Quantum Physics · Cross-listed from math-ph
The Classification of Pauli Stabilizer Codes: A Lattice and Continuum Treatise
Bowen Yang and Matthew Yu
We classify mobile Pauli stabilizer codes up to gapped interfaces and coarse-graining using the framework of algebraic $\mathrm{L}$-theory. We compare this classification with that of framed TQFTs, theories that arise naturally in the continuum, highlighting a close structural relationship between the two. Our approach is formulated in the category of perfect chain complexes equipped with quadratic functor over the Laurent
polynomial ring $R = \mathbb{Z}/p[x_1^{\pm 1}, \ldots, x_n^{\pm 1}]$, within which the collection of topological operators of Pauli stabilizer codes arise naturally as objects. In particular, we establish a bulk-boundary correspondence for
lattice theories: the equivalence class of a Pauli stabilizer code up to gapped interface is described by a Clifford QCA in one dimension higher. This is done using the universal target category for stabilizer codes, which is the categorical spectrum whose existence and universal properties are introduced in this work. We conclude by highlighting subtle differences between the classification of Pauli stabilizer codes and TQFTs, leading to qualitative distinctions between
lattice and continuum theories.
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Quantum Physics · Cross-listed from physics.hist-ph
The Legacy of Enrico Fermi to Varenna
Vladislav Gavryusev and Massimo Inguscio
The Varenna school is a hub where generations of physicists, including numerous Nobel laureates, have shaped the field, often through collaborative exchanges across political and cultural boundaries. We examine the scientific legacy of Enrico
Fermi and its influence on modern atomic, molecular, and optical physics. Beginning with
Fermi's 1954 lectures at the Varenna school, key developments are traced from high-energy physics to
laser spectroscopy, precision metrology, and the control of ultracold atoms. Milestones such as Doppler-free spectroscopy, optical frequency combs, Bose-Einstein condensation, and degenerate
Fermi gases are highlighted as turning points leading to
quantum simulation and
quantum computation.
Fermi's early advocacy for building a computer, rather than buying it, can be viewed as a precursor to today's efforts in
quantum science and technologies. This historical trajectory and legacy continues to inform current research in
quantum matter and information science.
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Quantum Physics · Cross-listed from cs.LG
Query-Efficient Quantum Approximate Optimization via Graph-Conditioned Trust Regions
Molena Huynh
In low-depth implementations of the
Quantum Approximate Optimization
Algorithm (QAOA), the dominant cost is often the number of objective evaluations rather than
circuit depth. We introduce a graph-conditioned trust-region method for reducing this query cost. A graph neural network predicts a Gaussian distribution N(mu, Sigma) over QAOA angles. The mean initializes a local optimizer, the covariance defines an ellipsoidal trust region that constrains the search, and the predicted uncertainty determines an instance-dependent evaluation budget. Thus the learned distribution defines a search policy rather than only an initial parameter estimate. Under explicit assumptions on local smoothness,
curvature, calibration, and noise, we derive bounds on objective degradation within the trust region, lower bounds on gradient variance, preservation of expected objective ordering under depolarizing noise, and finite-sample coverage guarantees. We evaluate the method for MaxCut at depth p = 2 on Erdos-Renyi, 3-regular, Barabasi-Albert, and Watts-Strogatz graphs with n = 8-16 vertices. Relative to random restarts and the strongest learned point-prediction baseline, the method reduces the mean number of
circuit evaluations from 343 and 85 to 45 +/- 7, while maintaining sampled approximation ratios within 3 percentage points of concentration-based heuristics. The method does not improve absolute approximation ratios; its advantage is reduced query cost at comparable solution quality. The predictive uncertainty is calibrated in the experiments, with ECE = 0.052 and Spearman correlation rho = 0.770, and the learned trust regions transfer to graph sizes not used during training. The results identify a low-depth, query-dominated regime in which graph-conditioned trust regions reduce the query cost of QAOA without modifying the ansatz.
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Quantum Physics
Contracting Tensor Networks with Generalized Belief Propagation
Joseph Tindall, Grace M. Sommers and Hilbert Kappen
Recent years have seen a growing interest in the use of belief propagation - an
algorithm originally introduced for performing statistical inference on graphical models - for approximate, but highly efficient, tensor network contraction. Here, we detail how to apply generalized belief propagation (GBP) - where messages are passed within a hierarchy of overlapping regions of the tensor network - to approximately contract tensor networks and obtain accurate results. The original belief propagation
algorithm is a corner case of this approach, corresponding to a particularly simple choice of regions of the tensor network. We implement the GBP
algorithm for a number of different region choices on a range of two- and three-dimensional, infinite and finite tensor networks, solving the corresponding fixed point equations both numerically and, in certain tractable cases, analytically. Our examples include calculating the partition function of the fully frustrated Ising model, computing the ground state degeneracy of three-dimensional ice models, measuring observables on the deformed AKLT
quantum state and evaluating the norm of randomly generated tensor network states.
