Quantum Physics · Cross-listed from cs.LG
QAdapt: A Noise-Adaptive Neural Pre-Decoding Framework for Quantum Error Correction
Ran Miao, Rui Luo, Xiaohan Shan et al. (+1 more)
Fault-tolerant
quantum computing (FTQC) relies on
quantum error correction to suppress physical errors and preserve logical information at scale. In practice, however, performance is constrained not only by physical noise but also by the latency of classical decoders processing rapidly generated syndrome data. This challenge is exacerbated by hardware noise that is strong, heterogeneous, and nonstationary, as well as by the simulation-to-hardware distribution shift that can substantially degrade fixed neural decoders. We present QAdapt, a noise-adaptive neural pre-decoding framework for surface-code
quantum error correction. QAdapt captures local spatiotemporal correlations in syndrome data, sequentially adapts to evolving noise conditions while mitigating catastrophic forgetting, and forwards the residual syndrome to a conventional global decoder. Across 110 synthetic out-of-distribution noise configurations for rotated surface-code
memory circuits, QAdapt consistently reduces the logical error rate relative to the neural pre-decoding baseline. On Google's Willow
benchmark data, without target-domain fine-tuning, it achieves reductions of up to 5.79 percent in logical error rate and 9.32 percent in backend decoding latency on the residual syndrome. These results demonstrate that QAdapt provides a practical and decoder-compatible approach to improving the robustness and backend decoding efficiency of
quantum error correction under evolving hardware noise.
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All authors Ran Miao, Rui Luo, Xiaohan Shan, Xiaoming Sun
Quantum Physics
Nearly optimal quantum circuits for Boolean oracles
Junhong Nie and Wei Zi
Quantum oracle of Boolean functions is one of the central bridges between classical and
quantum algorithms, but the study focusing at
quantum circuit optimization of such oracle is yet closed. In this paper, we propose nearly optimal tradeoffs among
circuit size,
circuit depth and ancilla count, for
quantum oracles of three kinds of Boolean functions: general total Boolean functions with output size $b$: with $1\le m\le\Theta\left(\frac{2^n}{n}\right)$ ancilla, size $\mathcal{O}\left(\frac{b2^n}{\log(n+m)}\right)$, depth $\mathcal{O}\left(\frac{b2^n}{n+m}\right)$; partial Boolean functions of effective support size $d$ and output size $b$: with $\Theta\left(\log d\right)\le m\le \Theta\left(d\right)$ ancilla, size $\mathcal{O}\left(n\log d+bd\right)$, depth $\mathcal{O}\left(\frac{n\log n\log d}{n+m}+\log n+\frac{d(\log d+b\log m)}{m}\right)$; sparse Boolean functions of true input size $d$: with $\Theta\left(\log n+\log d\right)\le m\le\Theta\left(\frac{nd}{\log d}\right)$ ancilla, size $\mathcal{O}\left(n^2\log d+\frac{nd}{\log(\log d+m/n)}\right)$, depth $\mathcal{O}\left(\frac{n^2\log n\log d}{n+m}+\log n+\frac{nd}{m}\right)$. All the size and depth bounds are asymptotically optimal up to logarithmic factors in the corresponding ancilla count regions. We hope these results find applications in scenarios where classical procedures are needed to be embedded into
quantum circuits, such as QROM implementation and
quantum algorithm design.
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Quantum Physics
Counterdiabatic Driving under Variational Frame Dressing
Boxi Li, Franco Nori and Felix Motzoi
Counterdiabatic (CD) driving accelerates adiabatic protocols by prescribing auxiliary control fields, but often fails to map them to physically available operations. We derive a general formalism for such a mapping. We formulate CD driving in a variational dressed frame, where an unconstrained auxiliary generator reshapes the effective adiabatic problem while simultaneously forcing the applied correction to stay restricted to the native laboratory controls. This yields a laboratory-frame commutator equation that can be solved without constructing instantaneous eigenstates or the adiabatic and dressed-frame
unitaries. The additional dressed-frame freedom reveals solutions that are inaccessible in the conventional adiabatic frame. We illustrate this mechanism in three settings: suppression of spectator errors in a multi-
qubit chain driven by a single quadrature, an analytical acceleration of adiabatic Bell-state preparation with a fixed entangling interaction, and implementation of a fast adiabatic holonomic gate in a degenerate tripod
manifold with correction pulses confined to the native couplings. Our results provide a systematic nonperturbative framework for constructing implementable counterdiabatic protocols beyond the conventional adiabatic frame.
