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Monday 13 October

Quantum Physics

Disorder-Induced Strongly Correlated Photons in Waveguide QED

Guoqing Tian, Li-Li Zheng, Zhi-Ming Zhan et al. (+2 more)

Strongly correlated photons play a crucial role in modern quantum technologies. Here, we investigate the probability of generating strongly correlated photons in a chain of N qubits coupled to a one-dimensional (1D) waveguide. We found that disorder in the transition frequencies can induce photon antibunching, and especially nearly perfect photon blockade events in the transmission and reflection outputs. As a comparison, in ordered chains, strongly correlated photons cannot be generated in the transmission output, and only weakly antibunched photons are found in the reflection output. The occurrence of nearly perfect photon blockade events stems from the disorder-induced near completely destructive interference of photon scattering paths. Our work highlights the impact of disorder on photon correlation generation and suggests that disorder can enhance the potential for achieving strongly correlated photon.
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All authors Guoqing Tian, Li-Li Zheng, Zhi-Ming Zhan, Franco Nori, Xin-You Lü

Quantum Physics

Relative Information, Relative Facts

Andrea Di Biagio and Carlo Rovelli

We offer a fresh perspective on the relational interpretation of quantum mechanics as a way of thinking about the world described by quantum theory based on quantifiable notions of information. This allows us to provide a definition of a relative fact, with no addition to orthodox quantum theory and no fundamentally special role for observers. By associating perspectives with commutative observables rather than entire quantum systems, several previous problems with the interpretation are dissolved. As a side result, we show how a quantum measurement, properly described, is a continuous process.
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Quantum Physics

Basis-independent Coherence in Noninertial Frames

Ming-Ming Du Yi-Hao Fan, Hong-Wei Li, Shu-Ting Shen et al. (+5 more)

We investigate the behavior of basis-independent quantum coherence between two modes of a free Dirac field as observed by relatively accelerated observers. Our findings reveal three key results: (i) the basis-independent coherence between modes A and BI decreases with increasing acceleration but remains finite even in the limit of infinite acceleration; (ii) at zero acceleration, the coherence between modes $A$ and $B_II$ is nonzero contrasting with the behavior of basis-dependent coherence, which typically vanishes in this case; and (iii) the basis-independent coherence between modes BI and BII remains constant regardless of acceleration, exhibiting a freezing phenomenon. These results demonstrate the intrinsic robustness of basis-independent coherence under Unruh effects.
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All authors Ming-Ming Du Yi-Hao Fan, Hong-Wei Li, Shu-Ting Shen, Xiao-Jing Yan, Xi-Yun Li, Wei Zhong, Yu-Bo Sheng, Lan Zhou

Quantum Physics

Bright Single-Photon Emission from Individual Tin-Vacancy Centers in Multi-Cone Diamond Waveguides

Pablo Tieben, Jan Rhensius, Takuya F. Segawa et al. (+5 more)

Diamonds containing color centers have recently gathered significant attention for photonic quantum technologies, including quantum sensing, photonic quantum computers, and quantum networks. Among the various color centers, tin-vacancy (SnV) centers are particularly promising due to the high emission efficiency from the zero-phonon line and due to their long spin coherence times. However, the extraction of photons from diamond remains a key challenge. Here we demonstrate high photon extraction from a single SnV center incorporated in a diamond nanopillar with tapered sidewalls and a multi-cone structure. A sharp emission peak with a full width at half maximum (FWHM) of $6\,$nm was observed at a wavelength of $619\,$nm. Furthermore, the second-order correlation function exhibited an antibunching dip well below $g^{(2)}(0) = 0.5$, indicating single-photon emission. Remarkably, the emitter achieved a high saturation count rate of approximately $9\,$Mcps. These results establish our nanopillar platform as a promising candidate for bright and stable quantum sources and sensors based on SnV centers in diamond.
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All authors Pablo Tieben, Jan Rhensius, Takuya F. Segawa, Risei Abe, Konosuke Shimazaki, Shigeki Takeuchi, Andeas W. Schell, Hideaki Takashima

