Quantum Physics
Arqon: A suite of control applications enabling a reliable quantum network
Scarlett Gauthier, Thomas R. Beauchamp and Stephanie Wehner
A
quantum network's purpose is to enable users to execute applications on end nodes. This requires the network to provide the service of creating entangled links between those nodes. Users of mature networks, such as the internet or the telephone network expect accepted service demands to be met reliably. We first define reliability requirements that extend classical computer network concepts to
quantum network service delivery. We then introduce Arqon, a suite of control applications designed to deliver reliable service in centrally controlled
quantum networks. We demonstrate through both analytic and numerical evaluation that Arqon satisfies all reliability requirements for accepted demands. These evaluations consider static network
topologies. We provide a complete Python implementation and perform complexity analysis showing that admission control scales as $O(k^3)$ in the number of incoming demands $k$ and schedule computation scales as ${O(N^3)}$ in the number of accepted demands to schedule $N$.
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Quantum Physics
The MQT Compiler Collection: A Blueprint for a Future-Proof Quantum-Classical Compilation Framework
Lukas Burgholzer, Daniel Haag, Yannick Stade et al. (+3 more)
As the capabilities of
quantum computing hardware continue to rise,
algorithms that exploit them are becoming increasingly complex. These developments increase the need for sophisticated compilation frameworks that translate high-level
algorithms into executable code. In the past, most solutions were built with a
quantum-first approach and handled mostly pure
quantum programs without classical elements such as structured control flow. However, developments in
quantum algorithms, error correction, and optimization, as well as the integration into high-performance computing (HPC) environments, depend on such classical elements. As
quantum-first approaches increasingly struggle to handle these concepts, classical-first approaches are becoming a promising alternative. In this work, we present the MQT Compiler Collection, a blueprint for a future-proof
quantum-classical compilation framework built on the Multi-Level Intermediate Representation (MLIR). After years of experience with the
quantum-first approach and its shortcomings, we propose a framework that embraces core MLIR concepts to support the full compilation pipeline from high-level
algorithms to hardware-specific instructions. The proposed architecture is designed from the ground up to support complex optimizations beyond, e.g., simple gate cancellation. It is publicly available at https://github.com/munich-
quantum-toolkit/core.
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All authors Lukas Burgholzer, Daniel Haag, Yannick Stade, Damian Rovara, Patrick Hopf, Robert Wille
Quantum Physics
Hardware-Efficient Erasure Qubits With Superconducting Transmon Qutrits
Bao-Jie Liu, Ying-Ying Wang, Yu-Xin Wang et al. (+3 more)
Quantum error correction using erasure
qubits offers higher fault-tolerant thresholds and improved scaling by converting dominant physical errors into detectable erasures. In superconducting
circuits, erasure
qubits can be constructed using the dual-rail approach, which, however, requires additional
qubit-count overhead and tailored coupling elements. Here, we demonstrate a hardware-efficient scheme that operates transmon qutrits as erasure
qubits, which is compatible with standard superconducting
circuit-QED hardware. The logical states $\ket{0_\text{L}}$ and $\ket{1_\text{L}}$ are represented by the ground and second excited states, while the dominant relaxation errors can be detected via an ancilla
qubit using a microwave-activated two-qutrit SWAP gate. We demonstrate a logical
qubit $T_1$ lifetime exceeding $500\,\mu\mathrm{s}$, post-selected with repeated mid-
circuit erasure detection, which is ten times longer than the $T_1$ time of the transmon physical
qubit. Coherence times beyond $300\,\mu\mathrm{s}$ are achieved using dynamical decoupling. Single-
qubit gate operations reach average Clifford gate infidelity on the order of $10^{-4}$. We further demonstrate dual-purposing an ancilla
qubit for both erasure detection and parity checking, showing heralded generation of Bell states between erasure
qubits. These results suggest that mainstream architectures of transmon
qubit arrays may already be capable of implementing erasure-based QEC strategies for hardware-efficient fault-tolerant
quantum computing.
