Quantum Physics
Navigating Quantum Missteps in Agent-Based Modeling: A Schelling Model Case Study
C. Nico Barati, Arie Croitoru, Ross Gore et al. (+5 more)
Quantum computing promises transformative advances, but remains constrained by recurring misconceptions and methodological pitfalls. This paper demonstrates a fundamental incompatibility between traditional
agent-based modeling (ABM) implementations and
quantum optimization frameworks like Quadratic Unconstrained Binary Optimization (QUBO). Using Schelling's segregation model as a case study, we show that the standard practice of directly translating ABM state observations into QUBO formulations not only fails to deliver
quantum advantage, but actively undermines computational efficiency. The fundamental issue is architectural. Traditional ABM implementations entail observing the state of the system at each iteration, systematically destroying the
quantum superposition required for computational advantage. Through analysis of Schelling's segregation dynamics on lollipop networks, we demonstrate how abandoning the QUBO reduction paradigm and instead reconceptualizing the research question, from "simulate
agent dynamics iteratively until convergence" to "compute minimum of
agent moves required for global satisfaction", enables a faster classical solution. This structural reconceptualization yields an
algorithm that exploits network
symmetries obscured in traditional ABM simulations and QUBO formulations. It establishes a new lower bound which
quantum approaches must outperform to achieve advantage. Our work emphasizes that progress in
quantum agent-based modeling does not require forcing classical ABM implementations into
quantum frameworks. Instead, it should focus on clarifying when
quantum advantage is structurally possible, developing best-in-class classical baselines through problem analysis, and fundamentally reformulating research questions rather than preserving classical iterative state change observation paradigms.
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All authors C. Nico Barati, Arie Croitoru, Ross Gore, Michael Jarret, William Kennedy, Andrew Maciejunes, Maxim A. Malikov, Samuel S. Mendelson
Quantum Physics
Gleason's Theorem for a Qubit as Part of a Composite System
Vincenzo Fiorentino and Stefan Weigert
We extend Gleason's
theorem to the two-dimensional
Hilbert space of a
qubit by invoking the standard axiom that describes composite
quantum systems. The tensor-product structure allows us to derive density
matrices and Born's rule for $d=2$ from a simple requirement: the probabilities assigned to measurement outcomes must not depend on whether a system is considered on its own or as a subsystem of a larger one. In line with Gleason's original
theorem, our approach assigns probabilities only to projection-valued measures, while other known extensions rely on considering more general classes of measurements. This extension of Gleason's
theorem to two-dimensional systems is shown to remain valid for some foil theories of
quantum theory.
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Quantum Physics
The impact of multimode sources on DLCZ type quantum repeaters
Emil R. Hellebek and Anders S. Sørensen
Long distance
entanglement generation at a high rate is a major
quantum technological goal yet to be fully realized, with the promise of many interesting applications, such as secure
quantum computing on remote servers and
quantum cryptography. One possible implementation is using a variant of the DLCZ-scheme by combining atomic-ensemble
memories and linear optics with spontaneous parametric down conversion (SPDC) sources. As we edge closer to the realization of such a technology, the complete details of the underlying components become crucial. In this paper we consider the impact of the multimode emission from the SPDC source on
quantum repeaters based on the DLCZ-scheme. We consider two cases, driving the SPDC using short Gaussian pulses and continuously. For pulsed driving, we find that the use of very narrow
laser pulses to drive SPDC source is crucial to obtain high fidelity end-to-end entangled states but this puts demands on the peak intensity. By introducing a maximally allowed
laser intensity, we find optimal pulse widths for each swap depth. For continuous driving, we find the temporal acceptance window of clicks relative to the heralding time to be a crucial parameter, and we can similarly optimize the acceptance window for each swap depth. For both cases, we thus identify optimal parameters given experimental limitations and aims. We have thus provided helpful knowledge towards the realization of long distance
entanglement generation using the DLCZ-scheme.
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Quantum Physics
QTIS: A QAOA-Based Quantum Time Interval Scheduler
José A. Tirado-Domínguez, Eladio Gutiérrez and Oscar Plata
Task scheduling with constrained time intervals and limited resources remains a fundamental challenge across domains such as manufacturing, logistics, cloud computing, and healthcare. This study presents a novel variant of the
Quantum Approximate Optimization
Algorithm (QAOA) designed to address the task scheduling problem formulated as a Quadratic Unconstrained Binary Optimization (QUBO) model. The proposed method, referred to as
Quantum Time Interval Scheduler (QTIS), integrates an ancilla-assisted
quantum circuit to dynamically detect and penalize overlapping tasks, enhancing the enforcement of scheduling constraints. Two complementary implementations are explored for overlap detection: a
quantum approach based on RY rotations and CCNOT gates, and a classical alternative relying on preprocessed interval comparisons. QTIS decomposes the problem
Hamiltonian, Hp, into two components, each parameterized by a distinct angle. The first component encodes the objective function, while the second captures penalty terms associated with overlapping intervals, which are controlled by the auxiliary
circuit. Subsequently, three minimization strategies are evaluated: standard QAOA, T-QAOA, and HT-QAOA, showing that employing separate parameters for the different components of the problem
Hamiltonian leads to lower energy values and improved solution quality. Results confirm the efficiency of QTIS in scheduling tasks with fixed temporal windows while minimizing conflicts, demonstrating its potential to advance hybrid
quantum-classical optimization in complex scheduling environments.
