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Tuesday 7 October

Quantum Physics

Blind quantum computing with different qudit resource state architectures

Alena Romanova and Wolfgang Dür

We discuss how blind quantum computing generalizes to multi-level quantum systems (qudits), which offers advantages compared to the qubit approach. Here, a quantum computing task is delegated to an untrusted server while simultaneously preventing the server from retrieving information about the computation it performs, the input, and the output, enabling secure cloud-based quantum computing. In the standard approach with qubits, measurement-based quantum computing is used: single-qubit measurements on cluster or brickwork states implement the computation, while random rotations of the resource qubits hide the computation from the server. We generalize finite-sized approximately universal gate sets to prime-power-dimensional qudits and show that qudit versions of the cluster and brickwork states enable a similar server-blind execution of quantum algorithms. Furthermore, we compare the overheads of different resource state architectures and discuss which hiding strategies apply to alternative qudit resource states beyond graph states.
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Quantum Physics

Quantum advantage from random geometrically-two-local Hamiltonian dynamics

Yihui Quek

Classical hardness-of-sampling results are largely established for random quantum circuits, whereas analog simulators natively realize time evolutions under geometrically local Hamiltonians. Does a typical such Hamiltonian already yield classically-intractable dynamics? We answer this question in the affirmative for the ensemble of geometrically-2-local Hamiltonians with Gaussian coefficients, evolved for constant time. This naturally leads to a quantum advantage scheme with clear prospects for experimental realization, necessitating only course-grained control. We give strong evidence of hardness for this physically-relevant ensemble. We develop the first worst-to-average-case reduction for approximating output probabilities of (time-independent) geometrically-2-local Hamiltonian evolutions. Our reduction proceeds by nonstandard means: while we also leverage polynomial interpolation, unlike previous works, we reduce directly to an evaluator for the exact distribution over Hamiltonians, from which we are trying to prove that sampling is hard. Previous works instead sampled from various perturbations of the true distribution, introducing additional constraints meant to keep the perturbation, measured in total variation distance, under control. We dispense with this step. Our reduction consists in a robust multivariate polynomial interpolation, reduced to sequential robust univariate interpolations via the symmetries of the Gaussian. We circumvent the fact that random Hamiltonians lack a hiding symmetry, a key property in previous proofs. We also contribute an algorithmic version of Berlekamp-Welch to deal with errored evaluations, solving an open problem from the RCS literature. We expect the machinery we develop to find use in average-case Hamiltonian complexity, filling in a gap in this literature which has thus far focussed on worst-case hardness results.
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Quantum Physics

An efficient algorithm to compute entanglement in states with low magic

ChunJun Cao, Gong Cheng and Tianci Zhou

A bottleneck for analyzing the interplay between magic and entanglement is the computation of these quantities in highly entangled quantum many-body magic states. Efficient extraction of entanglement can also inform our understanding of dynamical quantum processes such as measurement-induced phase transition and approximate unitary designs. We develop an efficient classical algorithm to compute the von Neumann entropy and entanglement spectrum for such states under the condition that they have low stabilizer nullity. The algorithm exploits the property of stabilizer codes to separate entanglement into two pieces: one generated by the common stabilizer group and the other from the logical state. The low-nullity constraint ensures both pieces can be computed efficiently. Our algorithm can be applied to study the entanglement in sparsely $T$-doped circuits with possible Pauli measurements as well as certain classes of states that have both high entanglement and magic. Combining with stabilizer learning subroutines, it also enables the efficient learning of von Neumann entropies for low-nullity states prepared on quantum devices.
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Quantum Physics

High-dimensional detection-loophole-free measurement-device-independent quantum random number generator

Joakim Argillander, Daniel Spegel-Lexne, Martin Clason et al. (+4 more)

Certifying random number generators is challenging, especially in security-critical fields like cryptography. Here, we demonstrate a measurement-device-independent quantum random number generator (MDI-QRNG) using high-dimensional photonic path states. Our setup extends the standard qubit beam-splitter QRNG to a three-output version with tunable fiber-optic interferometers acting as tunable beam splitters and superconducting detectors. This setup generates over 1.2 bits per round and 1.77 Mbits per second of certifiably secure private randomness without requiring any trust in the measurement apparatus, a critical requirement for the security of real-world cryptographic applications. Our results demonstrate certifiably secure high-dimensional quantum random-number generation, paving the way for practical, scalable QRNGs without the need for complex devices.
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All authors Joakim Argillander, Daniel Spegel-Lexne, Martin Clason, Pedro R. Dieguez, Marcin Pawłowski, Anubhav Chaturvedi, Guilherme B. Xavier