Quantum Physics

Experimental Demonstration of the PBR Test on a Superconducting Processor

Songqinghao Yang, Haomu Yuan and Crispin H. W. Barnes

We present an experimental implementation of the Pusey-Barrett-Rudolph (PBR) no-go theorem on IBM's 156-qubit Heron2 Marrakesh superconducting quantum processor. By preparing qubits in a set of non-orthogonal states and evolving them under carefully compiled unitary circuits, we test whether one can interpret the hidden variable model for quantum states as merely epistemic – reflecting ignorance about some underlying physical reality. To account for realistic hardware imperfections, we derive noise-aware error tolerance based on decoherence models calibrated to the device's performance. Our results show that a significant majority of adjacent qubit pairs and adjacent five-qubit configurations yield outcome statistics that violate the epistemic bound, thus ruling out the epistemic interpretation of quantum mechanics. Furthermore, we observe a clear trend: the probability of passing the PBR test decreases as the spatial separation within the quantum processor between qubits increases, highlighting the sensitivity of this protocol to connectivity and coherence in Noisy Intermediate-Scale Quantum (NISQ) systems. These results demonstrate the PBR test as a promising device-level benchmark for quantumness in the presence of realistic noise.
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Quantum Physics

Comparison and optimisation of hybridization algorithms for onboard classical and quantum accelerometers

Benoit Kaczmarczuk, Yannick Bidel, Alexandre Bresson et al. (+6 more)

We study two hybridization algorithms used for the combination of a quantum inertial sensor based on atom interferometry with a classical inertial sensor for onboard acceleration measurements. The first is based on the direct extraction of the interferometer phase, and was previously used in seaborne and airborne gravity measurement campaigns. The second is based on the combination of three consecutive measurements and was originally developed to increase the measurement range of the quantum sensor beyond its linear range. After comparing their performances using synthetic data, we implement them on acceleration data collected in a recent airborne campaign and evaluate the bias and the scale factor error of the classical sensor. We then extend their scope to the dynamical evaluation of other key measurement parameters (e.g. alignment errors). We demonstrate an improvement in the correlation between the two accelerometers' measurements and a significant reduction of the error in the estimation of the bias of the classical sensor.
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All authors Benoit Kaczmarczuk, Yannick Bidel, Alexandre Bresson, Nassim Zahzam, Alexis Bonnin, Malo Cadoret, Tim Enzlberger Jensen, Quentin Beaufils, Franck Pereira Dos Santos

Quantum Physics

Tensor-Network-Based Unraveling of Non-Markovian Dynamics in Large Spin Chains via the Influence Martingale Approach

Sujay Mondal, Siddhartha Dutta and Abhijit Bandyopadhyay

Classical simulation of open quantum system dynamics remains challenging due to the exponential growth of the Hilbert space, the need to accurately capture dissipation and decoherence, and the added complexity of memory effects in the non-Markovian regime. We develop an efficient algorithm for simulating both Markovian and non-Markovian dynamics in large one-dimensional quantum systems. Extending the Tensor Jump Method, which combines TDVP-based tensor-network evolution with a Suzuki–Trotter decomposition of stochastic trajectories, our approach incorporates time-dependent decay rates–treating positive rates as time-inhomogeneous Markovian processes and negative rates via the Influence Martingale formalism to unravel time-local non-Markovian dynamics. We further introduce the concept of the `influence radius' to achieve a resource-efficient framework enabling scalable simulations of open-system dynamics in the non-Markovian regime, as demonstrated for a one-dimensional transverse-field Ising chain comprising up to 100 spin qubits.
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Quantum Physics

Tunable multi-photon correlations from a coherently driven quantum dot

Thomas K. Bracht, Rachel N. Clark, Petros Androvitsaneas et al. (+7 more)

Mixing the fields generated by different light sources has emerged as a powerful approach for engineering non-Gaussian quantum states. Understanding and controlling the resulting photon statistics is useful for emerging quantum technologies that are underpinned by interference. In this work, we investigate intensity correlation functions arising from the interference of resonance fluorescence from a quantum emitter with a coherent laser field. We show that the observed bunching behavior results from a subtle interplay between quantum interference and the normalization of the correlation functions. We show that by adjusting the mixing ratio and phase one can achieve full tunability of the second-order correlation, ranging from anti-bunching to bunching. We further extend our analysis to third-order correlation functions, both experimentally and theoretically, to provide new insights into the interpretation of higher-order correlations and offer practical tools for shaping quantum optical fields.
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All authors Thomas K. Bracht, Rachel N. Clark, Petros Androvitsaneas, Matthew Jordan, Samuel G. Bishop, Harry E. Dyte, Moritz Cygorek, Ian A. Farrer, Doris E. Reiter, Anthony J. Bennett