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Quantum Physics
Experimental demonstration of non-local magic in a superconducting quantum processor
Halima Giovanna Ahmad, Gianluca Esposito, Viviana Stasino et al. (+9 more)
Non-local magic is the non-stabilizerness that no local
unitary operation can erase. It captures the joint action of
entanglement and magic underlying
quantum advantage, and it has never been measured on
quantum hardware. Here we report its first experimental demonstration, on a superconducting
quantum processing unit, through two independent routes: an optimal local-erasure protocol and a direct, state-agnostic measurement of subsystem purity. The two agree with each other and with theory. Exploiting direct access to the device, we construct a noise model with no free parameters that identifies readout error and a depolarizing controlled-Z channel as the dominant mechanisms, and we show that local and non-local magic can be addressed separately, erasing local magic in situ while preserving the non-local part. Non-local magic provides a hardware
benchmark beyond standard gate-fidelity protocols and points toward more reliable pre-fault tolerant devices.The same tools underlie a purity-estimation protocol with exponential speedup and the decoding of Hawking radiation in a black-hole toy model.
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All authors Halima Giovanna Ahmad, Gianluca Esposito, Viviana Stasino, Jovan Odavic, Carlo Cosenza, Alessandro Sarno, Pasquale Mastrovito, Michele Viscardi, Stefano Cusumano, Francesco Tafuri, Davide Massarotti, Alioscia Hamma
Quantum Physics
Constraint-preserving quantum algorithm for the multi-frequency antenna placement problem
Matteo Vandelli, Francesco Ferrari and Daniele Dragoni
Quantum algorithms for combinatorial optimization typically encode constraints as soft penalties within the objective function, which can reduce efficiency and scalability compared to state-of-the-art classical methods that instead exploit constraints to guide the search toward high-quality solutions. Although solving this issue for an arbitrary problem is inherently a hard task, we address this challenge for a specific problem in the field of telecommunications, the multi-frequency
antenna placement problem, by introducing a constraint-preserving
quantum adiabatic
algorithm (QAA). To this aim, we construct a
quantum circuit that prepares an initial state comprising an equal superposition of all feasible solutions, and define a custom mixer that preserves both the one-hot encoding constraint for vertex coloring and the cardinality constraint on the number of
antennas. This scheme can be extended to a broader range of applications characterized by similar constraints. We first
benchmark the performance of this
quantum algorithm against a basic version of QAA, demonstrating superior performance in terms of feasibility and success probability. We then apply this
algorithm to large problem sizes with hundreds of variables using a constraint-aware decomposition method based on the SPLIT framework. Our results indicate competitive performance against other large-scale classical approaches, such as branch-and-bound and simulated annealing. This work supports previous claims that constraint-aware
algorithms are crucial for the practical and efficient application of
quantum methods in industrial settings.
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Quantum Physics
Design and Optimization of Adaptive Diversity Schemes in Quantum MIMO Channels
Shehbaz Tariq and Symeon Chatzinotas
As
quantum networks evolve toward a full
quantum Internet, reliable transmission in
quantum multiple-input multiple-output (QuMIMO) settings becomes essential, yet remains difficult due to noise, crosstalk, and the mixing of
quantum information across subchannels. To improve reliability in such settings, we study an adaptive diversity strategy for discrete-variable QuMIMO systems based on universal asymmetric cloning at the transmitter and probabilistic purification at the receiver. An input
qubit is encoded into M approximate clones, transmitted over an N x N multi-mode
quantum channel, and recovered through a purification map optimized using available channel state information (CSI). For the given cloning asymmetry parameters, we derive an
eigenvalue-based expression for the decoder-optimal end-to-end fidelity in the form of a generalized Rayleigh quotient, which enables efficient tuning of the cloner without iterative optimization. As a design choice, we employ semidefinite program (SDP) to construct the purification map for only a targeted success probability p. This numerical framework is used to study fixed-noise, dimension-scaling noise, and
stochastic depolarization regimes. A cloning asymmetry index is introduced to quantify the distribution of
quantum information across the multiple subchannels across these operating conditions. The results show that the proposed scheme yields significant fidelity gains in crosstalk-dominated settings and automatically adapts to channel
symmetry and channel conditions. This work provides design guidelines for future QuMIMO systems and establishes a robust baseline for more advanced transmission and decoding strategies.