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Quantum Physics
Emergence of a Macroscopic Cat State and Multi-Channel Entanglement in a Frustrated Cluster Spin Chain
Mohit Lal Bera, Andreu Anglés-Castillo, Alberto Acevedo Meléndez et al. (+8 more)
We study a one-dimensional frustrated
spin chain, which combines cluster-Ising and anisotropic next-nearest neighbor Ising models. We first offer a historical perspective that justifies the studied model. Then we study in detail the two
quantum phases and prove that they are separated by a first order
quantum phase transition. On one side, the ground state corresponds to a
ferromagnetic phase, shows the presence of macroscopic cat states, and a small gap that closes in the thermodynamic limit. On the other phase, competing interactions avoid the establishment of a topological phase, though it conserves large incommensurate
quantum correlations. We prove it is fundamentally distinct from a simple paramagnet, and we name it an incommensurate phase. This is a gapped phase, which gap does not close in the thermodynamic limit. While in the
ferromagnetic phase there are two dominant Schmidt coefficients, in the incommensurate phase there are four. This corresponds to four distinct bipartite
entanglement channels contributing substantially to the ground state. Finally, we discuss the utility of the macroscopic cat states for
quantum metrology applications and the experimental feasibility of the system.
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All authors Mohit Lal Bera, Andreu Anglés-Castillo, Alberto Acevedo Meléndez, Rafael Gómez-Lurbe, Luca Ion, Rodrigo M. Sanz, Somayeh Mehrabankar, Tanmoy Pandit, Carmen G. Almudéver, Armando Pérez, Miguel Angel Garcia-March
Quantum Physics · Cross-listed from physics.comp-ph
Performance Benchmarking: Software for the Density Matrix Renormalization Group
Per Sehlstedt, Paolo Bientinesi and Lars Karlsson
The performance of scientific software often determines the scale of problems that can be solved in practice. As multiple implementations of the same
algorithm emerge, systematic evaluation is needed to compare their strengths and limitations. The density
matrix renormalization group (DMRG)
algorithm, widely used to study
quantum systems, has over 50 software implementations. These implementations vary in multiple aspects that can strongly affect performance. However, despite the need, performance evaluations of these implementations are scarce and lack a consistent standard; many existing evaluations are either too incomplete to enable meaningful comparisons or focus on objectives other than direct performance comparisons, thereby limiting understanding of how the implementations compare. Here, we present a performance-oriented benchmarking framework to facilitate meaningful comparisons of DMRG implementations, and we apply it to quantify the performance of eight implementations, highlighting similarities and differences among them. Furthermore, we examine multiple parameter settings, optimization strategies, and implementation-specific features to demonstrate how parameter configuration can affect performance and how systematic evaluation can reveal non-obvious trade-offs. The results show significant performance differences, up to two orders of magnitude in some cases, not only between different implementations when aligning parameters, but also within the same implementation when comparing different parameter configurations. Hence, our results demonstrate the significant value and insight that can be gained from conducting rigorous performance evaluations. Using our results and framework as a starting point, more rigorous benchmarking will ultimately help users and developers make informed decisions and support future development efforts to build better, more efficient software.