Quantum Physics

Quantum Information Processing with Spatially Structured Light

Suraj Goel, Bohnishikha Ghosh and Mehul Malik

Qudits have proven to be a powerful resource for quantum information processing, offering enhanced channel capacities, improved robustness to noise, and highly efficient implementations of quantum algorithms. The encoding of photonic qudits in transverse-spatial degrees of freedom has emerged as a versatile tool for quantum information processing, allowing access to a vast information capacity within a single photon. In this review, we examine recent advances in quantum optical circuits with spatially structured light, focusing particularly on top-down approaches that employ complex mode-mixing transformations in free-space and fibers. In this context, we highlight circuits based on platforms such as multi-plane light conversion, complex scattering media, multimode and multi-core fibers. We discuss their applications for the manipulation and measurement of multi-dimensional and multi-mode quantum states. Furthermore, we discuss how these circuits have been employed to perform multi-party operations and multi-outcome measurements, thereby opening new avenues for scalable photonic quantum information processing.
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Quantum Physics

Hot-Starting Quantum Portfolio Optimization

Sebastian Schlütter, Tomislav Maras, Alexander Dotterweich et al. (+1 more)

Combinatorial optimization with a smooth and convex objective function arises naturally in applications such as discrete mean-variance portfolio optimization, where assets must be traded in integer quantities. Although optimal solutions to the associated smooth problem can be computed efficiently, existing adiabatic quantum optimization methods cannot leverage this information. Moreover, while various warm-starting strategies have been proposed for gate-based quantum optimization, none of them explicitly integrate insights from the relaxed continuous solution into the QUBO formulation. In this work, a novel approach is introduced that restricts the search space to discrete solutions in the vicinity of the continuous optimum by constructing a compact Hilbert space, thereby reducing the number of required qubits. Experiments on software solvers and a D-Wave Advantage quantum annealer demonstrate that our method outperforms state-of-the-art techniques.
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All authors Sebastian Schlütter, Tomislav Maras, Alexander Dotterweich, Nico Piatkowski

Quantum Physics

Quantum phase transition of sub-Ohmic spin-boson models: An approach by the multiple Davydov D2 Ansatz

Justin Tan, Nengji Zhou and Yang Zhao

The ground state properties and quantum phase transitions of sub-Ohmic spin-boson models are investigated using the multiple Davydov D2 Ansatz in conjunction with the variational principle. Three variants of the model are studied: (i) a single bath with diagonal coupling, (ii) two independent baths with diagonal and off-diagonal couplings, and (iii) a single bath with simultaneous diagonal and off-diagonal couplings. For the purely diagonal model, the multiple Davydov D2 Ansatz yields critical coupling strengths that are consistent with other methodologies, validating its accuracy and efficiency. In the two-bath model, the competition between diagonal and off-diagonal couplings drives a first-order transition for both symmetric and asymmetric spectral exponents, with von-Neumann entropy showing a continuous peak only under exact symmetry. Finally, for a single bath with simultaneous diagonal and off-diagonal couplings, we demonstrate that a rotational transformation maps the system to an equivalent purely diagonal model, enabling simpler and intuitive physical interpretation and reduced computational complexity.
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Quantum Physics

Efficient and Robust Spatial-to-Fiber Coupling forMultimode Quantum Networks via CascadedAdaptive Feedback Control

Ya Li, WanRu Wang, Weizhe Qiao et al. (+5 more)

Duan-Lukin-Cirac-Zoller (DLCZ)-based multimodequantum networks rely on efficient spatial-to-fiber coupling, yetenvironmental perturbations compromise this performance. Wedevelop a cascaded adaptive feedback control system integratedinto the quantum entanglement source preparation path.Leveraging a power-feedback hillclimbing algorithm, itdynamically regulates piezoelectric-actuated mirrors to achieveautonomous multi-dimensional beam alignment, Experimentsshow it rapidly boosts single-mode fiber (SMF) coupling efficieneyto over 70% within 20 seconds and entering the most efficient andstable transmission state after 75 seconds.Importantly, it enhancesthe stability of the atom-photon interfacecritical for quantumlight-matter interactionsproviding a practical framework forefficient, robust spatial light transmission in scalable quantumnetworks.
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All authors Ya Li, WanRu Wang, Weizhe Qiao, Qizhou Wu, Changqing Niu, Xiaolong Zou, Youxing Chen, Xin Guo

Quantum Physics · Cross-listed from gr-qc

A novel quantum memory effect and thermal modulation in graviton-mediated entanglement