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Quantum Physics · Cross-listed from math-ph
Inhomogeneous SSH models and the doubling of orthogonal polynomials
Nicolas Crampé, Quentin Labriet, Lucia Morey et al. (+2 more)
We analyze Su-Schrieffer-Heeger (SSH) models using the doubling method for orthogonal
polynomial sequences. This approach yields the analytical spectrum and exact eigenstates of the models. We demonstrate that the standard SSH model is associated with the doubling of Chebyshev
polynomials. Extending this technique to the doubling of other finite sequences enables the construction of
Hamiltonians for inhomogeneous SSH models which are exactly solvable. We detail the specific cases associated with Krawtchouk and $q$-Racah
polynomials. This work highlights the utility of
polynomial-doubling techniques in obtaining exact solutions for physical models.
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All authors Nicolas Crampé, Quentin Labriet, Lucia Morey, Gilles Parez, Luc Vinet
Quantum Physics
Stochastic unravelings for Heisenberg picture and trace-nonpreserving dynamics
Federico Settimo, Kimmo Luoma, Dariusz Chruściński et al. (+3 more)
Stochastic unravelings allow to efficiently simulate open system dynamics, yet their application has traditionally been restricted to master equations that preserve both Hermiticity and trace. In this work, we introduce a general framework that extends piecewise-deterministic unravelings to arbitrary trace-nonpreserving master equations, requiring only positivity and Hermiticity of the dynamics. Our approach includes, as special cases, unravelings of arbitrary dynamics in the Heisenberg picture, evolutions interpolating between fully Lindblad and non-Hermitian
Hamiltonian generators, and equations employed in the derivation of full counting statistics, for which we show it can be used to obtain the moments of the associated probability distribution. The framework is suitable for both trace-decreasing and trace-increasing processes through
stochastic disappearance and replication of the
stochastic realizations, and it is compatible with different unraveling schemes and with reverse jumps in the non-Markovian regime. Thereby, our approach provides a powerful and versatile simulation method that significantly broadens the applicability of
stochastic techniques for open system dynamics.
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All authors Federico Settimo, Kimmo Luoma, Dariusz Chruściński, Bassano Vacchini, Andrea Smirne, Jyrki Piilo
Quantum Physics
Optomechanical disk resonator in the quantum ground state of motion
Andrea Barbero, Samuel Pautrel, Bertrand Evrard et al. (+5 more)
Although they have enabled several advances in the field of optomechanics, optomechanical disk resonators have not yet been operated in the
quantum regime. We present the first experimental demonstration of an optomechanical disk resonator prepared in the
quantum ground state. With a gigahertz frequency, the mechanical breathing mode of the investigated
semiconductor disk reaches a level of excitation below a single
phonon when cooled in a dilution refrigerator. We quantify the
phonon occupancy of the mechanical mode by performing Brillouin sideband spectroscopy: a conical optical fiber is evanescently coupled to the disk optical whispering-gallery mode, and Stokes and anti-Stokes
photons scattered by
phonon emission and absorption are counted on a single-
photon detector. We measure a suppression of the absorption process corresponding to a
phonon occupancy of $0.66\pm0.20$. We experimentally investigate the mechanisms ruling
laser-induced heating, which limits the lowest measurable
phonon occupancy, and notably witness an extra-cavity heating effect.
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All authors Andrea Barbero, Samuel Pautrel, Bertrand Evrard, Jérémy Bon, Romain Dezert, Aristide Lemaître, Adrien Borne, Ivan Favero
Quantum Physics
Multi-Criteria Routing for Cryptographic Key Exchange in QKD Networks: Survey, Taxonomy, and a Conceptual Optimization Framework
Ivan Cvitic, Dragan Perakovic and Armando Nolasco Pinto
Quantum Key Distribution (QKD) networks enable unconditionally secure key exchange using
quantum mechanical principles. However, routing cryptographic keys across multi-hop
quantum networks introduces challenges unique to
quantum communication. This survey analyzes and classifies 26 routing strategies proposed between 2013 and 2024 for terrestrial, satellite, and hybrid QKD infrastructures. Dynamic, key-aware routing
algorithms have been shown to reduce service rejection rates by 25-40% compared to static shortest-path methods by incorporating real-time key pool availability and link error rates. Multi-path strategies improve resilience against trusted-node compromise by distributing keys across disjoint routes, albeit with an increase in key consumption of 30-60%. SDN-based orchestration frameworks emerge as essential enablers of flexible, QoS-aware routing for hybrid networks integrating terrestrial fibers and intermittent satellite links. Critical research directions include reducing dependence on trusted nodes through
quantum repeaters or post-
quantum cryptography, developing standardized key management interfaces for interoperability, and adopting AI-driven predictive routing for adapting to fluctuating network states. This comprehensive synthesis of routing
algorithms, performance trade-offs, and unresolved issues offers practical guidance for engineering secure, adaptable, and large-scale QKD networks capable of meeting real-world deployment demands.
