states becomes locally indistinguishable through orthogonality-preserving LOCC transformations. In this letter, we establish that any complete orthogonal product basis that is initially locally distinguishable remains so under all orthogonality-preserving local projective measurements, thereby ruling out activation via orthogonality-preserving local projective measurements and classical communication. We further introduce and formalise the notions of "strongly local sets", namely locally distinguishable sets that remain non-activable under all bipartitions. Interestingly, the study of "local activability" of distinguishable sets is useful to characterise the boundary between LOCC distinguishability and its irreversible loss in multipartite systems. Our results provide a rigorous structural understanding of local-to-nonlocal transitions in
quantum state discrimination.
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All authors Atanu Bhunia, Saronath Halder, Preeti Parashar, Ritabrata Sengupta
Quantum Physics
Quantum algorithms for density functional theory with minimal readout
Yuansheng Zhao, Hirofumi Nishi, Taichi Kosugi et al. (+5 more)
While
quantum computers have shown significant promise for electronic structure calculations, their potential to accelerate density functional theory (DFT) calculations remains unclear. In this work, we present a
qubit-efficient encoding scheme for wavefunctions in Kohn–Sham (KS) DFT, together with a
quantum algorithm that computes all occupied orbitals simultaneously. We further show that our
algorithm is particularly well suited to the Harris functional, enabling the total energy to be evaluated with a potential exponential speedup over classical approaches by entirely avoiding the costly readout of the electronic density. In addition, we propose a second method for achieving self-consistent DFT calculations using multiple copies of the wavefunction, which likewise circumvents density readout. The applicability of our
algorithms is demonstrated through several numerical examples, and their efficiency is compared with that of existing approaches.
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All authors Yuansheng Zhao, Hirofumi Nishi, Taichi Kosugi, Satoshi Hirose, Hiroki Sakagami, Tatsuki Oikawa, Tatsuya Okayama, Yu-ichiro Matsushita