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Quantum Physics
Noise-aware emulation and cross-device validation of neutral atom analog quantum processing units
Constantin Dalyac, Sergi Julià-Farré, Lucas Leclerc et al. (+26 more)
Analog
quantum processors based on Rydberg atom arrays are a powerful platform for many-body
quantum simulation, combinatorial optimization, and graph machine learning. As these devices become increasingly accessible, establishing confidence in their outputs requires predictive models that quantitatively connect microscopic hardware imperfections to empirical results. Here, we present a noise-aware emulation framework that propagates the dominant noise mechanisms throughout the full computation cycle to predict device behavior. We validate the framework by benchmarking two representative protocols,
quantum annealing and post-quench dynamics, on three Pasqal
quantum processors where classical simulations still provide ground truth. Across all three devices, the measured observables fall within the uncertainty envelopes predicted by the emulator. Beyond reproducing the data, the framework isolates which physical mechanism dominates in each operating regime, provides quantitative guidance for
algorithm design and hardware improvements, and establishes a foundation for verifying analog processors in regimes beyond classical reach.
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All authors Constantin Dalyac, Sergi Julià-Farré, Lucas Leclerc, Vittorio Vitale, Boris Albrecht, Lucas Béguin, Kemal Bidzhiev, Petru Borta, Clémence Briosne-Frejaville, Daniel J. Campbell, Dorian Claveau, Makrem Chatti, Antoine Cornillot, Julius de Hond, Anita Devi, Thomas Eritzpokhoff, Gaétan Hercé, Fergus Hayes, Soufiane Kaghad, Arun Kumar Abhimanyu, Lucas Lassablière, Mauro Mendizabal, Anton Quelle, Julien Ripoll, Henrique Silvério, Joseph Vovrosh, Guillaume Villaret, Adrien Signoles, Alexandre Dauphin
Quantum Physics
No-Go Theorems for Quantum Transport Metrics from Fixed Cost Operators
Minbo Gao, Zhengfeng Ji and Tianshi Yu
Coupling-based
quantum optimal transport generalizes classical optimal transport by representing transport plans as bipartite states with prescribed marginals and evaluating their cost as the expectation of a fixed Hermitian operator. Friedland et al. [Phys. Rev. Lett. 129, 110402 (2022)] conjectured that the square root of the optimal cost associated with the SWAP projector is a metric in every dimension and that this property persists for nearby
quantum cost
matrices. Miller [arXiv:2607.07764] disproved both
conjectures by constructing explicit diagonal qutrit counterexamples. Building on his analysis, we prove a uniform no-go
theorem for standard couplings. In every dimension $d\geq3$, no fixed cost operator makes either the optimal cost or its square root a metric, with violations occurring already among commuting states. The obstruction persists under stabilization of the SWAP cost. For channel-induced couplings, global nonnegativity and vanishing self-cost force the cost operator to be zero, precluding point separation when $d\geq2$. Taken together, these no-go results show that fixed-cost coupling formulations do not lead to metrics on the full
quantum state space.
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Quantum Physics · Cross-listed from hep-ph
Tengyu Ai, Qi Bi, Yuxin He et al. (+3 more)
Missing
neutrinos need not destroy
collider spin tomography. We formulate the visible measurement under kinematic ambiguities arising from invisible particles as a coarse-grained positive-operator-valued measure on the production
spin density
matrix. We show that information loss is governed by the null space of the resulting visible-data map, not by the number of kinematic solutions. In $e^+e^-\to\tau^+\tau^-\to\pi^+\pi^-+\nu\bar\nu$, the twofold ambiguity leaves only the antisymmetric
spin-correlation combination $C_{nr}-C_{rn}$ unidentifiable, while the differential production rate and the remaining fourteen
spin coefficients are identifiable. For practical reconstruction under kinematic ambiguities, we develop a self-consistent fixed-point unfolding method using only visible data, without assuming a theoretical production template. Closure tests in Standard Model and anomalous tau-dipole
benchmarks show that the method reproduces the truth-level differential production rate and all identifiable
spin coefficients, whereas the usual flat average over kinematic folds gives significantly biased reconstructions. When a nontrivial null space is present, the reconstructed identifiable subspace together with positivity yields controlled ranges for concurrence and the CHSH parameter.