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All authors Bao-Jie Liu, Ying-Ying Wang, Yu-Xin Wang, Manthan Badbaria, Shruti Puri, Chen Wang
Quantum Physics · Cross-listed from cond-mat.quant-gas
An Algorithm for Fast Assembling Large-Scale Defect-Free Atom Arrays
Tao Zhang, Xiaodi Li, Hui Zhai et al. (+1 more)
It is widely believed that tens of thousands of physical
qubits are needed to build a practically useful
quantum computer. Atom arrays formed by optical tweezers are among the most promising platforms for achieving this goal, owing to the excellent scalability and mobility of atomic
qubits. However, assembling a defect-free atom array with ~ 10^4
qubits remains algorithmically challenging, alongside other hardware limitations. This is due to the computationally hard path-planning problems and the time-consuming generation of suffciently smooth trajectories for optical tweezer potentials by spatial light modulators (SLM). Here, we present a unified framework comprising two innovative components to fully address these algorithmic challenges: (1) a path-planning module that employs a supervised learning approach using a graph neural network combined with a modified auction decoder, and (2) a potential-generation module called the phase and profile-aware Weighted Gerchberg-Saxton
algorithm. The inference time for the first module is nearly a size-independent constant overhead of ~ 5 ms, and the second module generates a potential frame with about 0.5 ms, a timescale shorter than the current commercial SLM refresh time. Altogether, our
algorithm enables the assembly of an atom array with 10^4
qubits on a timescale much shorter than the typical
vacuum lifetime of the trapped atoms.
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All authors Tao Zhang, Xiaodi Li, Hui Zhai, Linghui Chen
Quantum Physics · Cross-listed from cond-mat.mes-hall
Fluctuation engineering in cavity quantum materials
Hope M Bretscher, Lorenzo Graziotto, Marios H Michael et al. (+14 more)
Coupling tailored electromagnetic fluctuations to materials provides a resource for controlling correlated
quantum matter. By structuring the frequency, spatial, and modal distribution of fluctuations through a new generation of cavity
quantum materials,
vacuum and thermal spectra can shift phase boundaries and stabilize or suppress orders. This review organizes the field around a fluctuation-focused perspective, surveying a practical design toolbox and recent milestones, and outlining theory-experiment challenges in realistic, multimode, beyond-long-wavelength regimes. We highlight photonic observables and map opportunities for
equilibrium and driven control across superconducting,
magnetic, moire, and topological platforms.
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All authors Hope M Bretscher, Lorenzo Graziotto, Marios H Michael, Angela Montanaro, I-Te Lu, Andrey Grankin, James W McIver, Jerome Faist, Daniele Fausti, Martin Eckstein, Michael Ruggenthaler, Angel Rubio, DN Basov, Mohammad Hafezi, Martin Claassen, Dante M Kennes, Michael A Sentef
Quantum Physics
Every Little Thing Heat Does Is Magic
Rafael A. Macêdo, A. de Oliveira Junior, Naim E. Comar et al. (+4 more)
How can one certify that an unknown
quantum state possesses magic without resorting to full state
tomography? We address this question by introducing two thermodynamic witnesses that rely solely on energy and heat measurements. First, we define the stabilizer ground-state energy as the lowest energy achievable by any stabilizer state, and the stabilizer gap as the separation between this value and the true ground-state energy. Any state whose energy lies below the stabilizer ground-state energy is therefore necessarily nonstabilizer. This leads to a direct witness of magic using only average-energy measurements. To overcome the limitations when direct energy measurements are inconclusive, we further develop a nonlinear witness based on heat exchange with a thermal ancilla. Specifically, we derive fundamental bounds on heat that are satisfied by all stabilizer states; therefore, their violation certifies the presence of magic. We demonstrate the effectiveness of our approach through several examples, ranging from few-body systems where heat exchange reveals nonstabilizerness even when energy measurements alone fail, to the transverse-field Ising chain, where the stabilizer gap becomes maximal at the
quantum critical point.
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All authors Rafael A. Macêdo, A. de Oliveira Junior, Naim E. Comar, Luna Lima Keller, Jonatan Bohr Brask, Lucas C. Céleri, Rafael Chaves
Quantum Physics
Geometry-Induced Long-Range Correlations in Recurrent Neural Network Quantum States
Asif Bin Ayub, Amine Mohamed Aboussalah and Mohamed Hibat-Allah
Neural
Quantum States based on autoregressive recurrent neural network (RNN) wave functions enable efficient sampling without Markov-chain autocorrelation, but standard RNN architectures are biased toward finite-length correlations and can fail on states with long-range dependencies. A common response is to adopt
transformer-style self-
attention, but this typically comes with substantially higher computational and
memory overhead. Here we introduce dilated RNN wave functions, where recurrent units access distant sites through dilated connections, injecting an explicit long-range inductive bias while retaining a favorable $\mathcal{O}(N \log N)$ forward pass scaling. We show analytically that dilation changes the correlation geometry and can induce power-law correlation scaling in a simplified linearized and perturbative setting. Numerically, for the critical 1D transverse-field Ising model, dilated RNNs reproduce the expected power-law connected two-point correlations in contrast to the exponential decay typical of conventional RNN ansätze. We further show that the dilated RNN accurately approximates the one-dimensional Cluster state, a paradigmatic example with long-range conditional correlations that has previously been reported to be challenging for RNN-based wave functions. These results highlight dilation as a simple
geometric mechanism for building correlation-aware autoregressive neural
quantum states.