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Quantum Physics · Cross-listed from gr-qc
Semiclassical phases of charged spin-$1/2$ matter-wave interferometers in gravitational wave backgrounds
Nontapat Wanwieng and Apimook Watcharangkool
A matter wave propagating through curved spacetime accumulates phase that encodes both geometry and gauge structure. We develop a semiclassical framework for charged
spin-$1/2$ matter-wave interferometers based on a WKB expansion of the covariant Dirac equation, in which the phase decomposes into dynamical,
spin, and electromagnetic Aharonov-Bohm (AB) contributions. In a freely falling detector frame, all three channels are governed by local
tidal fields. In a weak gravitational-wave (GW) background, the dynamical and
spin phases probe the gravitoelectric and gravitomagnetic sectors of
curvature, while the AB phase arises from
curvature-induced electromagnetic fields obtained from Maxwell's equations in curved spacetime. For a Mach-Zehnder interferometer (MZI), all three responses are determined by the same
tidal scale, $\ddot{h}_A \sim \Omega^2_{gw}h_0$, and filtered by a common
geometric kernel, while entering through distinct physical couplings. In particular, the AB contribution depends not only on the enclosed flux but also on spatial variations of the induced fields and exhibits an intrinsic frequency dependence set by the traversal time. These results provide a unified description of matter-wave interferometric phases in time-dependent GW backgrounds and identify complementary dynamical,
spin, and electromagnetic pathways through which spacetime
curvature imprints itself on
quantum interference.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Dynamical preparation of U(1) quantum spin liquids in an analogue quantum simulator
Simon Karch, Melissa Will, Irene Prieto Rodriguez et al. (+7 more)
Locally constrained gauge theories underpin our understanding of fundamental interactions in particle physics and the emergent behaviour of
quantum materials. In strongly correlated systems, they can give rise to
quantum spin liquids that lack conventional order and are defined by coherent superpositions of an extensive number of many-body configurations. Realising and probing such exotic states experimentally is an outstanding challenge both in solid-state and synthetic
quantum systems, not least due to the difficulty of detecting the fragile coherences between many-body states. Here, we report a large-scale (>3,000 sites) realisation of a two-dimensional U(1)
lattice gauge theory with ultracold atoms in a square optical superlattice and demonstrate non-
equilibrium preparation of extended regions of U(1)
quantum spin liquids. We demonstrate Gauss's law validity in a quench experiment, enabled by a new microscopy technique for detecting doubly occupied sites. We observe characteristic real-space correlations and momentum-space pinch points, hallmarks of the emergent U(1) gauge structure. Using round-trip interferometric protocols, we directly observe large-scale coherence between many-body configurations, providing strong evidence for
quantum spin liquid regions extending over ~100
lattice sites. Our results establish non-
equilibrium quantum simulation protocols as a powerful route for accessing and probing exotic, highly-entangled states beyond those hosted by the engineered
Hamiltonian in thermal
equilibrium.
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All authors Simon Karch, Melissa Will, Irene Prieto Rodriguez, Nikolas Liebster, SeungJung Huh, Michael Knap, Frank Pollmann, Clemens Kuhlenkamp, Immanuel Bloch, Monika Aidelsburger
Quantum Physics
Application of a Quantum Amplitude Redistribution Algorithm to the Data Filtering Problem
Karina Zakharova, Artem Chernikov and Sergey Sysoev
This paper presents an analysis of the applicability of a
quantum amplitude redistribution
algorithm to the data filtering problem and the results of modeling the
algorithm's operation in comparison with a median filter.
Quantum Physics
Experimental high-dimensional multi-qubit Bell non-locality on a superconducting quantum processor
Yousef Mafi, Ali G. Moghaddam and Teemu Ojanen
Combining recent advances in superconducting
quantum hardware, we explore
quantum correlations in a previously inaccessible regime by observing simultaneously high-dimensional and many-body Bell non-locality. We report a high-confidence Bell violation in the correlations between two $d=64$-dimensional systems encoded in twelve
qubits. For system sizes up to $d=32$, the strength of the observed nonlocal correlations exceeds the
quantum upper bound for $d=2$ systems, providing direct evidence of high-dimensional nonlocality. Furthermore, we demonstrate that the observed violation is genuinely collective: all
qubits contribute to the nonlocal correlations, while most pairwise correlations across the bipartition remain Bell-local. Our work illustrates how present-day
quantum processors enable the exploration of fundamental predictions of
quantum mechanics in previously inaccessible regimes and, in turn, how fundamental
quantum effects can be used to
benchmark their performance.
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Quantum Physics
CAbLECAR: efficiently scheduling QLDPC codes on a tileable spin qubit chip with shuttling
Jason D. Chadwick and Frederic T. Chong
Semiconductor spin qubits are a promising platform for large-scale
quantum computing, but have yet to take full advantage of the broad class of
quantum low-density parity check (QLDPC) codes, which promise high encoding rates and efficient logic but require nonlocal connectivity between physical
qubits. In this work, we investigate the implementation of QLDPC codes on a tileable, shuttling-based
spin qubit architecture. By tailoring syndrome extraction
circuits to the shuttling noise model, we significantly improve on previous surface code proposals and extend the feasible shuttling range of the architecture by 5-10x, enabling the implementation of more complex codes with long-range interactions. Taking inspiration from the field of robotics, we develop a coordinated shuttle scheduling
algorithm that supports arbitrary codes and use it to
benchmark the logical performance of a variety of promising code families. We find that the optimized schedules are up to 86% faster than hand-optimized schedules for certain code families. Through detailed
circuit-level simulations, we identify specific QLDPC codes that improve upon prior surface code implementations by orders of magnitude, increasing encoding efficiency and reducing logical error rates. This work demonstrates the potential of shuttling-based
spin qubit hardware platforms for scalable and efficient fault-tolerant
quantum computation.