Quantum Physics

Quantum matrix arithmetics with Hamiltonian evolution

Christopher Kang and Yuan Su

The efficient implementation of matrix arithmetic operations underpins the speedups of many quantum algorithms. We develop a suite of methods to perform matrix arithmetics – with the result encoded in the off-diagonal blocks of a Hamiltonian – using Hamiltonian evolutions of input operators. We show how to maintain this $\textit{Hamiltonian block encoding}$, so that matrix operations can be composed one after another, and the entire quantum computation takes $\leq 2$ ancilla qubits. We achieve this for matrix multiplication, matrix addition, matrix inversion, Hermitian conjugation, fractional scaling, integer scaling, complex phase scaling, as well as singular value transformation for both odd and even polynomials. We also present an overlap estimation algorithm to extract classical properties of Hamiltonian block encoded operators, analogous to the well known Hadamard test, at no extra cost of qubit. Our Hamiltonian matrix multiplication uses the Lie group commutator product formula and its higher-order generalizations due to Childs and Wiebe. Our Hamiltonian singular value transformation employs a dominated polynomial approximation, where the approximation holds within the domain of interest, while the constructed polynomial is upper bounded by the target function over the entire unit interval. We describe a circuit for simulating a class of sum-of-squares Hamiltonians, attaining a commutator scaling in step count, while leveraging the power of matrix arithmetics to reduce the cost of each simulation step. In particular, we apply this to the doubly factorized tensor hypercontracted Hamiltonians from recent studies of quantum chemistry, obtaining further improvements for initial states with a fixed number of particles. We achieve this with $1$ ancilla qubit.
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Quantum Physics

Extended validations on photon number resolving detector based Gaussian boson sampling with low noises

Yang Ji, Yongzheng Wu, Shi Wang et al. (+3 more)

Gaussian boson sampling (GBS) is a variety of boson sampling overcoming the stable single-photon preparation difficulty of the later. However, like those in the original version, noises in GBS will also result in the deviation of output patterns and the reduction of classical simulation complexity. We extend the pattern recognition validation, together with the correlation approach as a comparison, on GBS using photon number resolving detectors, with noises of both photon loss and distinguishability, to quantificationally evaluate noise levels. As for the classical simulation with noises to be used during validations, it is actually a simulation of mixed states where we employ an existing photon-pair strategy to realize polynomial speedup locally. Furthermore, we use an output-binning strategy to realize validation speedup. Our simulation indicates that the pattern recognition protocol is useful for noise evaluations of GBS even when noises are sufficiently low.
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All authors Yang Ji, Yongzheng Wu, Shi Wang, Jie Hou, Zijian Wang, Bo Jiang

Quantum Physics · Cross-listed from q-fin.PR

Quantum-Theoretical Re-interpretation of Pricing Theory

Tian Xin

Motivated by Heisenberg's observable-only stance, we replace latent "information" (filtrations, hidden diffusions, state variables) with observable transitions between price states. On a discrete price lattice with a Hilbert-space representation, shift operators and the spectral calculus of the price define observable frequency operators and a translation-invariant convolution generator. Combined with jump operators that encode transition intensities, this yields a completely positive, translation-covariant Lindblad semigroup. Under the risk-neutral condition the framework leads to a nonlocal pricing equation that is diagonal in Fourier space; in the small-mesh diffusive limit its generator converges to the classical Black-Scholes-Merton operator. We do not propose another parametric model. We propose a foundation for model construction that is observable, first-principles, and mathematically natural. Noncommutativity emerges from the observable shift algebra rather than being postulated. The jump-intensity ledger determines tail behavior and short-maturity smiles and implies testable links between extreme-event probabilities and implied-volatility wings. Future directions: (i) multi-asset systems on higher-dimensional lattices with vector shifts and block kernels; (ii) state- or flow-dependent kernels as "financial interactions" leading to nonlinear master equations while preserving linear risk-neutral pricing; (iii) empirical tests of the predicted scaling relations between jump intensities and market extremes.
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Quantum Physics

Anonymous Quantum Tokens with Classical Verification

Dmytro Gavinsky, Dar Gilboa, Siddhartha Jain et al. (+2 more)

The no-cloning theorem can be used as a basis for quantum money constructions which guarantee unconditionally unforgeable currency. Existing schemes, however, either (i) require long-term quantum memory and quantum communication between the user and the bank in order to verify the validity of a bill or (ii) fail to protect user privacy due to the uniqueness of each bill issued by the bank, which can allow its usage to be tracked. We introduce a construction of single-use quantum money that gives users the ability to detect whether the issuing authority is tracking them, employing an auditing procedure for which we prove unconditional security. Bill validation is classical, and hence does not require long-term quantum memory or quantum communication, making the protocol relatively practical to deploy. We discuss potential applications beyond money, including anonymous one-time pads and voting.
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All authors Dmytro Gavinsky, Dar Gilboa, Siddhartha Jain, Dmitri Maslov, Jarrod R. McClean

Quantum Physics

Optimised spectral purity of unfiltered photons via pump and nonlinearity shaping

Tommaso Faleo, Christopher L. Morrison, Roméo Beignon et al. (+6 more)