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Quantum Physics
High-Fidelity Transmon Reset with a Multimode Acoustic Resonator
Andraž Omahen, Simon Storz, Igor Kladarić et al. (+1 more)
Achieving sufficiently low residual excited-state populations remains a key challenge in superconducting
quantum circuits, particularly for protocols operating close to noise limits or requiring repeated
qubit initialization. Existing protocols primarily address this challenge through sophisticated control, engineered dissipation, or feedback mechanisms. Here, we demonstrate an alternative approach in which a superconducting
qubit is reset using a physically distinct, intrinsically colder phononic bath. Specifically, we interface a transmon with a high-overtone bulk acoustic resonator (HBAR), enabling cooling of the
qubit into GHz-frequency modes. Using this approach, we achieve a residual excited-state population of the
qubit below $10^{-4}$, representing an improvement of one to two orders of magnitude compared to existing reset schemes. These results highlight the potential of phononic baths as a resource for high-fidelity
qubit initialization in superconducting
circuits.
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All authors Andraž Omahen, Simon Storz, Igor Kladarić, Yiwen Chu
Quantum Physics · Cross-listed from cond-mat.stat-mech
Decoding coherent errors in toric codes on honeycomb and square lattices: duality to Majorana monitored dynamics and symmetry classes
Zhou Yang, Andreas W. W. Ludwig and Chao-Ming Jian
Topological stabilizer codes, such as the toric and surface codes, are leading candidates for fault-tolerant
quantum computation. While their decodability under
stochastic noise has been extensively studied, the effects of coherent errors, which involve
quantum interference, remain less explored. In this work, we study the decodability of toric codes on honeycomb and square
lattices subject to $X$- and $Z$-type coherent errors generated by the $X$- and $Z$-rotations on each
qubit. We establish a duality between these decoding problems and 1+1D monitored dynamics of non-interacting Majorana
fermions. This duality shows that the Altland-Zirnbauer
symmetry class of the dual Majorana dynamics governs the universal structure of the decodability phase diagram. We show that the honeycomb-
lattice toric code (hTC) with $X$-type error is dual to class-DIII dynamics, while the hTC with $Z$-type error and the square-
lattice toric code (sTC) with both error types are dual to class-D dynamics. The key distinction arises from time-reversal
symmetry. In class DIII, the generic transition out of the decodable phase is dual to a measurement-induced transition between dynamical phases with area-law and logarithmic
entanglement scaling. In contrast, in class D, the generic decodability transition corresponds to a transition between two topologically distinct area-law phases. To explore these transitions in microscopic models, we consider hTC and sTC with $X$-type errors as representatives and introduce a minimal two-parameter coherent error model with spatially varying rotation angles. Using analytical and numerical methods, we map out the decodability phase diagrams and characterize the universal behavior of the transitions. We find that the decodability of sTC is more vulnerable to spatially varying coherent errors than uniform ones.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Floquet Engineering of a Quasiequilibrium Superradiant Phase Transition in Landau Polaritons
Wen-Hua Wu, Fuyang Tay, Mengqian Che et al. (+3 more)
Superradiant phase transitions (SRPTs), characterized by
photon condensation and macroscopic matter polarization, are forbidden in
equilibrium for homogeneous fields by no-go
theorems. Here, we show that Floquet driving can circumvent this constraint in a Landau polariton system consisting of a two-dimensional electron gas coupled to a terahertz cavity in a DC
magnetic field. An off-resonant AC
magnetic field modulates the cyclotron frequency and light–matter coupling strength while leaving the diamagnetic term unchanged, generating an additional DC coupling contribution. This drives the system across a critical threshold into a superradiant phase, characterized by
photon condensation and Landau-level polarization in the ground state of the Floquet
Hamiltonian. This quasiequilibrium approach offers a route to SRPTs distinct from driven-dissipative schemes.
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All authors Wen-Hua Wu, Fuyang Tay, Mengqian Che, Andrey Baydin, Junichiro Kono, David Hagenmüller
Quantum Physics · Cross-listed from hep-lat
Comments on "Ether of Orbifolds"
Masanori Hanada
We comment on a recent manuscript "Ether of Orbifolds" by Henry Lamm. In the first version, it was mistakenly claimed that the orbifold
lattice Hamiltonian is not gauge invariant, and a quantity $\epsilon_g$, which has nothing to do with a non-existent "gauge violation", was introduced. The scaling of this $\epsilon_g$ was used to claim a huge simulation cost. In fact, $\epsilon_g$ characterizes the shift of the effective
lattice spacing – because, in the orbifold
lattice formulation, the
lattice is generated dynamically from the
vacuum expectation value of the complex
matrices. In the second version, the claim about the gauge
symmetry was partially corrected, based on our comments. However, $\epsilon_g$ is still mistakenly interpreted as a measure of "departure from SU($N$)", inconsistently with the foundational results by Kaplan, Katz, and Ünsal, and also by Arkani-Hamed, Cohen, and Georgi. This interpretation plays a central role in sustaining the argument introduced in the first version.