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Quantum Physics
Topological Quantum Transducers in a Hybrid Rydberg Atom System
Pei-Yao Song, Jin-Lei Wu, Weibin Li et al. (+1 more)
We propose a topological transport platform for microwave-to-optical conversion at the single-
photon level in a Rydberg atom-cavity setting. This setting leverages a hybrid dual-mode Jaynes-Cummings (JC) configuration, where a microwave resonator couples an optical cavity mediated by a Rydberg atom ensemble. Our scheme uniquely enables the formation of Fock-state
lattices (FSLs), where
photon hopping rates depend on
photon numbers in individual sites. We identify an inherent zero-energy mode corresponding to the dark state of the dual-mode JC model. This enables to build a high-efficiency single-
photon transducer, which realizes topologically protected
photon transport between the microwave and optical modes. Crucially, we show analytically that the FSL features continuous variations of the winding number. Our work establishes a robust mechanism for efficient
quantum transduction in synthetic dimensions and opens avenues for exploring topological physics with continuous winding numbers in the atom-cavity system.
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All authors Pei-Yao Song, Jin-Lei Wu, Weibin Li, Shi-Lei Su
Quantum Physics
Bell Inequality Violation with Vacuum-One-Photon Number Superposition States
Zi-Qi Zeng, Jian Wang, Xiu-Bin Liu et al. (+8 more)
Entanglement is a central resource in
quantum technologies, and the realization of photonic
entanglement necessarily relies on interaction with matter. Resonance fluorescence (RF), originating from the coherent interaction between a driving field and a two-level system, plays a pivotal role in
quantum optics. Here, we demonstrate a novel route to
entanglement generation based on RF from a single
quantum dot. Rather than relying on generation of multiphoton states, our approach directly exploits
vacuum-one-
photon number superposition states created under resonant excitation. By delocalizing this superposition via a beam splitter, we realize time-bin
entanglement and observe a clear violation of the Clauser-Horn-Shimony-Holt Bell inequality using Franson-type interferometry. Our scheme removes the need for multiphoton generation, simplifies the experimental requirements, and establishes a scalable pathway toward solid-state entangled
photon sources.
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All authors Zi-Qi Zeng, Jian Wang, Xiu-Bin Liu, Xu-Jie Wang, Li Liu, Hanqing Liu, Haiqiao Ni, Zhichuan Niu, Carlos Antón-Solanas, Bang Wu, Zhiliang Yuan
Quantum Physics · Cross-listed from math.CT
Axiomatising the dagger category of complex Hilbert spaces
Jan Paseka and Thomas Vetterlein
We axiomatise the dagger category of complex
Hilbert spaces and bounded linear maps, using exclusively purely categorical conditions. Our axioms are chosen with the aim of an easy interpretability: two of them describe the composition of objecs, two further ones deal with the decomposition of objects, and a final axiom expresses a
symmetry property. The categorical reconstruction of complex
Hilbert spaces addresses foundational issues in
quantum physics. We present a simplified alternative to recent characterisations.
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Quantum Physics
QSentry: Backdoor Detection for Quantum Neural Networks via Measurement Clustering
Shuolei Wang, Zimeng Xiao, Jinjing Shi et al. (+3 more)
Quantum neural networks (QNNs) are an important model for implementing
quantum machine learning (QML), while they demonstrate a high degree of vulnerability to
backdoor attacks similar to classical networks. To address this issue, a
quantum backdoor attack detection framework called QSentry is proposed, in which a
quantum Measurement Clustering method is introduced to detect
backdoors by identifying statistical anomalies in measurement outputs. It is demonstrated that QSentry can effectively detect anomalous distributions induced by
backdoor samples with extensive experiments. It achieves a 75.8% F1 score even under a 1% poisoning rate, and further improves to 85.7% and 93.2% as the poisoning rate increases to 5% and 10%, respectively. The integration of silhouette coefficients and relative cluster size enable QSentry to precisely isolate
backdoor samples, yielding estimates that closely match actual poisoning ratios. Evaluations under various
quantum attack scenarios demonstrate that QSentry delivers superior robustness and accuracy compared with three state-of-the-art detection methods. This work establishes a practical and effective framework for mitigating
backdoor threats in QML.