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All authors Tengyu Ai, Qi Bi, Yuxin He, Zekun Li, Jia Liu, Xiao-Ping Wang
Quantum Physics · Cross-listed from math.CO
The cycle C9 does not admit uniform mixing
Alison Gray, Pransu Patel and Isaiah Young
We study continuous-time
quantum walks (CTQWs) on cycles. In particular, we prove that the cycle $C_9$ does not admit uniform mixing at any time via algebraic geometry and Gröbner basis techniques to rule out all cyclic 9-roots.
Quantum Physics
Optimal T Counts under Sparsity: from QROM to State Preparation and Block Encoding
Tongyang Li, Fengning Ou, Xinzhao Wang et al. (+3 more)
Many
quantum algorithms require coherent access to classical data, often modeled by
quantum read-only
memory (QROM). We initiate the study of the $\mathrm{T}$ count of sparse QROM, in which only $s$ of the $2^n$ addresses store nonzero data. We prove that the optimal $\mathrm{T}$ count is $\Theta\left(n+\min\left\{s,\sqrt{s\left(m+\log(2^{n+1}/s)\right)}\right\}\right)$. Our upper bounds use a multilevel hashing scheme, while our lower bounds reduce sparse QROM to state preparation and use counting arguments for adaptive Clifford+$\mathrm{T}$
circuits. The lower bounds thus hold even when mid-
circuit measurements and classically controlled operations are allowed. As applications, we obtain matching $\mathrm{T}$-count bounds $\Theta\left(\min\left\{s,\sqrt{s\log(2^{n+1}/s)}\right\} +\sqrt{s\log(1/\varepsilon)}+\log(1/\varepsilon)\right)$ for $s$-sparse state preparation and $\Theta\left(\sqrt{2^n s\left(n+\log(1/\varepsilon_{\rm BE})\right)} +\log(1/\varepsilon_{\rm BE})\right)$ for block encoding of $s$-sparse
matrices, where $\varepsilon$ and $\varepsilon_{\rm BE}$ are the precision of state preparation and block encoding, respectively.
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All authors Tongyang Li, Fengning Ou, Xinzhao Wang, Penghui Yao, Pei Yuan, Shengyu Zhang
Quantum Physics · Cross-listed from cond-mat.stat-mech
Interacting Quantum Symmetric Exclusion Process
Denis Bernard, Friedrich Hübner and Stefano Scopa
We introduce and solve the Interacting
Quantum Symmetric Exclusion Process (IQSEP), a family of models describing the
stochastic quantum hopping of charged particles along the edges of a
lattice, with hopping amplitudes that depend on the occupations of neighbouring sites. In the absence of interactions, they reduce to the standard
quantum simple symmetric exclusion process, exhibiting coherent diffusive transport. For interactions of order one, they capture incoherent diffusive transport and its fluctuations, characterized by density-dependent diffusivity and mobility, making contact with the macroscopic fluctuation theory. By rescaling the interaction strength appropriately with the
lattice mesh, we define a mesoscopic scaling regime that retains a finite coherence length in the continuous thermodynamic limit. This regime interpolates between coherent behavior at small length scales and incoherent behavior at large scales. The resulting scaling theory accounts for fluctuations of
quantum coherences in interacting diffusive systems, going beyond the scope of standard fluctuating hydrodynamics.
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Quantum Physics
Displaced photon helicity states: a quantum realization of Arago-Fresnel interference
Yan Zhang, Xiang Liu and Hou Ian
The laws of Arago and Fresnel relate the polarization of light to its capacity for interference. We investigate their
quantum counterpart at the level of the fundamental helicity states of a single
photon. To this end, we introduce displaced
photon helicity states (DPHS), constructed by coherently displacing single-
photon helicity eigenstates. The displacement bridges the microscopic
spin degree of freedom and macroscopic polarization observables, enabling interference effects that would otherwise be inaccessible to isolated
photons. We derive analytical expressions for the Stokes parameters associated with the interference between DPHS and coherent optical fields and show that opposite helicities generate distinct parameter dependences in the resulting fringe structure. The interference constitutes a
quantum realization of Arago-Fresnel interference and permits the helicity of an unknown
photon to be inferred from the Stokes response of the combined field. Beyond this application, DPHS generalize the framework of displaced nonclassical states from
photon-number excitations to the helicity degree of freedom of light.