Photonic quantum technologies rely on the efficient generation and interference of indistinguishable photons. Exceptional achievements in this respect have been obtained by domain engineering of quasi-phase-matched parametric down-conversion sources, demonstrating high two-photon interference visibility using only moderate bandpass spectral filtering. Here, we optimised the spectral purity and indistinguishability of photons from telecom-wavelength sources by combining Gaussian quasi-phase-matching with Gaussian pump spectral shaping. Without spectral filtering, we used time-of-flight spectrometry to estimate an upper bound spectral purity of 99.9272(6)%, and achieved visibilities of up to 98.5(8)% in two-photon interference experiments with independent sources.
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All authors Tommaso Faleo, Christopher L. Morrison, Roméo Beignon, Francesco Graffitti, Vikas Remesh, Stefan Frick, Alessandro Fedrizzi, Gregor Weihs, Robert Keil

Quantum Physics

Quantum $f$-divergences and Their Local Behaviour: An Analysis via Relative Expansion Coefficients

Shreyas Iyer, Peixue Wu, Paula Belzig et al. (+1 more)

Any reasonable measure of distinguishability of quantum states must satisfy a data processing inequality, that is, it must not increase under the action of a quantum channel. We can ask about the proportion of information lost or preserved and this leads us to study contraction and expansion coefficients respectively, which can be combined into a single relative expansion coefficient. We focus on two prominent families: (i) standard quantum $f$ divergences and (ii) their local (second-order) behaviour, which induces a monotone Riemannian semi-norm (that is linked to the $\chi^2$ divergence). Building on prior work, we identify new families of $f$ for which the global ($f$ divergence) and local (Riemannian) relative expansion coefficients coincide for every pair of channels, and we clarify how exceptional such exact coincidences are. Beyond equality, we introduce an equivalence framework that transfers qualitative properties such as strict positivity uniformly across different relative expansion coefficients. Leveraging the link between equality in the data processing inequality (DPI) and channel reversibility, we apply our framework of relative expansion coefficients to approximate recoverability of quantum information. Using our relative expansion results for primitive channels, we prove a reverse quantum Markov convergence theorem, converting positive expansion coefficients into quantitative lower bounds on the convergence rate.
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All authors Shreyas Iyer, Peixue Wu, Paula Belzig, Graeme Smith

Quantum Physics · Cross-listed from physics.optics

An integrated photonic millimeter-wave receiver with sub-ambient noise temperature

Junyin Zhang, Shuhang Zheng, Jiachen Cai et al. (+7 more)

Growing demands for timing precision in positioning systems, and resolution in atmospheric sensing and automotive radar, have extended receiver frontends into the millimetre-wave and THz regimes. At these frequencies, the noise performance of field-effect transistors used in low-noise amplifiers degrades rapidly due to parasitic effects, limited carrier mobility, hot electrons, and shot noise. Parametric transducers that couple electromagnetic signals to optical fields can offer quantum-limited sensitivity at room temperature. Electro-optic materials can enable receivers that transduce radiofrequency signals into optical phase shifts. Yet to date, room-temperature receivers using electro-optical detection have not attained a noise figure comparable to the electronic amplifiers. Here we demonstrate a room-temperature cavity electro-optic-based millimetre-wave receiver on a lithium tantalate photonic integrated circuit, achieving an on-chip photon-number transduction efficiency of 10%. The equivalent input-referred receiver-added noise temperatures reach $110\,\mathrm{K}$ on-chip and $202^{+35}_{-33}\,\mathrm{K}$ for the full system at 60.5 GHz, with noise figure matching state-of-the-art V-band electronic low-noise amplifiers. We resolve the thermal noise in cavity electro-optic transducers, showing that the system is limited by thermal photon occupation ($\sim$100) in the millimetre-wave cavity and optical vacuum noise, with no measurable excess noise. Our work demonstrates that integrated photonics can reach state-of-the-art electronic receiver noise performance while offering high-radiofrequency-power survivability and compatibility with remote optical readout. This establishes cavity electro-optics as a low-noise, electromagnetic interference-resilient, chip-scale receiver frontend for millimetre-wave applications, enabling scalable analogue processing in the optical domain.
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All authors Junyin Zhang, Shuhang Zheng, Jiachen Cai, Connor Denney, Zihan Li, Yichi Zhang, Changhao Han, Xin Ou, Gabriel Santamaria-Botello, Tobias J. Kippenberg

Quantum Physics

Anchor: Reducing Temporal and Spatial Output Performance Variability on Quantum Computers

Yuqian Huo, Daniel Leeds, Jason Ludmir et al. (+2 more)

Quantum computing, which has the power to accelerate many computing applications, is currently a technology under development. As a result, the existing noisy intermediate-scale quantum (NISQ) computers suffer from different hardware noise effects, which cause errors in the output of quantum programs. These errors cause a high degree of variability in the performance (i.e., output fidelity) of quantum programs, which varies from one computer to another and from one day to another. Consequently, users are unable to get consistent results even when running the same program multiple times. Current solutions, while focusing on reducing the errors faced by quantum programs, do not address the variability challenge. To address this challenge, we propose Anchor, a first-of-its-kind technique that leverages linear programming to reduce the performance variability by 73% on average over the state-of-the-art implementation focused on error reduction.
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All authors Yuqian Huo, Daniel Leeds, Jason Ludmir, Nicholas S. DiBrita, Tirthak Patel