Mainak Dutta, Partha Nandi and Bibhas Ranjan Majhi

A central challenge in probing the quantum nature of gravity is to distinguish effects that are genuinely quantum from those that can be explained classically. In this work, we study how quantized gravitational waves interact with thermal quantum systems, modeled as harmonic oscillators. We show that, unlike classical waves, quantized gravitons generate entanglement and leave behind a persistent “graviton-induced quantum memory” even after the wave has passed. This effect is further shaped by the presence of thermal noise, which does not simply wash out quantum correlations but can in fact amplify them in distinctive ways. Our analysis reveals clear signatures - such as nonlinear thermal corrections and a prethermal time-crystal-like phase-that cannot arise from any classical treatment. These results identify experimentally relevant markers of gravitons and provide a framework for exploring how finite-temperature environments may help uncover the quantum nature of gravity.
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Quantum Physics

Analytical Control of Quantum Coherence: Markovian Revival via Basis Engineering and Exact Non-Markovian Criteria

Na-Na Zhang, Chao-Yi Wu, Ming Li et al. (+4 more)

The preservation of quantum coherence is besieged by a fundamental dogma: its revival necessitates non-Markovian memory effects from structured environments. This paradigm has constrained quantum control strategies and obscured simpler paths to coherence protection. Here, we shatter this belief by demonstrating unambiguous coherence revival even in strictly Markovian regimes, achieved solely through basis engineering in the $\sigma_x/\sigma_y$ bases. We establish a comprehensive analytical framework for predictive coherence control, delivering three universal design principles. First, we derive a minimum critical noise based frequency, $\omega_{0}^{c} = 1.57/(0.4996 \cdot t_{\max})$, serving as a universal criterion for engineering non-Markovian dynamics over any interval $[0, t_{\max}]$. Crucially, we show that Markovian environments ($\omega_0 < \omega_0^c$) can exhibit coherence revival when the Zeeman energy satisfies $\omega_k > \pi/(2t_{\max})$, decoupling revival from environmental memory. Furthermore, for non-Markovian environments, we provide exact conditions for periodic and complete revival: setting $\omega_0 = n \cdot 6.285/t_{\max}$ guarantees revival in the $\sigma_z$ basis, while combining it with $\omega_k = \pi \omega_0 / 6.285$ ensures perfect revival in the $\sigma_x/\sigma_y$ bases. Our results, validated by rigorous quantum simulations, provide a predictive toolkit for coherence control, offering immediate strategies for enhancing quantum memory, sensing, and error mitigation.
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All authors Na-Na Zhang, Chao-Yi Wu, Ming Li, Wei-Xuan Cao, Jun-Hao Zhang, Yong-Rui Guo, Ren-Pu Li

Quantum Physics

Assessing the Role of Communication in Modular Multi-Core Quantum Systems

Maurizio Palesi, Enrico Russo, Giuseppe Ascia et al. (+8 more)

The scalability of quantum computing is constrained by the physical and architectural limitations of monolithic quantum processors. Modular multi-core quantum architectures, which interconnect multiple quantum cores (QCs) via classical and quantum-coherent links, offer a promising alternative to address these challenges. However, transitioning to a modular architecture introduces communication overhead, where classical communication plays a crucial role in executing quantum algorithms by transmitting measurement outcomes and synchronizing operations across QCs. Understanding the impact of classical communication on execution time is therefore essential for optimizing system performance. In this work, we introduce \qcomm, an open-source simulator designed to evaluate the role of classical communication in modular quantum computing architectures. \qcomm{} provides a high-level execution and timing model that captures the interplay between quantum gate execution, entanglement distribution, teleportation protocols, and classical communication latency. We conduct an extensive experimental analysis to quantify the impact of classical communication bandwidth, interconnect types, and quantum circuit mapping strategies on overall execution time. Furthermore, we assess classical communication overhead when executing real quantum benchmarks mapped onto a cryogenically-controlled multi-core quantum system. Our results show that, while classical communication is generally not the dominant contributor to execution time, its impact becomes increasingly relevant in optimized scenarios – such as improved quantum technology, large-scale interconnects, or communication-aware circuit mappings. These findings provide useful insights for the design of scalable modular quantum architectures and highlight the importance of evaluating classical communication as a performance-limiting factor in future systems.
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All authors Maurizio Palesi, Enrico Russo, Giuseppe Ascia, Hamaad Rafique, Davide Patti, Vincenzo Catania, Sergi Abadal, Abhijit Das, Pau Escofet, Eduard Alarcon, Carmen G. Almudéver

Quantum Physics

On the Potential of Quantum Computing in Classical Program Analysis

Yicheng Guang, Pietro Zanotta, Kai Zhou et al. (+2 more)