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Quantum Physics
How Quantum Contextuality disappears in the Classical Limit
Arthur C. R. Dutra, Roberto D. Baldijão and Marcelo Terra Cunha
The emergence of classicality is fundamentally driven by the interaction between a
quantum system and its environment. Foundational open-system approaches, notably the Caldeira-Leggett model, successfully captured how these interactions lead to macroscopic effects like
quantum dissipation and decoherence. However, these approaches often leave the precise definitions of classicality and quantumness ambiguous. In
quantum information theory, this boundary is a heavily scrutinized question, and Kochen-Specker contextuality emerges as a hallmark of nonclassicality. It is therefore natural to investigate whether decoherence can actually suppress this property. Taking this path creates an apparent conundrum, once there exist two distinct manifestations of
quantum contextuality: state-dependent and state-independent ones. While state-dependent contextuality naturally vanishes under state degradation, state-independent contextuality could persist for any
quantum state, since it shows up even for the maximally mixed state! In this paper, we resolve this apparent paradox by analyzing sequential measurement implementations of the paradigmatic Klyachko, Can, Binicioğlu, and Shumovsky (KCBS) and Peres-Mermin prepare-and-measure scenarios under the influence of depolarizing channels. By introducing noise both prior to and in between measurements, and by analyzing the resulting sequential correlators in both the Schrödinger and Heisenberg pictures, we show how open-system dynamics suppress the correlations required to witness contextuality, leading to classicalization.
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Quantum Physics
Operating a contextual Stern-Gerlach apparatus
Th. K. Mavrogordatos
We propose a contextual cavity/
circuit QED analogue and extension of the Stern-Gerlach experiment, where the pseudo-
spin of a two-state `atomic' transition plays the role of the “
spin”, while the resonant field driving the transition stands for the “
magnetic field”. A phase-sensitive continuous detection of the cavity field coupled to the induced `atomic' dipole affects the stability of the two distinct outcomes. The dressed states comprising the latter give their place to a self-consistent spontaneous dressed-state polarization as the driving strength is lowered. The associated
evolution proves anew highly contextual, underpinned by a persistent production of coherent-state superpositions for a particular setting of the monitoring device. Finally, when bistability is absent, we employ the photoelectron `atomic' emission statistics as a diagnostic tool of the cavity field fluctuations.
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Quantum Physics · Cross-listed from cond-mat.mes-hall
Gate-dependent offset charge shifts and anharmonicity in gatemon qubits in the weak tunneling regime
Utkan Güngördü, Rusko Ruskov, Silas Hoffman et al. (+3 more)
Gatemon
qubits are based on a
superconductor-
quantum dot-
superconductor (S-QD-S) junction which enables in situ electrostatic tuning via a gate electrode. For a single-channel QD this structure gives rise to two subgap Andreev bound states (ABSs), and generally leads to a richer
quantum phase dynamics as compared to conventional transmons. In a recent work [Phys. Rev. B 111, 214503 (2025)] we derived the
quantum phase dynamics from a many-body treatment which leads to an effective gate voltage-dependent
Hamiltonian that self-consistently incorporates the phase quantization. It predicts (i) a renormalization of the junction's effective capacitance and (ii) the presence of gate voltage and occupation-dependent charge offsets in junctions with tunneling asymmetry. Here, we quantify the observable impact of these effects on the
qubit's energy spectrum and anharmonicity, by studying the interplay of the two Andreev branches as a function of dot-gate voltages and junction transparencies. We show the relation of these predictions to simplified gatemon models and propose a protocol to experimentally detect the predicted charge offsets.
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All authors Utkan Güngördü, Rusko Ruskov, Silas Hoffman, Kyle Serniak, Andrew J. Kerman, Charles Tahan
Quantum Physics
Encoding strategies for quantum enhanced fluid simulations: opportunities and challenges
Omer Rathore, Alastair Basden, Nicholas Chancellor et al. (+1 more)
Quantum computing has emerged as a powerful potential accelerator for computational fluid dynamics (CFD), but whether this promise can be realized in practice depends on how fluid information is encoded on
quantum hardware. This review provides an architecture-agnostic assessment of encoding strategies for
quantum-enhanced fluid simulation, focusing on the trade-offs they impose on state preparation, measurement, boundary treatment, nonlinear dynamics, and temporal
evolution. We examine the principal encoding paradigms used in the literature and relate them to representative
quantum algorithms for fluid simulation. Through these examples, we show that encoding choices fundamentally shape both the
algorithm itself and also the practical feasibility of
quantum CFD. For example, highly compact encodings can offer attractive asymptotic advantages but might introduce severe bottlenecks in readout, state preparation, and nonlinear processing, whereas less compact representations may simplify interactions and improve compatibility with analog and near-term hardware. No single encoding is universally optimal, rather the most suitable choice depends strongly on the structure of the fluid problem, the computational objective and the constraints of the target
quantum platform. We therefore argue that encoding should be treated as a primary design variable in
quantum CFD and revisited iteratively throughout the design pipeline, as different algorithmic components interact and influence one another.