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All authors Shuolei Wang, Zimeng Xiao, Jinjing Shi, Heyuan Shi, Shichao Zhang, Xuelong Li
Quantum Physics
Fidelity-Preserving Quantum Encoding for Quantum Neural Networks
Yuhu Lu and Jinjing Shi
Efficiently encoding classical visual data into
quantum states is essential for realizing practical
quantum neural networks (QNNs). However, existing encoding schemes often discard spatial and semantic information when adapting high-dimensional images to the limited
qubits of Noisy Intermediate-Scale
Quantum (NISQ) devices. We propose a Fidelity-Preserving
Quantum Encoding (FPQE) framework that performs near lossless data compression and
quantum encoding. FPQE employs a convolutional encoder-decoder to learn compact multi-channel representations capable of reconstructing the original data with high fidelity, which are then mapped into
quantum states through amplitude encoding. Experimental results show that FPQE performs comparably to conventional methods on simple
datasets such as MNIST, while achieving clear improvements on more complex ones, outperforming PCA and
pruning based encodings by up to 10.2% accuracy on Cifar-10. The performance gain grows with data complexity, demonstrating FPQE's ability to preserve high-level structural information across diverse visual domains. By maintaining fidelity during classical to
quantum transformation, FPQE establishes a scalable and hardware efficient foundation for high-quality
quantum representation learning.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Refraction-induced transverse charge transport
Ronika Sarkar, Arka Bandyopadhyay, Awadhesh Narayan et al. (+1 more)
We introduce a mechanism that produces a
Hall-like transverse response in time-reversal-invariant materials, driven entirely by
geometric effects. Specifically, we demonstrate that a tilted potential interface causes electron wave packets to undergo a refractionlike deflection upon transmission through the barrier, leading to a finite transverse current and a corresponding
Hall-like conductance. Our analytical framework captures the essential features of this refraction-driven charge transport, and the resulting transverse conductance profile is corrob- orated by numerical simulations across different
lattice models and device geometries. We further visualize our predicted effect through real-time wave packet dynamics, which reveals its purely
geometric origin and the robustness of the transverse response. These findings establish a fundamentally distinct class of
Hall-like transport phenomena in mesoscopic systems that preserve time-reversal
symmetry.
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All authors Ronika Sarkar, Arka Bandyopadhyay, Awadhesh Narayan, Diptiman Sen
Quantum Physics
Thermalizing channel states for rapid qubit heating
Ziyang You, Wenhui Huang, Libo Zhang et al. (+4 more)
Although known for negatively impacting the operation of superconducting
qubits, thermal baths are shown to exert
qubit control in a positive way, provided they are properly engineered. We demonstrate an experimental method to engineer the transduction of microwave driving into heat flow through a leaky resonator. Given the precise conversion, a
qubit receiving the heat flow obtains a quasi-thermal
equilibrium with arbitrary target temperature in hundreds of nanoseconds. We show that the dynamics of the
quantum transducing process is described by thermalizing channel states, generated from the double dressings of the resonator by the semi-classical driving and the
qubit-resonator coupling. Their spectrum, coupling, and driving strength determine the channel rate of energy flow, along with the relaxation rates of
photon leakage into the bath. The analytical prediction is shown to match well with the experimental measurements on an Xmon
qubit circuit.
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All authors Ziyang You, Wenhui Huang, Libo Zhang, Song Liu, Youpeng Zhong, Yibo Gao, Hou Ian
Quantum Physics
Calibration of single-photon cameras using radioluminescent light sources
Radek Machulka, Václav Michálek, Ondřej Haderka et al. (+1 more)
In this paper, we address the calibration of the
quantum efficiency of single-
photon cameras using radioluminescent light sources. The proposed methods are subsequently compared with absolute calibration techniques based on the detection of correlated
photon pairs. Furthermore, we propose a method for transferring absolute calibration using the aforementioned radioluminescent emitters.
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All authors Radek Machulka, Václav Michálek, Ondřej Haderka, Jan Peřina
Quantum Physics
Tensor-network approach to quantum optical state evolution beyond the Fock basis
Nikolay Kapridov, Egor Tiunov and Dmitry Chermoshentsev
Understanding the
quantum evolution of light in nonlinear media is central to the development of next-generation
quantum technologies. Yet, modeling these processes remains computationally demanding, as the required resources grow rapidly with
photon number and phase-space resolution. Here, we introduce a tensor-network approach that efficiently captures the dynamics of nonlinear optical systems in a continuous-variable representation. Using the
matrix product state (MPS) formalism, both
quantum states and operators are encoded in a highly compressed form, enabling direct numerical integration of the Schrödinger equation. We demonstrate the method by simulating degenerate spontaneous parametric down-conversion (SPDC) and show that it accurately reproduces established theoretical
benchmarks - energy conservation, pump depletion, and quadrature squeezing - even in regimes where conventional Fock-basis simulations become infeasible. For high-intensity pump fields $(\alpha=100)$, the MPS representation achieves compression ratios below $3\times10^{-4}$ while preserving physical fidelity. This framework opens a scalable route to modeling multimode
quantum light and nonlinear optical phenomena beyond the reach of traditional methods.