Quantum Physics

Quantumness and its hierarchies in PT-symmetric down-conversion models

Jan Peřina Jr., Karol Bartkiewicz, Grzegorz Chimczak et al. (+4 more)

We investigate the hierarchy of quantum correlations in a quadratic bosonic parity-time-symmetric system (PTSS) featuring distinct dissipation and amplification channels. The hierarchy includes global nonclassicality, entanglement, asymmetric quantum steering, and Bell nonlocality. We elucidate the interplay between the system physical nonlinearity – which serves as a source of quantumness – and the specific dynamics of bosonic PTSSs, which are qualitatively influenced by their damping and amplification characteristics. Using a set of quantifiers – including local and global nonclassicality depths, negativity, steering parameters, and the Bell parameter – we demonstrate that the standard PTSS typically exhibits weaker quantumness than its counterparts affected solely by damping or solely by amplification. Both the maximum values attained by these quantifiers and the speed and duration of their generation are generally lower in the standard PTSS. A comparative analysis of three two-mode PTSSs – standard, passive, and active – with identical eigenvectors and real parts of eigenfrequencies, but differing in their damping and amplification strengths, reveals the crucial role of quantum fluctuations associated with gain and loss. Among them, the passive PTSS yields the most strongly nonclassical states. Nevertheless, under suitable conditions, the standard PTSS can also generate highly nonclassical states. The supremacy of the passive PTSS is further supported by its fundamental advantages in practical realizations.
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All authors Jan Peřina Jr., Karol Bartkiewicz, Grzegorz Chimczak, Anna Kowalewska-Kudlaszyk, Adam Miranowicz, Joanna K. Kalaga, Wiesław Leonski

Quantum Physics

Batched high-rate logical operations for quantum LDPC codes

Qian Xu, Hengyun Zhou, Dolev Bluvstein et al. (+5 more)

High-rate quantum LDPC (qLDPC) codes reduce memory overhead by densely packing many logical qubits into a single block of physical qubits. Here we extend this concept to high-rate computation by constructing batched fault-tolerant operations that apply the same logical gate across many code blocks in parallel. By leveraging shared physical resources to execute many logical operations in parallel, these operations realize high rates in space-time and significantly reduce computational costs. For arbitrary CSS qLDPC codes, we build batched gadgets with constant space-time overhead (assuming fast classical computation) for (i) single-shot error correction, state preparation, and code surgeries (ii) code switching, and (iii) addressable Clifford gates. Using these batched gadgets we also construct parallel non-Clifford gates with low space-time cost. We outline principles for designing parallel quantum algorithms optimized for a batched architecture, and show in particular how lattice Hamiltonian dynamical simulations can be compiled efficiently. We also propose a near-term implementation using new self-dual Bivariate-Bicycle codes with high encoding rates ($\sim 1/10$), transversal Clifford gates, and global $T$ gates via parallel magic state cultivation, enabling Hamiltonian simulations with a lower space-time cost than analogous surface-code protocols and low-rate qLDPC protocols. These results open new paths toward scalable quantum computation via co-design of parallel quantum algorithms and high-rate fault-tolerant protocols.
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All authors Qian Xu, Hengyun Zhou, Dolev Bluvstein, Madelyn Cain, Marcin Kalinowski, John Preskill, Mikhail D. Lukin, Nishad Maskara

Quantum Physics · Cross-listed from physics.optics

General diffraction properties of aperiodic slit arrays

Thiago de Souza Ferreira, Daniel Jonathan, Antonio Z. Khoury et al. (+1 more)

Fraunhofer diffraction plays a vital role in experimental physics not only because it accurately describes the behaviour of light in the usual propagation limit, but also because it links the diffracted light with the scattering object through one of the most important mathematical transformations in physics: the Fourier transform. Acting as a probe in material characterisation as well as used as a tool for particle trapping or sensing, the pattern of interference maxima resulting from the Fraunhofer diffraction through periodic scattering is an ubiquitous routine. In this paper we analyse the Fraunhofer diffraction resulting from the much less studied aperiodic scatter of the light. We provide general conditions for the experimental observation of the peaks of interference maxima featured into patterns that display periodic structures on a number of distance scales. Our theoretical analysis is supported by thorough experimental demonstrations.
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All authors Thiago de Souza Ferreira, Daniel Jonathan, Antonio Z. Khoury, Daniel S. Tasca

Quantum Physics

Non-iid hypothesis testing: from classical to quantum

Giacomo De Palma, Marco Fanizza, Connor Mowry et al. (+1 more)