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Quantum Physics
Measurement-induced state transitions across the fluxonium qubit landscape
Alex A. Chapple, Boris M. Varbanov, Alexander McDonald et al. (+1 more)
Understanding the mechanisms that limit high-fidelity readout in
circuit quantum electrodynamics is essential for its optimization. Multi-
photon resonances are understood to be a limiting factor, causing population transfer from the computational states to higher-energy states under drive. This effect, known as measurement-induced state transitions, has been extensively studied for the transmon
qubit. While this exploration has begun for the fluxonium
qubit, a systematic study of this effect is lacking. Here, we bridge this gap by theoretically studying measurement-induced state transitions in the fluxonium
qubit over a wide range of parameters, comprising essentially all experimentally explored ranges. We find that lighter fluxoniums are less susceptible to these state transitions when compared to their heavier counterparts. We attribute this effect to the combination of lower density of multi-
photon resonances, a smaller requisite coupling for a given dispersive shift, and a more harmonic-like structure of the charge operator. We confirm the validity of our analysis by performing time-dependent readout simulations. Finally, we consider the impact of the superinductor's array modes on measurement-induced state transitions over a large range of parameters.
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All authors Alex A. Chapple, Boris M. Varbanov, Alexander McDonald, Alexandre Blais
Quantum Physics
Accelerating Quantum Tensor Network Simulations with Unified Path Variations and Non-Degenerate Batched Sampling
Taylor Lee Patti, Paavai Pari, Yang Gao et al. (+5 more)
Quantum trajectory methods reduce the computational overhead of simulating noisy
quantum systems, approximating them with $m$ stochastically sampled $2^n$-entry
quantum statevectors rather than exact $2^{2n}$-entry density
matrices. Recently, Pre-Trajectory Sampling with Batched Execution (PTSBE) has dramatically increased the data collection rate of these methods. While statevector PTSBE has demonstrated data collection speedups of over $10^6 \times$, tensor network implementations only achieved $\sim 15 \times$ speedup. This comparatively modest tensor network advantage stemmed from 1) contraction path recalculations, 2) sequential tensor network sampling, and 3) inflexible/unoptimized contraction hyperparameters. In this manuscript, we increase PTSBE's tensor network data collection rate to more than $10^8\times$ that of traditional trajectories methods by developing 1) error-independent unified path variation, 2) non-degenerate tensor network sampling, and 3) a flexible/optimized contraction framework. While our methods are particularly powerful for accelerating non-proportional sampling, we also demonstrate a more than $1000\times$ speedup for more general
quantum simulations.
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All authors Taylor Lee Patti, Paavai Pari, Yang Gao, Azzam Haidar, Thien Nguyen, Tom Lubowe, Daniel Lowell, Brucek Khailany
Quantum Physics
Time evolution of impurity models and their universality for quantum computation
N. C. Mai Pham and Raul A. Santos
Impurity Hamiltonians are systems of $N$ fermionic modes where $O(1)$ of them interact among themselves via quartic (or higher order)
fermion terms, while coupling quadratically with $O(N)$ bath modes. Without the quartic interactions, these systems are classically simulable with $O(N^3)$ resources. It was proved that the time-dependent
evolution of these systems can perform universal
quantum computation. The question of whether or not this remains true for time-independent
evolution remains open. Here, we prove that the time
evolution of generic time-independent
impurity Hamiltonians on $O(N)$
qubits is universal on $N$
qubits if the input state is a product state of
fermions in any single particle basis. In our proof we find that for a computation of depth $S$, the size of the
impurity scales as $O(S\log S)$.
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Quantum Physics
Multivariate quantum reservoir computing with discrete and continuous variable systems
Tobias Fellner, Jonas Merklinger and Christian Holm
Quantum reservoir computing is a promising paradigm for processing temporal data. So far, the primary focus has been on univariate time series. However, the most relevant and complex real-world data is multidimensional. In this paper, we establish an extensive framework for multivariate data processing in
quantum reservoir computing. We propose and evaluate three multivariate encoding schemes and introduce the mixing capacity as a novel metric to evaluate the effectiveness with which a reservoir combines independent data streams. The computational performance of these proposed schemes is systematically assessed using this metric, as well as on the chaotic Lorenz-63 system prediction task, for two
quantum reservoirs based on discrete and continuous-variable
quantum systems. Furthermore, we relate the computational performance on these tasks to the underlying
quantum properties of the reservoir. Our findings reveal that the optimal encoding method is highly dependent on the reservoir system and the specific task, underlining the importance of a task-specific input design. Moreover, we observe that peak computational performance coincides with the presence of non-classical effects, which indicates that
quantum resources play a role in processing multivariate data.