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Quantum Physics
QADR: A Scalable, Quantum-Resistant Protocol for Anonymous Data Reporting
Nilesh Vyas and Konstantin Baier
The security of future large-scale IoT networks is critically threatened by the “Harvest Now, Decrypt Later” (HNDL) attack paradigm. Securing the massive, long-lived data streams from these systems requires protocols that are both
quantum-resistant and highly scalable. Existing solutions are insufficient: post-
quantum classical protocols rely on computational assumptions that may not hold for decades, while purely
quantum protocols are too resource-intensive for the sheer scale of IoT. This paper introduces the
Quantum Anonymous Data Reporting (QADR) protocol, a hybrid framework that provides a theoretical
benchmark and high-performance architecture for this challenge, designed for future fully-connected
quantum networks. The protocol achieves scalable,
quantum-resistant anonymity through a hybrid security model; it leverages information-theoretically secure keys from
Quantum Key Distribution (QKD) to seed a
quantum-secure pseudorandom function (QS-PRF), grounding its long-term data protection in well-established computational hardness assumptions. We also propose and analyze an automated slot reservation mechanism by making a deliberate trade-off: achieving high performance by accepting a quantifiable information leak during the anonymous slot reservation phase while maintaining strong unlinkability for the final data submission. Our security analysis formally quantifies the anonymity reduction caused by the leak and discusses pathways to fully mitigate it at a significant performance cost. We prove the protocol's critical advantage as a performance
benchmark: its primary communication cost scales as $O(n^2)$, a dramatic improvement over
quantum-native alternatives ($O(n^4)$), establishing a high-performance goal for future
quantum-secured anonymity systems.
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Quantum Physics · Cross-listed from hep-th
Note on Logical Gates by Gauge Field Formalism of Quantum Error Correction
Junichi Haruna
The gauge field formalism, or operator-valued cochain formalism, has recently emerged as a powerful framework for describing
quantum Calderbank-Shor-Steane (CSS) codes. In this work, we extend this framework to construct a broad class of logical gates for general CSS codes, including the S, Hadamard, T, and (multi)controlled-Z gates, under the condition where fault-tolerance or
circuit-depth optimality is not necessarily imposed. We show that these logical gates can be expressed as exponentials of
polynomial functions of the electric and
magnetic gauge fields, which allows us to derive explicit decompositions into physical gates. We further prove that their logical action depends only on the (co)homology classes of the corresponding logical
qubits, establishing consistency as logical operations. Our results provide a systematic method for formulating logical gates for general CSS codes, offering new insights into the interplay between
quantum error correction, algebraic
topology, and
quantum field theory.
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Quantum Physics
Fail fast: techniques to probe rare events in quantum error correction
Michael E. Beverland, Malcolm Carroll, Andrew W. Cross et al. (+1 more)
The ultimate goal of
quantum error correction is to create logical
qubits with very low error rates (e.g. 1e-12) and assemble them into large-scale
quantum computers capable of performing many (e.g. billions) of logical gates on many (e.g. thousands) of logical
qubits. However, it is necessarily difficult to directly assess the performance of such high-quality logical
qubits using standard Monte Carlo sampling because logical failure events become very rare. Building on existing approaches to this problem, we develop three complementary techniques to characterize the rare-event regime for general
quantum low-density parity-check (qLDPC) codes under
circuit noise. (I) We propose a well-motivated, low-parameter ansatz for the failure spectrum (the fraction of fault sets of each size that fail) that empirically fits all the QEC systems we studied and predicts logical error rates at all physical error rates. (II) We find min-weight logical operators of syndrome measurement
circuits and exactly compute the number of min-weight failing configurations. (III) We generalize the splitting method to qLDPC codes using multi-seeded Metropolis sampling to improve convergence for systems with many inequivalent logical operators. We apply these tools to distance-6, -12, and -18 bivariate bicycle codes under
circuit noise, observing strong low-error-rate performance with the recently proposed Relay decoder but also considerable scope for further improvement.