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All authors Omer Rathore, Alastair Basden, Nicholas Chancellor, Halim Kusumaatmaja
Quantum Physics · Cross-listed from cond-mat.mes-hall
Singlet-triplet oscillations in multivalley Si double quantum dots
Łukasz Cywiński, Mats Volmer, Tom Struck et al. (+2 more)
Charge separation from the $(4,0)$ to the $(3,1)$ state in a Si/SiGe double
quantum dot is commonly used for initialization of
spin qubits and Pauli-
spin-blockade readout. It was used in recent experiments involving creation of the $(3,1)$ singlet, and subsequent shuttling of one of the electrons. We present a theoretical description of the process of charge separation and singlet-triplet mixing, arriving at expressions for the singlet return probability that take into account experimentally observed finite probabilities of the creation of singlets with various patterns of valley occupations. In our analysis we focus on
magnetic fields for which the electron
spin Zeeman splitting is close to the valley splitting in one of the dots, when the
spin-valley coupling causes a strong renormalization of the frequency of oscillations of singlet return probability. The latter effect has been recently used to perform valley splitting mapping by shuttling of one
quantum dot to various locations with respect to the other. We give a detailed description of singlet-triplet dynamics near these
spin-valley resonances and compare the results of calculations with measurements on double
quantum dots in two distinct Si/SiGe
heterostructures. Comparison of theory with experiments in which the presence of a few valley occupation patterns is visible, gives insight into the valley dependence of $g$-factors in these structures, providing support for a recently proposed theoretical model of this dependence. We also discuss how dephasing of singlet return probability oscillations near the
spin-valley resonances is affected by valley splitting fluctuations caused by electric field noise.
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All authors Łukasz Cywiński, Mats Volmer, Tom Struck, Giordano Scappucci, Lars R. Schreiber
Quantum Physics
Optical depth dictates universal bounds on many-body decay in atomic ensembles
Cosimo C. Rusconi, Eric Sierra, Wai-Keong Mok et al. (+3 more)
Cooperative emission is well understood for idealized symmetric systems, but its fundamental limits in spatially extended, free-space ensembles remain an open question. Here, we derive a universal law for the scaling of the maximum
photon emission rate with system size that unifies both ordered arrays and disordered atomic clouds in arbitrary dimensions at fixed density. We demonstrate that, for a fixed atomic density, the maximum emission rate scales universally as the product of the atom number and the system's
geometric optical depth. The
geometric optical depth differs from the result of the Beer-Lambert law at high density and encodes the dimensional scaling across all regimes from independent emission to the Dicke limit. Furthermore, we establish a scaling law for directional detection, revealing that the observed rate depends on the detector's numerical aperture: small apertures yield Dicke-like quadratic scaling, whereas large apertures recover our integrated universal bound. Our results establish optical depth as the parameter governing many-body cooperative emission in both ordered and disordered ensembles, and reveal that directional and total-emission scalings must be carefully distinguished in experimental settings.
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All authors Cosimo C. Rusconi, Eric Sierra, Wai-Keong Mok, Avishi Poddar, Simon B. Jäger, Ana Asenjo-Garcia
Quantum Physics
DiffQEC: A versatile diffusion model for quantum error correction
Tianyi Xu, Qinglong Liu, Maolin Wang et al. (+4 more)
Quantum computers could solve problems beyond the reach of classical devices, but this potential depends on
quantum error correction (QEC) to protect fragile
quantum states from noise. A central challenge in QEC is decoding: inferring likely physical errors from syndrome patterns generated by repeated stabilizer measurements. Existing decoders, including graph-based and neural approaches, typically return a single correction hypothesis and therefore discard the richer posterior structure of the error distribution conditioned on the observed syndrome. Here we recast QEC decoding as posterior inference using discrete denoising
diffusion, exploiting the analogy between
stochastic error accumulation and the forward
diffusion process. We introduce DiffQEC, a generative decoder that combines a syndrome processor for multi-round spatial-temporal syndrome histories with syndrome feature modulation to condition denoising on the observed syndrome throughout inference. On experimental data from Google's superconducting
quantum processor, DiffQEC reduces logical error rates by up to 10.2% relative to minimum-weight perfect matching and by about 5% relative to tensor-network decoding. These improvements persist for larger code distances up to 17 under depolarizing noise and for logical
circuits of increasing depth. Beyond accuracy, the learned posterior provides confidence estimates for post-selection and reveals physically meaningful error structure, establishing posterior generative decoding as a practical framework for QEC.