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Quantum Physics
Purifications for Convex Cones
Felix Campidell and Tim Netzer
Motivated by the importance of the purification principle in
quantum theory and generalized probabilistic theories, we study purifications using only the geometry of a finite-dimensional proper convex cone. We prove an existence
theorem for indecomposable cones and intermediate tensor cones containing the maximally entangled state; in particular, every interior point of an indecomposable homogeneous cone admits a purification. This applies to Lorentz cones, for example. We also give a criterion for uniqueness up to local
automorphisms. On the boundary, we show that if every proper face of $C$ is simplicial, then only pure points can admit purifications, and we demonstrate that this conclusion fails in the presence of non-simplicial faces. Examples involving positive semidefinite cones, Lorentz cones, $k$-positive maps, PPT tensors, and polyhedral cones illustrate both existence and non-uniqueness phenomena.
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Quantum Physics
Entanglement Swapping with Integrated Narrowband Photon Sources for Quantum Repeaters
Mingsong Wu, Leili Esmaeilifar, Rui Ning Wang et al. (+3 more)
Promising implementations of first generation
quantum repeaters are predicted to require atomic-based
quantum memory systems interfaced with photonic sources. Integrated photonics provides a promising solution for fibre-based, field-deployed operation of
quantum repeaters, however many leading
quantum memory platforms require narrow-bandwidth
photons that are challenging to generate with integrated photonics. Narrowband
photons also present significant technical challenges when implementing
entanglement-swapping, particularly with regards to systems-level stabilisation. This work addresses some of these fundamental and technical challenges, by demonstrating
entanglement-swapping using state-of-the-art integrated
photon sources with bandwidths compatible with multiple atomic-based
quantum memory platforms. We obtained a background-subtracted (net) HOM visibility of 0.99$\,\pm\,$0.01, showing high
photon indistinguishability and purity, with a net swapped state visibility of $\mathcal{V}$=0.88$\,\pm\,$0.06 demonstrating that the final
entanglement would be sufficient to violate a Bell inequality. The experiment used independent pump
lasers for each
photon pair source, with highly different frequencies to mimic
entanglement swapping between different repeater nodes or platforms. Phase and frequency stabilisation spanning 1.6 THz was achieved using all-fibre, commercially-available components. These results address important challenges in implementing field-deployed
quantum repeaters, from the integrated photonic solutions for narrowband
photon pairs, to systems-level stabilisation between independent
quantum repeater nodes.
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All authors Mingsong Wu, Leili Esmaeilifar, Rui Ning Wang, Tobias J. Kippenberg, Rob Thew, Tiff Brydges
Quantum Physics
Efficient atom rearrangements for quantum error correction primitives with a single AOD
Tom Hartweg, Asier Piñeiro Orioli, Hugo Perrin et al. (+1 more)
Neutral-atom
quantum computers offer arbitrary connectivity enabled by atom transport. Some logical operations can then be simplified or reduced entirely to
geometric rearrangements of the atoms. Minimizing the duration of these movements is therefore essential for high logical throughput. We introduce new primitives to shear, rotate and reflect 2D arrays of atoms in a static
lattice using sweeps of a single dynamic crossed acousto-optic deflector (AOD) pair. Using (nega-)binary and
geometric decompositions, we achieve an AOD stroke count scaling logarithmically in the linear size of the array. In one example, we use the Paeth decomposition to implement a $90^{\circ}$ rotation for a transversal Hadamard gate in a rotated surface code of distance $d$ in $3\lfloor\log_2(d-1)\rfloor + 4$ AOD strokes and $O(d^{1/3})$ constant-jerk time, against $O(d^2)$ strokes and $O(d^{7/3})$ time for atom-by-atom rearrangement.