We study hypothesis testing (aka state certification) in the non-identically distributed setting. A recent work (Garg et al. 2023) considered the classical case, in which one is given (independent) samples from $T$ unknown probability distributions $p_1, \dots, p_T$ on $[d] = \{1, 2, \dots, d\}$, and one wishes to accept/reject the hypothesis that their average $p_{\mathrm{avg}}$ equals a known hypothesis distribution $q$. Garg et al. showed that if one has just $c = 2$ samples from each $p_i$, and provided $T \gg \frac{\sqrt{d}}{\epsilon^2} + \frac{1}{\epsilon^4}$, one can (whp) distinguish $p_{\mathrm{avg}} = q$ from $d_{\mathrm{TV}}(p_{\mathrm{avg}},q) > \epsilon$. This nearly matches the optimal result for the classical iid setting (namely, $T \gg \frac{\sqrt{d}}{\epsilon^2}$). Besides optimally improving this result (and generalizing to tolerant testing with more stringent distance measures), we study the analogous problem of hypothesis testing for non-identical quantum states. Here we uncover an unexpected phenomenon: for any $d$-dimensional hypothesis state $\sigma$, and given just a single copy ($c = 1$) of each state $\rho_1, \dots, \rho_T$, one can distinguish $\rho_{\mathrm{avg}} = \sigma$ from $D_{\mathrm{tr}}(\rho_{\mathrm{avg}},\sigma) > \epsilon$ provided $T \gg d/\epsilon^2$. (Again, we generalize to tolerant testing with more stringent distance measures.) This matches the optimal result for the iid case, which is surprising because doing this with $c = 1$ is provably impossible in the classical case. We also show that the analogous phenomenon happens for the non-iid extension of identity testing between unknown states. A technical tool we introduce may be of independent interest: an Efron-Stein inequality, and more generally an Efron-Stein decomposition, in the quantum setting.
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All authors Giacomo De Palma, Marco Fanizza, Connor Mowry, Ryan O'Donnell

Quantum Physics

Self-concordant Schrödinger operators: spectral gaps and optimization without condition numbers

Sander Gribling, Simon Apers, Harold Nieuwboer et al. (+1 more)

Spectral gaps play a fundamental role in many areas of mathematics, computer science, and physics. In quantum mechanics, the spectral gap of Schrödinger operators has a long history of study due to its physical relevance, while in quantum computing spectral gaps are an important proxy for efficiency, such as in the quantum adiabatic algorithm. Motivated by convex optimization, we study Schrödinger operators associated with self-concordant barriers over convex domains and prove non-asymptotic lower bounds on the spectral gap for this class of operators. Significantly, we find that the spectral gap does not display any condition-number dependence when the usual Laplacian is replaced by the Laplace–Beltrami operator, which uses second-order information of the barrier and hence can take the curvature of the barrier into account. As an algorithmic application, we construct a novel quantum interior point method that applies to arbitrary self-concordant barriers and shows no condition-number dependence. To achieve this we combine techniques from semiclassical analysis, convex optimization, and quantum annealing.
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All authors Sander Gribling, Simon Apers, Harold Nieuwboer, Michael Walter

Quantum Physics

Quantum Strategies to Overcome Classical Multiplexing Limits

Tzula B. Propp, Jeroen Grimbergen, Emil R. Hellebek et al. (+5 more)

Near-term quantum networks face a bottleneck due to low quantum communication rates. This degrades performance both by lowering operating speeds and increasing qubit storage time in noisy memories, making some quantum internet applications infeasible. One way to circumvent this bottleneck is multiplexing: combining multiple signals into a single signal to improve the overall rate. Standard multiplexing techniques are classical in that they do not make use of coherence between quantum channels nor account for decoherence rates that vary during a protocol's execution. In this paper, we first derive semiclassical limits to multiplexing for many-qubit protocols, and then introduce two techniques: single click quantum multiplexing and multi-server multiplexing. These can enable beyond-classical multiplexing advantages. We illustrate these techniques through three example applications: 1) entanglement generation between two asymetric quantum network nodes (i.e., repeaters or quantum servers with inequal memories), 2) remote state preparation between many end user devices and a single quantum node, and 3) remote state preparation between one end user device and many internetworked quantum nodes. By utilizing many noisy internetworked quantum devices instead of fewer low-noise devices, our multiplexing strategies enable new paths towards achieving high-speed many-qubit quantum network applications.
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All authors Tzula B. Propp, Jeroen Grimbergen, Emil R. Hellebek, Junior R. Gonzales-Ureta, Janice van Dam, Joshua A. Slater, Anders S. Sørensen, Stephanie D. C. Wehner