Classical program analysis techniques, such as abstract interpretation and symbolic execution, are essential for ensuring software correctness, optimizing performance, and enabling compiler optimizations. However, these techniques face computational limitations when analyzing programs with large or exponential state spaces, limiting their effectiveness in ensuring system reliability. Quantum computing, with its parallelism and ability to process superposed states, offers a promising solution to these challenges. In this work, we present QEX, a design that uses quantum computing to analyze classical programs. By synthesizing quantum circuits that encode program states in superposition and trace data dependency between program variables through entanglement, QEX enables the simultaneous exploration of program behaviors, significantly improving scalability and precision. This advancement has broad applications, from debugging and security verification to optimizing compilers for next-generation hardware. As a proof-of-concept, we evaluated QEX on 22 benchmark programs, demonstrating its effectiveness in analyzing program states. To support more language features and make QEX realized sooner in Fault-Tolerant Quantum Computing (FTQC), we propose QEX-H which hybridizes QEX with classical analysis techniques. To our knowledge, this work is the first proposal to use quantum computing for classical program analysis.
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All authors Yicheng Guang, Pietro Zanotta, Kai Zhou, Yueqi Chen, Ramin Ayanzadeh

Quantum Physics · Cross-listed from gr-qc

Detecting gravitational waves with spin systems

Jiamin Liang, Mingqiu Li, Yu Gao et al. (+3 more)

The observation of gravitational waves has opened a new window into the Universe through gravitational-wave astronomy. However, high-frequency gravitational waves remain undetected. In this work, we propose that spin systems can be employed to detect gravitational waves in this unexplored frequency regime. We derive the spin's response to gravitational waves and identify three distinct effects: the well-known Gertsenshtein effect, a metric-induced interaction, and the gravitational spin Hall effect. We focus on nuclear spins and utilize nuclear magnetic resonance to enhance the gravitational response, leveraging the advantages of long coherence time, high polarization, and a small gyromagnetic ratio. The proposed experimental scheme is capable of probing gravitational waves in the kilohertz to gigahertz range, with projected sensitivities reaching $\sqrt{S_h}\approx10^{-20}~\mathrm{Hz}^{-1/2}$.
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All authors Jiamin Liang, Mingqiu Li, Yu Gao, Wei Ji, Sichun Sun, Qi-Shu Yan

Quantum Physics

Electron-positron pair creation in a supercritical static asymmetric potential well

Z. L. Li, A. R. Sun, J. H. Xia et al. (+2 more)

The electron-positron pair creation in a supercritical static asymmetric potential well, which is composed of a subcritical and a supercritical potential separated by a fixed distance, is investigated using computational quantum field theory. To explain the discrete peaks in the positron energy spectrum, an analytical formula for determining the positions of bound states in a subcritical asymmetric potential well is derived and extended to the supercritical asymmetric potential well in two ways. One of the two methods can not only predict the positions of bound states, but also offer the pair creation rate. This study also reveals that the subcritical potential height can optimize the energy spread of created electrons, providing a new way to produce high-energy electron beams with concentrated energy in experiments. Moreover, it is found that the pair creation rate in a supercritical asymmetric potential well, composed of a subcritical symmetric potential well and a supercritical Sauter potential, exceeds the sum of the pair creation rates produced by each potential individually. This finding suggests a potential method for enhancing pair yield.
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All authors Z. L. Li, A. R. Sun, J. H. Xia, J. X. Wu, Y. J. Li

Quantum Physics

Effective reorganization energy for electron transfer

Ethan Abraham, Junghyun Yoon, Troy Van Voorhis et al. (+1 more)

The Marcus theory expression for the rate of non-adiabatic electron transfer is widely used across a range of physical conditions. Although Marcus theory defines the reorganization energy classically, here we show that the reorganization parameter appearing in the activation barrier for normal-region electron transfer is most generally a quantum mechanical object that depends on the electronic coupling, coinciding with the Marcus picture only in the limit of vanishing electronic coupling. This result unifies the physical description of electron-transfer activation barriers across the adiabatic and non-adiabatic regimes and formally predicts that Marcus-like rate expressions remain accurate beyond their traditional non-adiabatic domain of validity. These insights allow us to derive a closed-form expression for the curvature of the current-overpotential relation for electron-transfer-limited reactions at the electrochemical interface, now formally applicable to both inner-sphere and outer-sphere processes.
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All authors Ethan Abraham, Junghyun Yoon, Troy Van Voorhis, Martin Z. Bazant