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Quantum Physics
Algorithmic Aspects of the Fermi–Hubbard Model
Ainesh Bakshi, Ankur Moitra and Xinyu Tan
Preparing Gibbs states on a
quantum computer offers a way to computationally probe how microscopic interactions give rise to macroscopic phenomena at finite temperature. However, an uncomfortable tension has set in: our strongest general guarantees for Gibbs state preparation hold in regimes where the resulting states can also be simulated classically. We propose investigating physical models with competing energy scales. The
Fermi–Hubbard model is a natural target: the competition between electron hopping, on-site repulsion and chemical potential encapsulates many mysteries of condensed matter physics, including Mott
insulators, striping and superconductivity. Our main result identifies a simple guiding principle: the temperature scale for efficient preparation is set by the couplings between sites, while the on-site energy scale is unrestricted. For the
Fermi–Hubbard model on an $n$-site nearest-neighbor
lattice of fixed dimension, we construct a
quantum Gibbs sampler that mixes in $\mathcal{O}(\log(n/\epsilon))$ time whenever the temperature is at least a constant factor larger than the hopping strength, but is allowed to be independent of the repulsion and chemical potential. We obtain analogous guarantees for local
qubit systems with bounded intersite interactions and arbitrary external fields. As a consequence, our
algorithms can efficiently prepare
Fermi–Hubbard Gibbs states that are provably outside the convex hull of fermionic Gaussian states and
qubit Gibbs states for locally interacting
Hamiltonians outside the set of product states.
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Quantum Physics
Sufficiency and Petz recovery for positive maps
Lauritz van Luijk and Henrik Wilming
We study the interconversion of families of
quantum states ("statistical experiments") via positive, trace-preserving (PTP) maps and clarify its mathematical structure in terms of minimal sufficient Jordan
algebras, which can be seen to generalize the Koashi-Imoto decomposition to the PTP setting. In particular, we show that Neyman-Pearson tests generate the minimal sufficient Jordan
algebra, and hence also the minimal sufficient *-
algebra corresponding to the Koashi-Imoto decomposition. As applications, we show that a) equality in the data-processing inequality for the relative
entropy or the $\alpha$-$z$
quantum Rényi divergence implies the existence of a recovery map also in the PTP case and b) that two dichotomies can be interconverted by PTP maps if and only if they can be interconverted by decomposable, trace-preserving maps. We thoroughly review the necessary mathematical background on Jordan
algebras. As a step beyond the finite-dimensional case, we prove Frenkel's formula for approximately finite-dimensional von Neumann
algebras.
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Quantum Physics
Per-Shot Evaluation of QAOA on Max-Cut: A Black-Box Implementation Comparison with Goemans-Williamson
Evgenii Dolzhkov, Franz G. Fuchs and Dirk Oliver Theis
The
Quantum Approximate Optimization
Algorithm (QAOA) has emerged as a promising approach for addressing combinatorial optimization problems on near-term
quantum hardware. In this work, we conduct an empirical evaluation of QAOA on the Max-Cut problem, using the Goemans-Williamson (GW)
algorithm as a classical baseline for comparison. Unlike many prior studies, our methodology treats QAOA implementations as black-box optimizers, relying solely on default parameter settings without manual fine-tuning. We evaluate specific off-the-shelf QAOA implementations under default settings, not the algorithmic potential of QAOA with optimized parameters. This reflects a more realistic use case for end users who may lack the resources or expertise for instance-specific optimization. To facilitate fair and informative evaluation, we construct
benchmark instances using well-known graph generation models that emulate practical graph structures, avoiding synthetic constructions tailored to either
quantum or classical
algorithms. A central component of our analysis is a per-shot statistical framework, which tracks the quality of QAOA outputs as a function of the number of
circuit executions. This enables probabilistic comparisons with the GW
algorithm by examining when and how frequently QAOA surpasses classical performance baselines such as the GW expectation and lower bound. Our results provide insight into the practical applicability of QAOA for Max-Cut and highlight its current limitations, offering a framework that can guide the assessment and development of future QAOA implementations.