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All authors Tianyi Xu, Qinglong Liu, Maolin Wang, Fei Zhang, Zhe Zhao, Yang Wang, Ye Wei
Quantum Physics
Optimization Using Locally-Quantum Decoders
Noah Shutty, Avijit Mandal, Seyoon Ragavan et al. (+8 more)
It was pointed out in [JSW+25] that widely-studied optimization problems such as D-regular max-k-XORSAT can be reduced to decoding of LDPC codes, using
quantum algorithms related to Regev's reduction. LDPC codes have very good decoders, such as Belief Propagation (BP), and this therefore makes D-regular max-k-XORSAT an enticing target for this class of
quantum algorithms. However, BP was found insufficient to achieve
quantum advantage. Here, we develop an intrinsically
quantum decoding technique, which decodes classical LDPC codes subject to coherent superpositions of bit flip errors. For average-case instances of D-regular max-k-XORSAT drawn from Gallager's ensemble, this
quantum decoder strongly outperforms classical belief propagation at many values of k and D. For some (k,D) the approximate optima achievable using this decoder surpass both Prange's
algorithm and simulated annealing. However, we stop short of achieving
quantum advantage because we identify an enhancement to Prange's
algorithm that recovers a precise tie, much as a precise tie was observed between the standard version of Prange's
algorithm and a more limited version of locally-
quantum decoding in [CT24].
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All authors Noah Shutty, Avijit Mandal, Seyoon Ragavan, Quentin Buzet, André Chailloux, Nicholas C. Rubin, Abid Khan, Sami Boulebnane, Ruslan Shaydulin, John Azariah, Stephen P. Jordan
Quantum Physics
Quantum vs. Classical Spin: A Comparative Study of Dipolar Spin Dynamics and the Onset of Chaos
Victor Henner, Alexander Nepomnyashchy and Tatyana Belozerova
We investigate the
spin dynamics of a dipole-coupled system by comparing a direct solution of the Schrodinger equation for
quantum spins with simulations of classical
spins. Although classical
spins have long been used in microscopic
spin dynamics simulations, we demonstrate that their results differ significantly from those of
quantum spins. Using Free Induction Decay as a
benchmark, we find that while the overall patterns are qualitatively similar, significant discrepancies emerge at both short and long timescales. We trace these differences to fundamental distinctions in the two descriptions.
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Quantum Physics
Quantum Kernel Advantage over Classical Collapse in Medical Foundation Model Embeddings
Sebastian Cajas Ordóñez, Felipe Ocampo Osorio, Dax Enshan Koh et al. (+10 more)
We provide evidence of
quantum kernel advantage under noiseless simulation in binary insurance classification on MIMIC-CXR chest radiographs using
quantum support vector machines (QSVM) with frozen
embeddings from three medical foundation models (MedSigLIP-448, RAD-DINO, ViT-patch32). We propose a two-tier fair comparison framework in which both classifiers receive identical PCA-q features. At Tier 1 (untuned QSVM vs. untuned linear SVM, C = 1 both sides), QSVM wins minority-class F1 in all 18 tested configurations (17 at p < 0.001, 1 at p < 0.01). The classical linear kernel collapses to majority-class prediction on 90-100% of seeds at every
qubit count, while QSVM maintains non-trivial recall. At q = 11 (MedSigLIP-448 plateau center), QSVM achieves mean F1 = 0.343 vs. classical F1 = 0.050 (F1 gain = +0.293, p < 0.001) without hyperparameter tuning. Under Tier 2 (untuned QSVM vs. C-tuned RBF SVM), QSVM wins all seven tested configurations (mean gain +0.068, max +0.112). Eigenspectrum analysis reveals
quantum kernel effective rank reaches 69.80 at q = 11, far exceeding linear kernel rank, while classical collapse remains C-invariant. A full
qubit sweep reveals architecture-dependent concentration onset across models. Code: https://github.com/sebasmos/qml-medimage
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All authors Sebastian Cajas Ordóñez, Felipe Ocampo Osorio, Dax Enshan Koh, Rafi Al Attrach, Aldo Marzullo, Ariel Guerra-Adames, J. Alejandro Andrade, Siong Thye Goh, Chi-Yu Chen, Rahul Gorijavolu, Xue Yang, Noah Dane Hebdon, Leo Anthony Celi
Quantum Physics
Isotopically enriched epitaxial CaWO$_{4}$ thin films for Er$^{3+}$ spin-photon quantum interfaces
Hanlin Tang, Kidae Shin, Ashwin K. Boddeti et al. (+10 more)
Rare earth ion (REI)-doped
oxide thin films are attractive for the application of
quantum interconnects due to their stable optical levels and scalability$^{1-3}$. Among them, Er$^{3+}$ doped CaWO$_{4}$ is promising because it possesses narrow optical linewidth transitions and a long
spin coherence time$^{4-6}$. The electron
spin coherence is limited at high temperatures by paramagnetic
impurities and by the presence of the 14.3% $^{183}$W
nuclear spin. To further increase the
spin coherence time at millikelvin temperatures, where the paramagnetic
impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er$^{3+}$ doped CaWO$_{4}$ thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO$_{4}$ thin films using an isotopically purified $^{186}$WO$_{3}$ source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes. $^{183}$W, the only W isotope that has a net
nuclear spin and is the major cause of
spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO$_{4}$ thin films as a promising platform for future studies of
nuclear-
spin-limited coherence and for scalable rare-earth-ion-based
quantum nanophotonic devices.