Quantum Physics

On the Quantum Equivalence between $S|LWE\rangle$ and $ISIS$

André Chailloux and Paul Hermouet

Chen, Liu, and Zhandry [CLZ22] introduced the problems $S|LWE\rangle$ and $C|LWE\rangle$ as quantum analogues of the Learning with Errors problem, designed to construct quantum algorithms for the Inhomogeneous Short Integer Solution ($ISIS$) problem. Several later works have used this framework for constructing new quantum algorithms in specific cases. However, the general relation between all these problems is still unknown. In this paper, we investigate the equivalence between $S|LWE\rangle$ and $ISIS$. We present the first fully generic reduction from $ISIS$ to $S|LWE\rangle$, valid even in the presence of errors in the underlying algorithms. We then explore the reverse direction, introducing an inhomogeneous variant of $C|LWE\rangle$, denoted $IC|LWE\rangle$, and show that $IC|LWE\rangle$ reduces to $S|LWE\rangle$. Finally, we prove that, under certain recoverability conditions, an algorithm for $ISIS$ can be transformed into one for $S|LWE\rangle$. We instantiate this reverse reduction by tweaking a known algorithm for $(I)SIS_\infty$ in order to construct quantum algorithm for $S|LWE\rangle$ when the alphabet size q is a small power of 2, recovering some results of Bai et al. [BJK+ 25]. Our results thus clarify the landscape of reductions between $S|LWE\rangle$ and $ISIS$, and we show both their strong connection as well as the remaining barriers for showing full equivalence.
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Quantum Physics

Noise-induced decoherence-free zones for anyons

Eric R. Bittner

We develop a stochastic framework for anyonic systems in which the exchange phase is promoted from a fixed parameter to a fluctuating quantity. Starting from the Stratonovich stochastic Liouville equation, we perform the Stratonovich–Itô conversion to obtain a Lindblad master equation that ties the dissipator directly to the distorted anyon algebra. This construction produces a statistics–dependent dephasing channel, with rates determined by the eigenstructure of the real symmetric correlation matrix $D_{ab}$. The eigenvectors of $D$ select which collective exchange currents – equivalently, which irreducible representations of the system – are protected from stochastic dephasing, providing a natural mechanism for decoherence-free subspaces and noise-induced exceptional points. The key result of our analysis is the universality of the optimal statistical angle: in the minimal two-site model with balanced gain and loss, the protected mode always minimizes its dephasing at $\theta^\star = \pi/2$, independent of the specific form of $D$. This robustness highlights a simple design rule for optimizing coherence in noisy anyonic systems, with direct implications for ultracold atomic realizations and other emerging platforms for fractional statistics.
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Quantum Physics · Cross-listed from cond-mat.str-el

Identifying chiral topological order in microscopic spin models by modular commutator

Avijit Maity, Aman Kumar and Vikram Tripathi

The chiral central charge $c_-$ is a key topological invariant of the edge characterizing the bulk two-dimensional chiral topological order, but its direct evaluation in microscopic spin models has long been a challenge, especially for non-abelian topological order. Building on the recently developed modular commutator formalism, we numerically obtain $c_-$ directly from single ground-state wave functions of two-dimensional interacting spin models that have chiral topological order. This provides a geometry-independent and bulk diagnostic of chirality. We study two nonintegrable systems – the Zeeman-Kitaev honeycomb model and the kagome antiferromagnet – both subjected to scalar spin chirality perturbations. We find that the modular commutator yields results consistent with the expected topological quantum field theories. We also compute the topological entanglement entropy which provides an independent diagnostic of the topological orders. Our work establishes modular commutators as a powerful numerical probe of chiral topological order in strongly correlated quantum magnets.
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Quantum Physics

Universal computational super-resolution framework for imaging of quantum dots

Dominik Vašinka, Jaewon Lee, Charlie Stalker et al. (+9 more)

We present a universal deep-learning method that reconstructs super-resolved images of quantum emitters from a single camera frame measurement. Trained on physics-based synthetic data spanning diverse point-spread functions, aberrations, and noise, the network generalizes across experimental conditions without system-specific retraining. We validate the approach on low- and high-density In(Ga)As quantum dots and strain-induced dots in 2D monolayer WSe$_2$, resolving overlapping emitters even under low signal-to-noise and inhomogeneous backgrounds. By eliminating calibration and iterative acquisitions, this single-shot strategy enables rapid, robust computational super-resolution for nanoscale characterization and quantum photonic device fabrication.
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All authors Dominik Vašinka, Jaewon Lee, Charlie Stalker, Victor Mitryakhin, Ivan Solovev, Sven Stephan, Sven Höfling, Falk Eilenberger, Seth Ariel Tongay, Christian Schneider, Miroslav Ježek, Ana Predojević

Quantum Physics

Quantum Annealing for Realistic Traffic Flow Optimization: Clustering and Data-Driven QUBO

Renáta Rusnáková, Martin Chovanec and Juraj Gazda

This article presents a scalable, data-driven formulation of city-wide Traffic Flow Optimization as a Quadratic Unconstrained Binary Optimization problem and evaluates its performance using quantum annealing and classical solvers on realistic urban networks. The framework builds a time-resolved congestion model from simulated mobility data by sampling vehicle trajectories at fixed intervals, identifying leader-follower interactions on shared road segments. In addition to congestion, the model incorporates route-duration penalties and an analytically derived penalty parameter that enforces one-hot route selection, ensuring feasible assignments while balancing network-wide congestion reduction and individual travel times. To mitigate the combinatorial growth of interactions in large-scale instances, the approach employs Leiden clustering to partition vehicles into dense communities that can be optimized independently. The resulting subproblems are solved using D-Wave's quantum annealer, exact mixed-integer programming via Gurobi, and several classical metaheuristics, and are evaluated on multiple city maps with up to 25,000 vehicles. Across large scenarios, the hybrid quantum annealing approach consistently produces feasible solutions within approximately 1% of Gurobi's objective values, while maintaining stable runtimes. Both methods outperform shortest-route baselines, achieving reductions in the proposed congestion-cost objective of up to 24.4% for the hybrid quantum approach and 29.4% for Gurobi. Finally, the study highlights the critical role of the underlying city map, showing that network structure directly influences interaction density, problem formulation, and the efficiency of embedding and solving on current quantum annealing hardware.
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Quantum Physics