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Quantum Physics
Scalable Neural Decoders for Practical Fault-Tolerant Quantum Computation
Andi Gu, J. Pablo Bonilla Ataides, Mikhail D. Lukin et al. (+1 more)
Quantum error correction (QEC) is essential for scalable
quantum computing. However, it requires classical decoders that are fast and accurate enough to keep pace with
quantum hardware. While
quantum low-density parity-check codes have recently emerged as a promising route to efficient fault tolerance, current decoding
algorithms do not allow one to realize the full potential of these codes in practical settings. Here, we introduce a convolutional neural network decoder that exploits the
geometric structure of QEC codes, and use it to probe a novel "waterfall" regime of error suppression, demonstrating that the logical error rates required for large-scale fault-tolerant
algorithms are attainable with modest code sizes at current physical error rates, and with latencies within the real-time budgets of several leading hardware platforms. For example, for the $[144, 12, 12]$ Gross code, the decoder achieves logical error rates up to $\sim 17$x below existing decoders - reaching logical error rates $\sim 10^{-10}$ at physical error $p=0.1\%$ - with 3-5 orders of magnitude higher throughput. This decoder also produces well-calibrated confidence estimates that can significantly reduce the time overhead of repeat-until-success protocols. Taken together, these results suggest that the space-time costs associated with fault-tolerant
quantum computation may be significantly lower than previously anticipated.
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All authors Andi Gu, J. Pablo Bonilla Ataides, Mikhail D. Lukin, Susanne F. Yelin
Quantum Physics
Thermal Time and Irreversibility from Non-Commuting Observables in Accelerated Quantum Systems
Marcello Rotondo
We investigate when temporal ordering becomes operationally meaningful in relativistic
quantum field theory using localized detector models. A time parameter alone does not ensure that different sequences of operations are physically distinguishable. We show that distinguishability arises when the state satisfies the Kubo–Martin–Schwinger (KMS) condition and the detector couples through non-commuting observables. We consider uniformly accelerated two-level detectors interacting with a
quantum field in the Minkowski
vacuum. The restriction of the
vacuum to the detector trajectory induces a thermal response characterized by the Unruh temperature and the Tolman profile. For sequential couplings through distinct observables, the reduced detector state depends on the ordering of interactions already at second order, with a dependence controlled by the KMS parameter. This asymmetry is quantified using
quantum relative
entropy. In a minimal model, the relevant states form a family of non-commuting Gibbs states with identical spectra and different generators, yielding a closed-form expression depending only on the dimensionless combination of temperature and detector energy scale.
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Quantum Physics
Kirkwood-Dirac distributions in classical optics
Alfredo Luis and Lorena Ballesteros Ferraz
We develop a comprehensive analysis of the Kirkwood-Dirac distributions in classical optics, revealing their deep connection with optical coherence as fundamental concept in optics. From their very definition, the Kirkwood-Dirac distributions emerge as generalized mutual coherence functions involving two different bases instead of just one. This perspective provides a unified interpretation of the so-called anomalous values, that are complex and negative values, as direct manifestations of coherence. We show that this interpretation consistently applies across all field variables considered in this work, including polarization, interference and wave propagation. Furthermore, we propose diverse methods of experimental determination of these distributions based on interference, in full agreement with their coherence-based interpretation.
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Quantum Physics
A Model Context Protocol Server for Quantum Execution in Hybrid Quantum-HPC Environments
Masaki Shiraishi, Ikko Hamamura, Tatsuya Ishigaki et al. (+1 more)
The integration of large
language models (LLMs) into scientific research is accelerating the realization of autonomous “AI Scientists.” While recent advancements have empowered AI to formulate hypotheses and design experiments, a critical gap remains in the execution of these tasks, particularly in the domain of
quantum computing (QC). Executing
quantum algorithms requires not only generating code but also managing complex computational resources such as QPUs and high-performance computing (HPC) clusters. In this paper, we propose an AI-driven framework specifically designed to bridge this execution gap through the implementation of a Model Context Protocol (MCP) server. Our system enables an LLM
agent to process natural
language prompts submitted as part of a job, autonomously executing
quantum computing workflows by invoking our tools via the MCP. We demonstrate the framework's capability by performing essential
quantum algorithmic primitives, including sampling and computation of expectation values. Key technical contributions include the development of an MCP server for
quantum execution, a pipeline for interpreting OpenQASM code, an automated workflow with CUDA-Q for the ABCI-Q hybrid platform, and an asynchronous execution pipeline for remote
quantum hardware using the Quantinuum emulator via CUDA-Q. This work validates that AI
agents can effectively abstract the complexities of hardware interaction through an MCP-based architecture, thereby facilitating the automation of practical
quantum research.