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All authors Hanlin Tang, Kidae Shin, Ashwin K. Boddeti, Sebastian P. Horvath, Adam Turflinger, Joseph Alexander, Jeffrey A. Dhas, Zihua Zhu, Shuhang Pan, Jeff D. Thompson, Yingge Du, Frederick J. Walker, Charles H. Ahn
Quantum Physics
Spectral Gap Informed Ramp QAOA
Kieran McDowall, Konstantinos Georgopoulos and Petros Wallden
A challenge with the
Quantum Approximate Optimisation
Algorithm (QAOA), and variational
algorithms in general, is finding good variational parameters, a task which in itself can be NP-hard. Recent work has sought to de-variationalise QAOA by picking well-informed guesses for the variational parameters. The Linear Ramp QAOA (LR-QAOA) achieves this by using parameter schedules inspired by the
quantum adiabatic
algorithm. In this work, we propose Spectral Gap Informed Ramp QAOA (SGIR-QAOA), a new QAOA variant that incorporates spectral gap information from an adiabatic
Hamiltonian, with the QAOA mixer
Hamiltonian as the initial
Hamiltonian, to construct smooth parameter schedules. SGIR-QAOA performs slow
evolution where the spectral gap of the adiabatic
Hamiltonian is small. We show that SGIR-QAOA has performance improvements over the LR-QAOA on Grover's problem at constant depth and that SGIR-QAOA requires shorter depths to achieve the same optimal solution probability. We then show that these performance benefits extend to a problem with potential practical applications - the Maximum Independent Set (MIS) problem. Finally, we demonstrate the scalability of the SGIR-QAOA method using extrapolated spectral gap information for scales that the spectral gap cannot be exactly evaluated, and show that the advantage appears to persist under mild depolarising noise.
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Quantum Physics
Gauge-covariant projected entangled paired states for interacting systems in a magnetic field
Wei Tang, Gunnar Möller, Frank Verstraete et al. (+1 more)
The
Hamiltonian for a system of itinerant particles on a two-dimensional
lattice in a uniform
magnetic field reduces the translational
symmetry to a
magnetic translation group, because of the need to choose a particular gauge for the vector potential. Nonetheless, in many situations all physical observables of the ground state remain entirely translation invariant. In this work, we introduce a projected entangled-pair state (PEPS) wavefunction with a pattern of virtual flux tensors, for which all physical expectation values are translation invariant by construction, possibly within an enlarged unit
cell reflecting any
symmetry breaking in the target state. Moreover, we show that the usual contraction and optimization methods for translation-invariant PEPS can be used, with the
magnetic flux per plaquette only entering as a continuous parameter in the tensor network contractions. Therefore, our approach provides a method for simulating an interacting many-body system in a uniform
magnetic field independently of the gauge choice for the vector potential and bypassing the need to consider extended
magnetic unit
cells.
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All authors Wei Tang, Gunnar Möller, Frank Verstraete, Laurens Vanderstraeten
Quantum Physics
Optimization of two-photon excitation by indistinguishable photons in a three-level atom
Masood Valipour, Gniewomir Sarbicki, Karolina Słowik et al. (+1 more)
We investigate the excitation of a three-level ladder-type atom by a unidirectional field with a pair of indistinguishable
photons. Starting from an analytical expression for the two-
photon absorption probability, we determine the two-
photon state that maximizes the population of the upper atomic state at a chosen time and show that in the limit of an infinitely long pulse, perfect excitation is possible. The optimal state is identified as the time-reversed counterpart of the two-
photon state emitted in spontaneous cascade decay. We then compare this ideal excitation strategy with experimentally accessible families of states, including symmetrized Gaussian product states, temporally correlated Gaussian states, and coherent pulses. We analyze how the optimal excitation conditions depend on the ratio of atomic decay rates and on the separation of the atomic transition frequencies. For indistinguishable
photons,
quantum interference may shift the maxima of the marginal spectral distribution away from the atomic resonances and qualitatively modify the optimal excitation strategy. Our results clarify the role of indistinguishability and correlations in two-
photon absorption and provide guidance for designing realistic excitation schemes in
quantum-optical light-matter interfaces.