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All authors Tom Hartweg, Asier Piñeiro Orioli, Hugo Perrin, Samuel Crew
Quantum Physics
Approximate sampling from decoded quantum interferometry via Markov chain Monte Carlo methods
Elies Gil-Fuster, Matan Ninio, Lennart Bittel et al. (+4 more)
Optimization problems are among the leading candidates for industrially relevant
quantum advantage. Decoded
quantum interferometry (DQI) has been proposed to tackle approximate optimization, establishing a connection to classical decoding problems. While previous work has primarily focused on the theoretical complexity of DQI, comparatively little is known about its empirical performance relative to classical
algorithms. In this work, we shed further light on the complexity of DQI and investigate numerically whether classical sampling methods can emulate the optimization capabilities of DQI. We first present a simplified analytical characterization of DQI that connects its expected performance to binomial statistics, and we identify concrete obstacles in further studying the complexity of DQI. Exploiting the fact that DQI output probabilities are efficiently computable, we apply Markov chain Monte Carlo (MCMC) techniques, particularly block-Gibbs sampling, to sample from the induced distribution. We study the runtime scaling of these methods for two optimization problems called max-XORSAT, where we reach beyond $1000$ effective
qubits; and OPI, where we reach beyond $150$ effective
qubits. Our results show that MCMC
algorithms can reliably attain the approximation ratios expected from DQI across a broad range of problem sizes. In OPI, in the regime where a super-
polynomial advantage is claimed for DQI, we observe an empirical runtime for MCMC that scales approximately as $1.1^{n}$, indicating exponential growth with a comparatively small base. Our findings do not refute existing
quantum advantage claims but provide new empirical evidence that classical sampling
algorithms can closely match DQI's optimization performance, offering a more nuanced perspective on the practical advantage of DQI.
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All authors Elies Gil-Fuster, Matan Ninio, Lennart Bittel, Yishai Shimoni, Jens Eisert, Stefan Woerner, Almudena Carrera Vázquez
Quantum Physics
Deterministic QKD source robust against side-channel attacks
Kiyoshi Tamaki, Marcos Curty, Akihiro Mizutani et al. (+1 more)
Quantum key distribution (QKD) is secure in principle, but practical security can be undermined by discrepancies between real devices and the idealized models assumed in security proofs. Source side channels, including those exploited by Trojan-horse attacks, are particularly detrimental: neglecting them compromises implementation security, whereas accounting for them reduces performance. Here we propose a QKD source that is intrinsically robust against side-channel attacks. Unlike existing passive and modulator-free schemes, it requires neither post-selection of the emitted pulses nor devices with a perfect extinction ratio to suppress side channels, and it does not introduce correlations between the intensity and the encoded bit or basis. Consequently, under idealized source assumptions commonly adopted in proposals for existing schemes, conventional decoy-state security analyses apply directly, yielding substantially higher key rates. Our proposal appears to be within reach of current technology and therefore provides a clear and practical path toward implementation-secure QKD.
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All authors Kiyoshi Tamaki, Marcos Curty, Akihiro Mizutani, Álvaro Navarrete
Quantum Physics · Cross-listed from math.FA
Poset-refined majorization relations
Alexander E. Guterman and Michael M. Wolf
Several classical majorization relations for sums or products of
matrices involve a majorizing vector of perfectly aligned
eigenvalues or singular values. By relaxing the order of
alignment to a partial order, we show that the majorization can be strengthened, provided the change-of-basis
matrices admit an LU-approximation with respect to this partial order. In this way, we obtain refined versions of Ky Fan's majorization relations, Horn's log-majorization relation, and von Neumann's trace inequality. As an application, we give a short proof of the separable Ky Fan majorization relation for an arbitrary number of tensor factors and extend it to a sum of tensor products of arbitrary
matrices. Further applications concern majorization relations for sums of (anti-)symmetric powers and for products of Kronecker sums.