Fermionic versus Spin Baths in non-Interacting Transport Models

Muhammad Zia, Moritz Cygorek, Erik M. Gauger et al. (+1 more)

We investigate how fermionic anticommutation shapes transport in a noninteracting resonant-level model where a single central site is coupled to an environment. To this end, we compare a fermionic reservoir with a bath of spin-half modes using exact diagonalization and a perturbative expansion of the master equation to identify the differences. Notably, we find different reduced dynamics for the spin and fermionic baths even though the particles remain noninteracting and both models enforce local Pauli blocking. These differences originate from higher-order terms in the system-bath coupling, where fermionic anticommutation introduces exchange signs in higher-order correlations. By contrast, all second-order contributions, set solely by two-point correlators, coincide. Deviations are largest for small to intermediate bath sizes, fading in the effectively Markovian regime where higher-order corrections are negligible. These results identify when spin baths can be a substitute for fermionic reservoirs and vice versa.
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All authors Muhammad Zia, Moritz Cygorek, Erik M. Gauger, Brendon W. Lovett

Quantum Physics

Angular–Momentum–Resolved Aharonov–Bohm Coupling Energy

Ju Gao and Fang Shen

We present an angular–momentum–resolved energetic formulation of the Aharonov–Bohm (AB) response for a confined Dirac electron based on two gauge–invariant interaction functionals: a magnetization–field functional and a current–potential functional. Using exact Dirac eigenmodes in a cylindrical cavity threaded by a solenoidal flux, we show that the magnetization–field functional yields a core–localized interaction energy restricted to the $l=0$ channel, with all higher angular–momentum contributions suppressed and vanishing entirely in the limit $a\!\to\!0$. The current–potential functional, by contrast, produces a finite, mode–dependent energy shift for $l\!\ge\!1$ in the same limit, arising from a local interaction between the solenoidal vector potential and the spatially distributed Dirac current, and explicitly encoding the geometric and topological structure of the coupling energy.
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Quantum Physics

Hybrid Quantum-Classical Policy Gradient for Adaptive Control of Cyber-Physical Systems: A Comparative Study of VQC vs. MLP

Aueaphum Aueawatthanaphisut and Nyi Wunna Tun

The comparative evaluation between classical and quantum reinforcement learning (QRL) paradigms was conducted to investigate their convergence behavior, robustness under observational noise, and computational efficiency in a benchmark control environment. The study employed a multilayer perceptron (MLP) agent as a classical baseline and a parameterized variational quantum circuit (VQC) as a quantum counterpart, both trained on the CartPole-v1 environment over 500 episodes. Empirical results demonstrated that the classical MLP achieved near-optimal policy convergence with a mean return of 498.7 +/- 3.2, maintaining stable equilibrium throughout training. In contrast, the VQC exhibited limited learning capability, with an average return of 14.6 +/- 4.8, primarily constrained by circuit depth and qubit connectivity. Noise robustness analysis further revealed that the MLP policy deteriorated gracefully under Gaussian perturbations, while the VQC displayed higher sensitivity at equivalent noise levels. Despite the lower asymptotic performance, the VQC exhibited significantly lower parameter count and marginally increased training time, highlighting its potential scalability for low-resource quantum processors. The results suggest that while classical neural policies remain dominant in current control benchmarks, quantum-enhanced architectures could offer promising efficiency advantages once hardware noise and expressivity limitations are mitigated.
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Quantum Physics

Robust Non-Adiabatic Holonomic Gating in Qutrits via Inverse-Engineered Pulse Shaping and Error Compensation

Jie Lu, Ji-Ze Han, Jie-Dong Huang et al. (+3 more)

Systematic Rabi-amplitude and detuning errors remain important sources of infidelity in high-fidelity quantum gates. We develop a robust pulse-engineering scheme for non-adiabatic holonomic quantum computing in a three-level $\Lambda$-type qutrit, combining inverse engineering with time-dependent perturbative analysis. Pulse shaping eliminates the leading second-order Rabi-amplitude contribution, while static detuning introduces a distinct population-mediated channel that cannot be removed within a single control loop. We therefore introduce a compensation loop that exactly cancels the dominant second-order $O_{13}^{\delta}$ contribution, with the residual $O_{12}^{\delta}$ channel further suppressed by pulse shaping. Using the logical average gate fidelity over the complete computational subspace, the optimized composite sequence reaches closed-system fidelities of $99.88\%$–$99.99\%$ for four representative single-qubit gates at $\epsilon=0.2$ and $\delta/2\pi=2$ MHz. With phenomenological decoherence at $T_1=T_2=30~\mu{\rm s}$, the NOT and S gates retain fidelities of $99.72\%$ and $99.79\%$, respectively, with a coherence-time crossover near $0.58~\mu{\rm s}$. These results identify the regime in which systematic-error suppression outweighs the decoherence cost of the additional control loop.
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All authors Jie Lu, Ji-Ze Han, Jie-Dong Huang, Yang Qian, Ying Yan, Zhi-Guo Huang