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All authors Masood Valipour, Gniewomir Sarbicki, Karolina Słowik, Anita Dąbrowska
Quantum Physics · Cross-listed from cond-mat.quant-gas
Entropy Signatures of Collective Modes and Vortex Dynamics in Rotating Two–Dimensional Bose–Einstein Condensates
L. A. Machado, N. D. Chavda, B. Chatterjee et al. (+4 more)
We investigate the nonequilibrium dynamics of a two-dimensional rotating Bose gas confined in a symmetric anharmonic trap, employing the multiconfigurational time-dependent Hartree method for
bosons (MCTDHB). We study states ranging from
vortex-free configurations to multicharged (
giant)
vortices, prepared by tuning the rotation frequency, and analyze their response to sudden interaction and trap quenches. In
vortex-free states, interaction quenches induce regular breathing–like dynamics, whereas in the presence of
giant vortices they lead to
symmetry-breaking surface excitations. In contrast, trap deformations that excite quadrupole-like modes produce stable oscillations in
vortex-free condensates but trigger rapid, irregular, and effectively chaotic splitting dynamics in multicharged
vortices. To characterize these processes beyond conventional density and phase observables, we employ information-theoretic measures, including marginal and joint
entropies, mutual information, and Kullback-Leibler (KL) divergence, supplemented by an angular-resolved KL measure that captures
symmetry breaking and azimuthal localization. We find that chaotic splitting is accompanied by a pronounced growth of information-theoretic indicators, signaling the buildup of many-body correlations and increasing complexity in the system dynamics. Our results demonstrate the extreme sensitivity of
giant vortices to excitation protocols and establish information-theoretic measures as a powerful framework to quantify correlations and complexity in rotating
quantum gases.
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All authors L. A. Machado, N. D. Chavda, B. Chatterjee, M. A. Caracanhas, B. Chakrabarti, A. Gammal, R. P. Sagar
Quantum Physics
Balancing Quantum Memories in Asymmetric Repeaters for High-Fidelity Entanglement Distribution
Karim S. Elsayed and Amr Rizk
At the core of the
quantum Internet lie
quantum repeaters that enable remote end-to-end
entanglement generation. Fundamentally, the
entanglement generation rate and fidelity of
quantum repeaters constitute the bottleneck for end-to-end performance. To achieve high rates,
quantum repeaters employ
quantum memory multiplexing. In a high-rate standard repeater, each
memory sequentially generates an
entanglement with its neighboring nodes and then applies
entanglement swapping. This, however, results in low fidelity due to decoherence of the first-formed
entanglement in the sequential generation process. By allocating different numbers of
memories to simultaneously form entanglements with the left and right adjacent nodes,
quantum repeaters reduce high waiting times and achieve high fidelity. In such a repeater, a mismatch problem arises due to the difference between the probabilistic number of generated entanglements on both sides. Consequently, some entanglements remain stored until opposite entanglements are available. The mismatch problem reduces the repeater rate and particularly the
entanglement fidelity. In this paper, we consider the mismatch problem in an asymmetric repeater with different distances to its adjacent nodes. To mitigate the mismatch problem, we derive a dynamic optimal
memory allocation. Under the optimal allocation, we derive statistical lower bounds on the achievable rate and fidelity. We demonstrate that the optimal allocation significantly improves the fidelity while maintaining a comparable rate to the standard repeater. In contrast, our results show that fixed
memory allocation may be detrimental to the fidelity.
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Quantum Physics
GSC-QEMit: A Telemetry-Driven Hierarchical Forecast-and-Bandit Framework for Adaptive Quantum Error Mitigation
Steven Szachara, Sheeraja Rajakrishnan, Dylan Jay Van Allen et al. (+3 more)
Quantum error mitigation (QEM) is essential for extracting reliable results from near-term
quantum devices, yet practical deployments must balance mitigation strength against runtime overhead under time-varying noise. We introduce GSC-QEMit, a telemetry-driven, context–forecast–bandit framework for adaptive mitigation that switches between lightweight suppression and heavier intervention as drift evolves. GSC-QEMit composes three coupled modules: (G) a Growing Hierarchical Self-Organizing Map (GHSOM) that clusters streaming telemetry into operating contexts; (S) an uncertainty-aware subsampled Gaussian-process forecaster that predicts short-horizon fidelity degradation; and (C) a cost-aware contextual multi-armed bandit (CMAB) that selects mitigation actions via Thompson sampling with explicit intervention cost. We evaluate GSC-QEMit on
benchmark circuit families (GHZ,
Quantum Fourier Transform, and Grover search) under nonstationary noise regimes simulated in Qiskit Aer, using an instrumented testbed where action labels correspond to graded mitigation intensity. Across Clifford, non-Clifford, and structured workloads, GSC-QEMit improves average logical fidelity by +9.0% relative to unmitigated execution while reducing unnecessary heavy interventions by reserving them for inferred noise spikes. The resulting policies exhibit a favorable fidelity–cost trade-off and transfer across the evaluated workloads without
circuit-specific tuning.