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All authors Masaki Shiraishi, Ikko Hamamura, Tatsuya Ishigaki, Tadashi Kadowaki
Quantum Physics · Cross-listed from cs.DC
Asynchronous Quantum Distributed Computing: Causality, Snapshots, and Global Operations
Siddhartha Visveswara Jayanti and Anand Natarajan
We initiate the study of asynchronous
quantum distributed systems, focusing on the case of implementing atomic
quantum global operations that can be decomposed into a collection of local operations on the components of the system. A simple example of such an operation is a
quantum snapshot in which the whole system is instantaneously measured. Based on the classical snapshot
algorithm of Chandy and Lamport, we design a
quantum distributed
algorithm to implement such decomposable global operations, which we call the QGO
Algorithm. The analysis of our
algorithm shows that arguments based on Lamport's computational causality remain valid in the
quantum world, even though, due to
entanglement, causality is not manifest from the standard description of the system in terms of a (global)
quantum state. Our other contributions include a formal model of
quantum distributed computing, and a formal specification for the desired behavior of a global operation, which may be of interest even in classical settings (such as in the setting of randomized
algorithms).
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Quantum Physics
QARIMA: A Quantum Approach To Classical Time Series Analysis
Nishikanta Mohanty, Bikash K. Behera, Badshah Mukherjee et al. (+3 more)
We present QARIMA, a
quantum state-similarity-based reconstruction of the classical ARIMA modelling pipeline. Rather than using a
quantum circuit as a standalone forecaster, QARIMA preserves ARIMA's interpretable
forecasting structure while reformulating its core building blocks through analogous
quantum-compatible modules. The framework integrates
quantum differencing assessment, QACF/QPACF lag discovery, compact-swap-test state projection, swap-test/VQC-based AR and MA coefficient estimation, and weak-lag refinement within a single ARIMA
forecasting workflow. QACF and QPACF serve the functional roles of ACF and PACF for MA and AR lag discovery, but construct lag relevance through
quantum measurement geometry rather than direct classical correlation. Given screened candidate orders $(p,d,q)$, AR and MA coefficients are estimated through state-
alignment losses incorporating cosine
alignment,
entropy regularization, phase correction, and norm control. We evaluate QARIMA across environmental, climatic, industrial, and weather time-series
datasets using rolling-origin out-of-sample testing against automated classical ARIMA baselines, with performance assessed through MSE, MAPE, and Diebold–Mariano tests. The results show that
quantum state-similarity modules can produce competitive and, in several cases, improved
forecasting behaviour while preserving ARIMA's transparency, modularity, and interpretability. QARIMA therefore establishes a distinct pathway for
quantum-enhanced statistical
forecasting: its modules remain functionally analogous to classical ARIMA subroutines, but are not algebraic replicas; they serve the same modelling roles through state overlap, projection, and measurement-driven parameter estimation.
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All authors Nishikanta Mohanty, Bikash K. Behera, Badshah Mukherjee, Pravat Dash, Giuseppe Sergioli, Roberto Giuntini
Quantum Physics
Optimized Gottesman-Kitaev-Preskill Error Correction via Tunable Preprocessing
Xiang-Jiang Chen, Hao-Miao Jiang, Liu-Jun Wang et al. (+1 more)
The Gottesman-Kitaev-Preskill (GKP) code is a promising bosonic candidate for realizing fault-tolerant
quantum computation. Among existing error-correction protocols for GKP code, the Steane-type scheme is a canonical and widely adopted paradigm, yet its intrinsic noise propagation pattern limits further performance improvement. In this work, we propose a preprocessing-based Steane-type (P-Steane) scheme, which introduces a tunable preprocessing stage with squeezing parameters $a$ and $b$ to actively reshape noise propagation, thereby constituting a parameter framework. This framework spans a spectrum of protocols beyond existing methods, reproducing the performance of both the ME-Steane scheme ($a=1$, $b=1$) and the teleportation-based scheme ($a=1/\sqrt{2}$, $b=\sqrt{2}$) as special cases. Crucially, in the small-noise regime and when the data
qubit is noisier than the ancilla
qubits, P-Steane scheme achieves the minimum product of position- and momentum-quadrature output noise variances when $2a = b$, and consistently outperforms the ME-Steane scheme within a specific squeezing-parameter range under this condition.
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All authors Xiang-Jiang Chen, Hao-Miao Jiang, Liu-Jun Wang, Qing Chen
Quantum Physics
Evaluating the performance of a weak-field homodyne receiver in quadrature phase-shift keying optical communication
Silvia Cassina, Alex Pozzoli, Michele N. Notarnicola et al. (+3 more)
Quantum communication protocols require efficient detection schemes to maximize the information transfer rate between the sender and the receiver. To this aim, we have demonstrated that weak-field receivers, merging wave-like and particle-like features, can be considered as a valid alternative to already existing receivers, such as optical homodyne detection. To better emphasize the potential of our receiver, in this work we consider a proof of concept for quaternary communication based on coherent states with the same amplitude and different phase values. The encoding in phase requires a fine control of phase noise obtained through a feedback system. The results achieved in terms of mutual information and secret key generation rate encourage further increase of the alphabet towards an approximately continuous phase modulation.