Quantum Physics

Hybrid quantum-classical analog simulation of two-dimensional Fermi-Hubbard models with neutral atoms

Sergi Julià-Farré, Antoine Michel, Christophe Domain et al. (+11 more)

We experimentally study the two-dimensional Fermi-Hubbard model using a Rydberg-based quantum processing unit in the analog mode. Our approach avoids encoding directly the original fermions into qubits and instead relies on reformulating the original model onto a system of fermions coupled to spins and then decoupling them in a self-consistent manner. We then introduce the auxiliary spin solver: this hybrid quantum-classical algorithm handles a free-fermion problem, which can be solved efficiently with a few classical resources, and an interacting spin problem, which can be naturally encoded in the analog quantum computer. This algorithm can be used to study both the equilibrium Mott transition as well as non-equilibrium properties of the original Fermi-Hubbard model, highlighting the potential of quantum-classical hybrid approaches to study strongly correlated matter.
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All authors Sergi Julià-Farré, Antoine Michel, Christophe Domain, Joseph Mikael, Jacques-Charles Lafoucriere, Joseph Vovrosh, Ahmed Chahlaoui, Dorian Claveau, Guillaume Villaret, Julius de Hond, Loïc Henriet, Antoine Browaeys, Thomas Ayral, Alexandre Dauphin

Quantum Physics

Learning stabilizer structure of quantum states

Srinivasan Arunachalam and Arkopal Dutt

We consider the task of learning a structured stabilizer decomposition of an arbitrary $n$-qubit quantum state $|\psi\rangle$: for $\epsilon > 0$, output a state $|\phi\rangle$ with stabilizer-rank $\textsf{poly}(1/\epsilon)$ such that $|\psi\rangle=|\phi\rangle+|\phi'\rangle$ where $|\phi'\rangle$ has stabilizer fidelity $< \epsilon$. We first show the existence of such decompositions using the recently established inverse theorem for the Gowers-$3$ norm of states [AD,STOC'25]. To learn this structure, we initiate the task of self-correction of a state $|\psi\rangle$ with respect to a class of states $S$: given copies of $|\psi\rangle$ which has fidelity $\geq \tau$ with a state in $S$, output $|\phi\rangle \in S$ with fidelity $|\langle \phi | \psi \rangle|^2 \geq \tau^C$ for a constant $C>1$. Assuming the algorithmic polynomial Frieman-Rusza (APFR) conjecture in the high doubling regime (whose combinatorial version was recently resolved [GGMT,Annals of Math.'25]), we give a polynomial-time algorithm for self-correction of stabilizer states. Given access to the state preparation unitary $U_\psi$ for $|\psi\rangle$ and its controlled version $cU_\psi$, we give a polynomial-time protocol that learns a structured decomposition of $|\psi\rangle$. Without assuming APFR, we give a quasipolynomial-time protocol for the same task. As our main application, we give learning algorithms for states $|\psi\rangle$ promised to have stabilizer extent $\xi$, given access to $U_\psi$ and $cU_\psi$. We give a protocol that outputs $|\phi\rangle$ which is constant-close to $|\psi\rangle$ in time $\textsf{poly}(n,\xi^{\log \xi})$, which can be improved to polynomial-time assuming APFR. This gives an unconditional learning algorithm for stabilizer-rank $k$ states in time $\textsf{poly}(n,k^{k^2})$. As far as we know, learning arbitrary states with even stabilizer-rank $2$ was unknown.
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Quantum Physics · Cross-listed from cond-mat.stat-mech

Full Eigenstate Thermalization in Integrable Spin Systems

Tanay Pathak

The Eigenstate Thermalization Hypothesis (ETH) is a standard tool to understand the thermalization properties of an isolated quantum system. Its generalization to higher order correlations of matrix elements of local operators, dubbed the full ETH, relates the ETH free cumulants to the corresponding thermal free cumulants in the thermodynamic limit. In this work, we numerically test the full ETH predictions using exact diagonalization of two integrable spin models: the Ising and the XXZ Heisenberg models. The differences from the behavior predicted by full ETH in chaotic systems are highlighted and contrasted along the way. We further study the out-of-time-ordered correlator (OTOC) through its approximate decomposition into contributions involving the second- and fourth-order ETH free cumulants. We find that, although the fourth-order contribution governs the late-time behavior of the OTOC in these integrable models, its dynamics differs significantly from that found in nonintegrable systems.
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