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All authors Steven Szachara, Sheeraja Rajakrishnan, Dylan Jay Van Allen, Jason Pollack, Travis Desell, Daniel Krutz
Quantum Physics
Witnessing entanglement between photon and matter due to graviton exchange
Arijit Dutta, Marko Toroš, Sougato Bose et al. (+1 more)
The paper presents a scheme to detect
entanglement arising from the
quantum nature of
gravity between a
spin qubit and
photons, using Stokes parameters. One of the crucial tests of the general theory of relativity is the bending of light due to the
curvature. Recently, a
quantum counterpart of this experiment to test the
quantum nature of the gravitational interaction has been proposed, in which the
spin-2, massless graviton yields
entanglement between matter and a
photon sector. Hence, it provides one of the most crucial experimental signatures for testing the
quantum nature of
gravity in a lab, since only
spin-2-induced
entanglement can yield the correct deflection of light due to matter. Here, we propose a positive partial-transpose (PPT) witness criterion for witnessing such an
entanglement. We scan the entangled states in this context by studying the overlap of the final state, which is proportional to the
entanglement phase. We exploit the Stokes observables to measure the
photon state and the
spins in the matter sector, thereby constructing a witness for the
quantum nature of
gravity in this setup. To quantify this
entanglement, we will couple the
photon to a local oscillator, whose phase need to be controlled to probe the orthogonal components of the macroscopic interference in the
laser beam. We have shown that for a non-maximally entangled state mediated by the
quantum nature of
gravity, the witness attains a maximal negativity of $-0.052$. Our findings indicate that this witness effectively detects
entanglement within the range $0.71 \leq |\gamma| < 1$, where $\gamma$ is the overlap between the two coherent states of the
photon, providing a clear signature of
quantum correlations.
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All authors Arijit Dutta, Marko Toroš, Sougato Bose, Anupam Mazumdar
Quantum Physics
Improving Zero-Noise Extrapolation via Physically Bounded Models
Andriy Miranskyy, Adam Sorrenti, Jasmine Thind et al. (+1 more)
Zero-noise extrapolation (ZNE) mitigates errors in near-term
quantum devices by extrapolating measurements obtained at amplified noise levels to estimate noise-free expectation values. In practice, commonly used extrapolation models are fitted without enforcing physical constraints, which can yield predictions outside the valid range of
quantum observables. In this work, we introduce physically bounded variants of
polynomial, exponential, and
polynomial–exponential extrapolation models by explicitly parameterizing the zero-noise estimate and constraining it during optimization. We evaluate the approach using a large synthetic
benchmark comprising 180,000
circuits and approximately 3.6 million ZNE experiments generated under realistic device noise models derived from IBM
quantum backends. We also perform preliminary validation on real
quantum hardware using GHZ and W-state
circuits. Across the synthetic
benchmark, bounded extrapolation substantially reduces unphysical predictions and improves the stability of exponential- and
polynomial–exponential-family models, whereas
polynomial models show little difference between bounded and unbounded variants. Hardware experiments show similar qualitative behaviour: bounded models generally avoid pathological extrapolations and often provide a more reliable balance between accuracy and usable coverage. At the same time, the results highlight practical limitations of current devices, including stronger-than-expected noise effects and variability not fully captured by simulation models. These results suggest that enforcing physical constraints during extrapolation improves the reliability of ZNE and that this approach can be incorporated into existing workflows with minimal modification.
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All authors Andriy Miranskyy, Adam Sorrenti, Jasmine Thind, Claude Gravel
Quantum Physics
Adaptive Tensor Network Sampling for Quantum Optimal Control
Zeki Zeybek, Rick Mukherjee and Peter Schmelcher
Quantum optimal control (QOC) provides a systematic framework for achieving high-fidelity operations in
quantum systems and plays a central role in tasks such as gate synthesis, state transfer, and pulse design. Existing QOC methods broadly fall into two categories: gradient-based and gradient-free
algorithms. The associated optimization landscape is often high-dimensional, non-convex, and populated by numerous local minima, making efficient gradient-free search strategies essential. To address this, we introduce a gradient-free
matrix product state/tensor train (MPS/TT) sampling heuristic for discrete
quantum optimal control. In our approach, the MPS defines a score function over the space of discrete control parameters, which in turn induces a sampling distribution over candidate control sequences. This distribution is iteratively refined through selection of better performing sequences and local tensor updates to bias the search toward high-performing sequences. We evaluate the method on a range of
benchmark problems, including single-
qubit state transfer, Bell-pair preparation, qutrit gate implementation, and open-system population transfer. Across these tasks, the method exhibits stable convergence behavior and competitive empirical performance relative to established gradient-free baselines. These results suggest that tensor network sampling offers a viable heuristic framework for discrete
quantum control.
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Quantum Physics
Noise-robust 1-copy distillation protocol for all distillable Bell-diagonal qutrits
Tobias C. Sutter, Christopher Popp and Beatrix C. Hiesmayr
Entanglement distillation is the process of converting noisy entangled states into maximally entangled pure states via local operations and classical communication. A long-standing, unresolved question is which entangled states are amenable to
distillation, known as the distillability problem. We solve this for Bell-diagonal qutrits with Weyl structure, and present a noise-robust scheme for
entanglement distillation. In particular, we find that violating the positive partial transposition (PPT) criterion is necessary and sufficient for the 1-distillability of these states. For this, we construct a Schmidt rank 2 eigenvector of the partially transposed density
matrix associated with its unique, three-fold degenerate negative
eigenvalue. This feature makes the derived
entanglement distillation protocol resilient to white-noise effects on the
quantum states. Our results thus make noisy entangled qutrit pairs more accessible for future
quantum technologies.
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