Quantum Physics

Verifiable Quantum Advantage via Optimized DQI Circuits

Tanuj Khattar, Noah Shutty, Craig Gidney et al. (+5 more)

Decoded Quantum Interferometry (DQI) provides a framework for superpolynomial quantum speedups by reducing certain optimization problems to reversible decoding tasks. We apply DQI to the Optimal Polynomial Intersection (OPI) problem, whose dual code is Reed-Solomon (RS). We establish that DQI for OPI is the first known candidate for verifiable quantum advantage with optimal asymptotic speedup: solving instances with classical hardness $O(2^N)$ requires only $\widetilde{O}(N)$ quantum gates, matching the theoretical lower bound. Realizing this speedup requires highly efficient reversible RS decoders. We introduce novel quantum circuits for the Extended Euclidean Algorithm, the decoder's bottleneck. Our techniques, including a new representation for implicit Bézout coefficient access, and optimized in-place architectures, reduce the leading-order space complexity to the theoretical minimum of $2nb$ qubits while significantly lowering gate counts. These improvements are broadly applicable, including to Shor's algorithm for the discrete logarithm. We analyze OPI over binary extension fields $GF(2^b)$, assess hardness against new classical attacks, and identify resilient instances. Our resource estimates show that classically intractable OPI instances (requiring $>10^{23}$ classical trials) can be solved with approximately 5.72 million Toffoli gates. This is substantially less than the count required for breaking RSA-2048, positioning DQI as a compelling candidate for practical, verifiable quantum advantage.
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All authors Tanuj Khattar, Noah Shutty, Craig Gidney, Adam Zalcman, Noureldin Yosri, Dmitri Maslov, Ryan Babbush, Stephen P. Jordan

Quantum Physics

On the Feasibility of Exact Unitary Transformations for Many-body Hamiltonians

Praveen Jayakumar, Tao Zeng and Artur F. Izmaylov

Exact unitary transformations play a central role in the analysis and simulation of many-body quantum systems, yet the conditions under which they can be carried out exactly and efficiently remain incompletely understood. We show that exact transformations arise whenever the adjoint action of a unitary's generator defines a linear map within a finite-dimensional operator space. In this regime, there exists a finite-degree polynomial that annihilates the adjoint map, rendering the Baker-Campbell-Hausdorff (BCH) expansion finite. We identify the role of Lie algebras and their modules in producing finite BCH expansions in all known cases. This perspective brings together previously disparate examples of exact transformations under a single unifying principle and clarifies how algebraic relations between generators and transformed operators determine the polynomial degree of the transformation. We illustrate this framework for previously known cases of efficient unitary transformations including unitary coupled-cluster and Pauli product generators. Using this framework, we propose a new class of fermionic generators that can be used for efficient transformations. The result establishes sufficient algebraic conditions for when exact unitary transformations are possible and provides new strategies for reducing their computational cost in quantum simulation and constructing feasible unitary transformations.
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Quantum Physics

Dominant spin-spin relaxation mechanism at clock transition of the $Ho_{x}Y_{1-x}W_{10}$ complex at different concentrations

Xiao Chen, Haechan Park, Silas Hoffman et al. (+2 more)

Spin decoherence poses a significant challenge in molecular magnets, with the nuclear spin bath serving as a prominent source. Intriguingly, spin qubits at the clock transition exhibit remarkable insensitivity to the surrounding nuclear spins. Recent experimental studies have unveiled a correlation between the decoherence time and the density of spin qubits, prompting our investigation into the contribution of the qubit bath to spin decoherence. In this paper, we present a comprehensive theoretical analysis of a few S=1 spin qubits, focusing on their interaction at the clock transition. Employing the exact diagonalization and the cluster correlation expansion (CCE) method, we simulate the dynamics of spin decoherence while varying the density of the qubit bath. To ensure the realism of our simulations, we incorporate structural and energetic parameters derived from previous studies on the HoW10 crystal. Our findings indicate that when the energy mismatch between the energy splittings of two qubits exceeds their interaction strength, they can become effectively insensitive to each other, offering an explanation for the absence of observed changes in the T2 time during experiments with lower qubit densities. Understanding the role of qubit bath density in spin decoherence at the clock transition not only advances our knowledge of decoherence mechanisms but also provides insights for the development of strategies to protect coherence in molecular magnets and other quantum systems. By optimizing the density of spin qubits, we can enhance the coherence properties and pave the way for improved performance of quantum devices. Overall, this study offers valuable insights into the relationship between qubit bath density and spin decoherence at the clock transition, contributing to the broader understanding and control of quantum systems in molecular magnets.
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All authors Xiao Chen, Haechan Park, Silas Hoffman, Shuanglong Liu, Hai-Ping Cheng