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All authors Silvia Cassina, Alex Pozzoli, Michele N. Notarnicola, Marco Lamperti, Stefano Olivares, Alessia Allevi
Quantum Physics
Charging Quantum Batteries via Dissipative Quenches
Riccardo Grazi, Donato Farina, Niccolò Traverso Ziani et al. (+1 more)
We investigate work extraction in open
quantum batteries composed of interacting
spin chains weakly coupled to engineered environments. Focusing on two- and four-
qubit XX models initially prepared in thermal Gibbs states, we analyze how dissipation and dephasing, acting either locally or collectively, can generate and shape ergotropy during both transient and steady-state dynamics. By introducing a continuous interpolation between parallel and collective noise channels, we systematically characterize the impact of environmental structure on work extractability. We show that purely dissipative dynamics can activate finite ergotropy from completely passive thermal states, giving rise to temperature-dependent transient regimes where hotter initial states temporarily outperform colder ones in an ergotropic Mpemba-like fashion. In contrast, collective dissipation leads to steady states whose passivity crucially depends on the initial temperature and system size, a behavior we trace back to the emergence of non-trivial dark subspaces. Finally, we demonstrate that dephasing channels suppress both transient advantages and steady-state work extraction, highlighting the qualitative difference between dissipative and dephasing environments.
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All authors Riccardo Grazi, Donato Farina, Niccolò Traverso Ziani, Dario Ferraro
Quantum Physics
Photon pairs, squeezed light and the quantum wave mixing effect in a cascaded qubit system
R. D. Ivanovskikh, W. V. Pogosov, A. A. Elistratov et al. (+6 more)
We develop a theoretical description of
quantum wave mixing (QWM) in a cascaded waveguide-QED system of two superconducting
qubits, where the probe is driven by an external coherent tone and by the resonance fluorescence of a strongly driven source
qubit. Starting from the field correlation functions of the source emission, we derive an effective master-equation treatment for the probe and identify the regime in which the incident fluorescence is characterized by anomalous correlations. When the coherent Rayleigh component of the source spectrum is suppressed, the probe equations of motion become equivalent to those for a
qubit driven by a coherent tone and broadband squeezed light, although the ligt emitted by the source is non-Gaussian and antibunched. This equivalence implies a selection rule for the peaks of the QWM spectrum, with a strong suppression of sidebands associated with processes involving an odd number of
photons taken from the source field. Numerical simulations of the full cascaded two-
qubit model for different ratios of radiative decay rates unambiguously confirm the participation of correlated
photon pairs in QWM processes. The current research illustrates that the analysis of peak amplitudes can be used to probe
photon statistics in the incident nonclassical field.
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All authors R. D. Ivanovskikh, W. V. Pogosov, A. A. Elistratov, S. V. Remizov, A. Yu. Dmitriev, T. R. Sabirov, A. V. Vasenin, S. A. Gunin, O. V. Astafiev
Quantum Physics
Divide et impera: hybrid multinomial classifiers from quantum binary models
Simone Roncallo, Angela Rosy Morgillo, Seth Lloyd et al. (+2 more)
We investigate how to combine a collection of
quantum binary models into a multinomial classifier. We employ a hybrid approach, adopting strategies like one-vs-one, one-vs-rest and a binary decision tree. We
benchmark each method, by emphasizing their computational overhead and their impact on the
quantum advantage. By comparison against a classical binary model (generalized using the same approach), we show that the decision tree represents a cost-effective solution, achieving similar accuracies to other methods with an overhead at most logarithmic in the total number of classes.
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All authors Simone Roncallo, Angela Rosy Morgillo, Seth Lloyd, Chiara Macchiavello, Lorenzo Maccone
Quantum Physics
Orthogonalised Self-Guided Quantum Tomography: Insights from Single-Pixel Imaging
Kiki Dekkers, Alice Ruget, Fazilah Nothlawala et al. (+6 more)
We introduce the concept of self-guided imaging (SGI) as a linear analogue of self-guided
quantum tomography (SGQT). We show that SGI is mathematically equivalent to single-pixel imaging (SPI). Taking inspiration from orthogonalised ghost imaging, a recent advance in SPI, we introduce orthogonalised SGQT. This requires no additional experimental overhead and leads to faster and more accurate final convergence, as we demonstrate numerically (fidelity $95.2\% \rightarrow 99.17\%$) and experimentally (fidelity $92.1\% \rightarrow 95.3\%$). This work suggests that further routines from SPI and SGQT can be interchanged to optimise measurements and convergence.
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All authors Kiki Dekkers, Alice Ruget, Fazilah Nothlawala, Sabrina Henry, Stirling Scholes, Miles Padgett, Andrew Forbes, Isaac Nape, Jonathan Leach