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All authors Michael E. Beverland, Malcolm Carroll, Andrew W. Cross, Theodore J. Yoder
Quantum Physics · Cross-listed from cs.LG
Vehicle Routing Problems via Quantum Graph Attention Network Deep Reinforcement Learning
Le Tung Giang, Vu Hoang Viet, Nguyen Xuan Tung et al. (+2 more)
The vehicle routing problem (VRP) is a fundamental NP-hard task in intelligent transportation systems with broad applications in logistics and distribution. Deep reinforcement learning (DRL) with Graph Neural Networks (GNNs) has shown promise, yet classical models rely on large multi-layer perceptrons (MLPs) that are parameter-heavy and
memory-bound. We propose a
Quantum Graph
Attention Network (Q-GAT) within a DRL framework, where parameterized
quantum circuits (PQCs) replace conventional MLPs at critical readout stages. The hybrid model maintains the expressive capacity of graph
attention encoders while reducing trainable parameters by more than 50%. Using proximal policy optimization (PPO) with greedy and
stochastic decoding, experiments on VRP
benchmarks show that Q-GAT achieves faster convergence and reduces routing cost by about 5% compared with classical GAT baselines. These results demonstrate the potential of PQC-enhanced GNNs as compact and effective solvers for large-scale routing and logistics optimization.
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All authors Le Tung Giang, Vu Hoang Viet, Nguyen Xuan Tung, Trinh Van Chien, Won-Joo Hwang
Quantum Physics · Cross-listed from physics.optics
Intelligent Inverse Design of Multi-Layer Metasurface Cavities for Dual Resonance Enhancement of Nanodiamond Single Photon Emitters
Omar A. M. Abdelraouf
Single-
photon emitters (SPEs) based on nitrogen-vacancy centers in nanodiamonds (neutral NV0 (wavelength 575 nm) and negative NV- (wavelength 637 nm)) represent promising platforms for
quantum nanophotonics applications, yet their emission efficiencies remain constrained by weak light-matter interactions. Multi-layer metasurfaces (MLM) offer unprecedented degrees of freedom for efficient light manipulation beyond conventional single-material metasurfaces, enabling dual-resonance cavities that can simultaneously enhance pump excitation and SPE collection. However, traditional trial-and-error and forward optimization methods face significant challenges in designing these complex structures due to the vast parameter space and computational demands. Here, we present NanoPhotoNet-Inverse, an artificial intelligence-driven inverse design framework based on a hybrid deep neural network architecture. This model efficiently performs inverse design of two dual-resonance MLM cavities to amplify pump and SPE emissions of different vacancies in nanodiamond and improve SPE collection. Our approach achieves inverse design prediction efficiency exceeding 98.7%, demonstrating three orders of magnitude amplification in SPE count rate and 50 picosecond lifetime, significantly surpassing conventional cavity designs. These remarkable enhancements in emission rates and collection efficiency position our platform as a transformative technology for advancing
quantum communication networks,
quantum computing architectures,
quantum sensing applications, and
quantum cryptography systems. Therefore, it opens new pathways for intelligent photonic device engineering in
quantum technologies.
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Quantum Physics
Radial Fast Entangling Gates Under Micromotion in Trapped-Ion Quantum Computers
Phoebe Grosser, Monica Gutierrez Galan, Isabelle Savill-Brown et al. (+7 more)
Micromotion in
radio-frequency ion traps is generally considered detrimental for
quantum logic gates, and is typically minimized in state-of-the-art experiments. However, as a deterministic effect, it can be incorporated into
quantum control frameworks aimed at designing high-fidelity
quantum logic controls. In this work, we demonstrate that micromotion can be beneficial to the design of fast gates utilizing the radial modes of a two-ion
crystal, particularly in the sub-trap-period regime where high-fidelity control sequences are identified with operation times ranging from hundreds of nanoseconds to microseconds. Through analysis of select fast gate solutions, we uncover the physical origin of micromotion enhancement and further study the induced gate error under experimental noises and control imperfections. This analysis establishes the feasibility of realising high-fidelity entangling gates in hundreds of nanoseconds using the micromotion-sensitive radial modes of trapped-ion
crystals.
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All authors Phoebe Grosser, Monica Gutierrez Galan, Isabelle Savill-Brown, Alexander K. Ratcliffe, Haonan Liu, Varun D. Vaidya, Simon A. Haine, C. Ricardo Viteri, Joseph J. Hope, Zain Mehdi
Quantum Physics · Cross-listed from cond-mat.str-el
Characterizing entanglement at finite temperature: how does a "classical" paramagnet become a quantum spin liquid?
Snigdh Sabharwal, Matthias Gohlke, Paul Skrzypczyk et al. (+1 more)
Quantum spin liquids (QSL) are phases of matter which are distinguished not by the
symmetries they break, but rather by the patterns of
entanglement within them. Although these
entanglement properties have been widely discussed for ground states, the way in which QSL form at finite temperature remains an open question. Here we introduce a method of characterizing both the depth and spatial structure of
entanglement, and use this to explore how patterns of
entanglement form as temperature is reduced in two widely studied models of QSL, the Kitaev honeycomb model, and the
spin-1/2 Heisenberg antiferromagnet on a Kagome
lattice. These results enable us to evaluate both the temperature at which
spins within the high-temperature paramagnet first become entangled, and the temperature at which the system first develops the structured, multipartite
entanglement characteristic of its QSL ground state.