Quantum Physics

Error mitigation for partially error-corrected quantum computers

Ben DalFavero and Ryan LaRose

We present a method for quantum error mitigation on partially error-corrected quantum computers - i.e., computers with some logical qubits and some noisy qubits. Our method is inspired by the error cancellation method and is implemented via a circuit for convex combinations of channels which we introduce in this work. We show how logical ancilla qubits can arbitrarily reduce the sampling complexity of error cancellation in a continuous space-time tradeoff, in the limiting case achieving $O(1)$ sample complexity which circumvents lower bounds for sample complexity with all known error mitigation techniques. This comes at the cost of exponential circuit depth, however, and leads us to conjecture that any error mitigation protocol with (sub-)polynomial sample complexity requires exponential time and/or space, even when logical qubits are utilized as a resource. We anticipate additional applications for our quantum circuits to implement convex combinations of channels, and to this end we discuss one application in simulating open quantum systems, showing an order of magnitude reduction in gate counts relative to current state-of-the-art methods for a canonical problem.
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Quantum Physics

A Simple and Efficient One-Shot Signature Scheme

Andrew Huang and Vinod Vaikuntanathan

One-shot signatures (OSS) are a powerful and uniquely quantum cryptographic primitive which allows anyone, given common reference string, to come up with a public verification key $\mathsf{pk}$ and a secret signing state $|\mathsf{sk}\rangle$. With the secret signing state, one can produce the signature of any one message, but no more. In a recent breakthrough work, Shmueli and Zhandry (CRYPTO 2025) constructed one-shot signatures, either unconditionally in a classical oracle model or assuming post-quantum indistinguishability obfuscation and the hardness of Learning with Errors (LWE) in the plain model. In this work, we address the inefficiency of the Shmueli-Zhandry construction which signs messages bit-by-bit, resulting in signing keys of $\Theta(\lambda^4)$ qubits and signatures of size $\Theta(\lambda^3)$ bits for polynomially long messages, where $\lambda$ is the security parameter. We construct a new, simple, direct, and efficient one-shot signature scheme which can sign messages of any polynomial length using signing keys of $\Theta(\lambda^2)$ qubits and signatures of size $\Theta(\lambda^2)$ bits. We achieve corresponding savings in runtimes, in both the oracle model and the plain model. In addition, unlike the Shmueli-Zhandry construction, our scheme achieves perfect correctness. Our scheme also achieves strong signature incompressibility, which implies a public-key quantum fire scheme with perfect correctness among other applications, correcting an error in a recent work of Çakan, Goyal and Shmueli (QCrypt 2025) and recovering their applications.
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Quantum Physics

Structural encoding with classical codes for computational-basis bit-flip correction in the early fault-tolerant regime

IlKwon Sohn, Changyeol Lee, Wooyeong Song et al. (+2 more)

Achieving reliable performance on early fault-tolerant quantum hardware will depend on protocols that manage noise without incurring prohibitive overhead. We propose a novel framework that integrates quantum computation with the functionality of classical error correction. In this approach, quantum computation is performed within the codeword subspace defined by a classical error correction code. The correction of various types of errors that manifest as bit flips is carried out based on the final measurement outcomes. The approach leverages the asymmetric structure of many key algorithms, where problem-defining diagonal operators (e.g., oracles) are paired with fixed non-diagonal operators (e.g., diffusion operators). The proposed encoding maps computational basis states to classical codewords. This approach commutes with diagonal operators, obviating their overhead and confining the main computational cost to simpler non-diagonal components. Noisy simulations corroborate this analysis, demonstrating that the proposed scheme serves as a viable protocol-level layer for enhancing performance in the early fault-tolerant regime.
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All authors IlKwon Sohn, Changyeol Lee, Wooyeong Song, Kwangil Bae, Wonhyuk Lee

Sunday 12 October

Quantum Physics

Sublogarithmic Distillation in all Prime Dimensions using Punctured Reed-Muller Codes

Tanay Saha and Shiroman Prakash

Magic state distillation is a leading but costly approach to fault-tolerant quantum computation, and it is important to explore all possible ways of minimizing its overhead cost. The number of ancillae required to produce a magic state within a target error rate $\epsilon$ is $O(\log^{\gamma} (\epsilon^{-1}))$ where $\gamma$ is known as the yield parameter. Hastings and Haah derived a family of distillation protocols with sublogarithmic overhead (i.e., $\gamma < 1$) based on punctured Reed-Muller codes. Building on work by Campbell et al. and Krishna-Tillich, which suggests that qudits of dimension $p>2$ can significantly reduce overhead, we generalize their construction to qudits of arbitrary prime dimension $p$. We find that, in an analytically tractable puncturing scheme, the number of qudits required to achieve sublogarithmic overhead decreases drastically as $p$ increases, and the asymptotic yield parameter approaches $\frac{1}{\ln p}$ as $p \to \infty$. We also perform a small computational search for optimal puncture locations, which results in several interesting triorthogonal codes, including a $[[519,106,5]]_5$ code with $\gamma=0.99$.
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Quantum Physics

Magnon-rotation enhanced nonreciprocity of multipartite entanglement in a magnomechanical system

Hamza Harraf, Noura Chabar, Mohamed Amazioug et al. (+2 more)

Nonreciprocal physics is attracting significant interest in quantum information processing. In this work, we propose a scheme to investigate the nonreciprocity of bi- and tripartite entanglement and generate squeezed states in a magnomechanical system. This is achieved through the Barnett effect, which originates from the rotation of the first magnon mode. The system consists of two YIG spheres, each supporting a magnon mode that represents collective spin motion, positioned inside a microwave cavity (MC). We show that the Barnett effect enhances entanglement under thermal effects and generates squeezed states for the two magnon modes and the photon mode. Moreover, we show that magnon-magnon coupling enhances entanglement between different two modes.
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All authors Hamza Harraf, Noura Chabar, Mohamed Amazioug, Rachid Ahl Laamara, Mojtaba Mazaheri

Quantum Physics · Cross-listed from gr-qc

Spinning into Quantum Geometry: Dirac and Wheeler-DeWitt Dynamics from Stochastic Helicity

Partha Nandi, Partha Ghose and Francesco Petruccione

Spin networks in loop quantum gravity provide a kinematical picture of quantum geometry but lack a natural mechanism for dynamical Dirac-type evolution, while the Wheeler–DeWitt equation typically enters only as an imposed constraint. We propose a stochastic framework in which each spin-network edge carries helicity-resolved amplitudes – two-state internal labels that undergo Poisson-driven flips. The resulting coupled master equations, after analytic continuation and the introduction of a fundamental length scale, generate Dirac-type dynamics on discrete geometry. At long times, the same process relaxes to helicity-symmetric equilibrium states, which are shown to satisfy a Wheeler–DeWitt-type condition. In this way, both quantum evolution and the gravitational constraint emerge within a single probabilistic framework. Our approach thus provides a background-independent and stochastic route to quantum geometry, offering an alternative to canonical quantization and a fresh perspective on the problem of time.
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Quantum Physics

Rigorous estimation of error thresholds of transversal Clifford logical circuits

Yichen Xu, Yiqing Zhou, James P. Sethna et al. (+1 more)

The threshold theorem promises a path to fault-tolerant quantum computation, provided the physical error rate is below a critical threshold. While transversal gates efficiently implement logical operations, they propagate errors and can lower this threshold relative to a static quantum memory. In this work, we generalize the statistical-mechanical (stat-mech) mapping from quantum memories to logical circuits with transversal gates, thereby enabling rigorous, decoder-independent thresholds for fault-tolerant logical computation. We first demonstrate the framework for two toric code blocks undergoing a transversal CNOT (tCNOT) gate, quantifying two independent error-spreading mechanisms. For persistent bit-flip errors with perfect syndromes, the stat-mech model is a 2D random Ashkin-Teller model. Monte Carlo simulation and finite-size scaling show that the tCNOT reduces the optimal bit-flip threshold to $p=0.080$, a $26\%$ decrease from the toric code memory threshold $p=0.109$. With syndrome errors included, the circuit maps to a 3D random 4-body Ising model with a plane defect, yielding a conservative estimate $p\geq 0.028$, a modest $15\%$ reduction from the memory threshold $p=0.033$. Beyond the tCNOT gate, we derive stat-mech models for all transversal Clifford gates of the toric code, including the fold-transversal Hadamard and $S$ gates, and generalize the framework to arbitrary CSS codes, proving that each transversal gate modifies the stat-mech model only locally in time. By reducing threshold analysis of fault-tolerant logical circuits to the study of classical spin models with local defects, our framework provides a systematic, decoder-independent benchmark for near-term fault-tolerant architectures.
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All authors Yichen Xu, Yiqing Zhou, James P. Sethna, Eun-Ah Kim