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All authors Snigdh Sabharwal, Matthias Gohlke, Paul Skrzypczyk, Nic Shannon
Quantum Physics
Beyond Trotterization: Variational Product Formulas for Quantum Simulation
Ibsal Assi, Michael Vogl, Meenu Kumari et al. (+1 more)
We propose a variational alternative to the Trotter-Suzuki decomposition that provides greater control over errors while preserving the
unitary structure of time
evolution. The variational parameters in our ansatz are derived from a global action principle, where Euler-Lagrange equations govern their optimal dynamics. Unlike conventional wavefunction-based variational methods, our approach specifically targets the time
evolution operation and this allows a single set of optimized parameters to be applied to any initial state for a fixed
Hamiltonian avoiding costly optimization procedures. Our method outperforms the standard Trotter-Suzuki formulas, typically achieving higher accuracy than higher-order Suzuki schemes. This translates directly to
quantum computing applications, where it enables the design of
quantum circuits with fewer gates which reduces noise and improves precision. Although we focus on
quantum dynamics, the method is broadly applicable to problems involving general time-
evolution operators. Applied to various model
Hamiltonians, our approach reduces errors by factors of 2 to 5 compared to Trotter-Suzuki decompositions, demonstrating its promise for accurate
quantum simulation with improved efficiency. In certain cases, the variational ansatz achieves higher accuracy than more complex higher-order Suzuki formulas while reducing the gate count by nearly half within a single
circuit layer. Furthermore, we derive approximate analytical expressions for the variational parameters up to cubic order in time, valid for generic
Hamiltonians. These approximations enable long-time
quantum simulations with improved accuracy over equivalent Suzuki decompositions, providing ready-to-use
evolution formulas that match Suzuki's gate complexity while delivering better performance.
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All authors Ibsal Assi, Michael Vogl, Meenu Kumari, J. P. F. LeBlanc
Quantum Physics
Work-minimizing protocols in driven-dissipative quantum systems: An impulse-ansatz approach
Masaaki Tokieda
The second law of thermodynamics sets a lower bound on the work required to drive a system between thermal
equilibrium states, with equality attained in the quasistatic limit. For finite-time processes, work in excess of this bound is inevitably dissipated, motivating the search for driving protocols that minimize this excess work. While classical
stochastic systems have been extensively explored,
quantum analyses remain limited and often rely on Markovian master equations valid only in the weak-coupling regime. Here, we study minimal work protocols for representative two-level systems coupled to a harmonic-oscillator bath using a numerically exact method. Inspired by known optimal solutions for Brownian oscillators, we introduce an impulse ansatz that incorporates possible boundary impulses and test it across a wide range of bath parameters. We find that impulse-like features remain nearly optimal in the
quantum, non-Markovian regime at short times. We also identify cases in which the widely used Markovian master equation fails even at weak coupling, highlighting the necessity of going beyond standard approximations in the
quantum regime for finite-time thermodynamic optimization.
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Quantum Physics
Generation of 10-dB squeezed light from a broadband waveguide optical parametric amplifier with improved phase locking method
Kazuki Hirota, Takahiro Kashiwazaki, Gyeongmin Ha et al. (+10 more)
We report generation of 10.1\pm0.2-dB squeezed light from a broadband periodically poled lithium niobate (PPLN) waveguide optical parametric amplifier (OPA). Based on our previous report where a similar PPLN waveguide shows 8.3-dB squeezing [T. Kashiwazaki et al., Appl. Phys. Lett. 122, 234003 (2023)], we reduce phase fluctuations and overall optical losses in the measurement system. In particular, we introduce a novel phase detection technique that does not require tapping a part of the squeezed light to get a phase locking signal. We use a phase-detection OPA seeded by a tapped probe and pump light before a squeezer OPA. This configuration breaks the conventional trade-off between generating a phase-locking signal with high-signal-to-noise ratio and suppressing degradation of squeezing level caused by optical tapping. With all these improvements, the phase fluctuation angle is reduced from 14 mrad to 9 mrad, and the total optical loss from 12% to 8%. Achieving more than 10 dB of squeezing by the broadband waveguide OPA is a significant step towards realization of fault-tolerant ultra-fast universal optical
quantum computation.
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All authors Kazuki Hirota, Takahiro Kashiwazaki, Gyeongmin Ha, Taichi Yamashima, Pawaphat Jaturaphagorn, Takumi Suzuki, Kazuma Takahashi, Akito Kawasaki, Asuka Inoue, Warit Asavanant, Mamoru Endo, Takeshi Umeki, Akira Furusawa