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Quantum Physics
Slow-light-enhanced Atomic Frequency Comb Quantum Memory in Stoichiometric EuCl$_3 \cdot$ 6D$_2$O
Zongfeng Li, Wanting Xiao, Mucheng Guo et al. (+3 more)
Rare-earth-doped
crystals are promising candidates for
quantum storage, yet their performance in free-space configurations is fundamentally restricted by low optical depth. Here, we demonstrate high-efficiency
quantum storage in a stoichiometric EuCl$_3 \cdot$ 6D$_2$O
crystal, which intrinsically provides high optical density without the complexity of cavity implementation. We show that in this high-density regime, the system exhibits significant slow-light-like effects, including dispersion-induced echo delays and finesse-dependent echo intensity modulation. We develop a unified theoretical framework showing how absorption and dispersion work in concert to mediate echo generation. We achieve storage efficiencies of 42.9% for classical light and 34.4% for weak coherent pulses, alongside 90% efficiency for slow-light storage. These findings validate EuCl$_3 \cdot$ 6D$_2$O as a robust platform, establishing a viable pathway for scalable solid-state
quantum memory.
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All authors Zongfeng Li, Wanting Xiao, Mucheng Guo, Shuping Liu, Fudong Wang, Manjin Zhong
Quantum Physics
Scaling theory of decoherence in Dicke superradiance
Nico S. Bassler, Julian Lyne and Javier Cuerda
The survival of many-body coherence depends on the competition between correlation buildup and decoherence. In Dicke superradiance, collective emission builds up correlations, producing a peak intensity scaling as $N^2$ for $N$ emitters. We develop a scaling theory including local dephasing and spontaneous emission and obtain fully collective, partially collective, and independent-emitter scaling regimes. The boundary of the fully collective regime defines a continuous phase transition in a transient observable. Local decoherence can prevent $N^2$ scaling despite increasing $N$.
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Quantum Physics · Cross-listed from math.OA
Unification of Quantum Graph Properties
Gian Luca Spitzer
Many properties of classical graphs are defined in terms of subsets of the vertex set. Examples include connected components, which are subsets $X \subseteq V(G)$ such that $X \times X^c$ and $E(G)$ are disjoint, or independent sets, for which $X \times X$ and $E(G)$ are disjoint. Direct generalisations of these definitions to
quantum graphs are difficult to achieve, since the natural notion of subsets of a
quantum set is much too rigid. As a consequence, approaches to generalising these classical properties to the
quantum setting have been eclectic. In some cases, multiple inequivalent definitions of the same notion are in use. We introduce a natural and well-motivated alternative definition of subsets of a
quantum set. Building on this, we propose unified definitions of
quantum graph properties as straightforward generalisations of the classical definitions. We recover this way the established notions of colourings and connected components. For independent sets and cliques, our approach suggests variations that diverge from existing definitions, but address some of their counterintuitive properties. We nevertheless show how to recover an important existing definition of independent sets in our framework.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Observation of Moiré Time Crystal in Floquet-driven Rydberg Atomic Gases
Shuai Shi, Dong-Yang Zhu, Yu Yang et al. (+8 more)
A Moiré time
crystal is a non-
equilibrium quantum phase emerging from the coherent interference of two distinct frequencies, at least one being the intrinsic oscillation of a
symmetry-broken time
crystal. Its hallmark is an ultra-long beat period, reflecting a time-domain mapping of the Moiré fringes that arise from mismatched spatial
lattices. However, to date, no experimental realization of such a Moiré time
crystal has been reported. In this work, by applying a bichromatic driving field with two distinct frequencies, we demonstrate that the interplay between long-range Rydberg interactions and dissipation gives rise to a unique comb-like Moiré pattern characterized by a beat-note comb, which superimposes subharmonic periodicity and fundamental frequencies. This Moiré pattern formed by two mismatched drives is staggered in the spectrum as the frequency of one driver changes. We experimentally map the phase diagram of the system and identify a robust region where the Moiré temporal order persists against perturbations in
laser detuning. The reported Moiré time
crystal not only provides a controllable platform for exploring emergent slow-fast dynamics and synthetic space-time
symmetries but also opens avenues for engineering complex temporal order in driven
quantum many-body systems.
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All authors Shuai Shi, Dong-Yang Zhu, Yu Yang, Chu-Rong Pan, Jing-Wen Tang, Ya-Peng Zhang, Yan-Li Zhou, Wei-Tao Liu, Li-Hua Zhang, Bang Liu, Dong-Sheng Ding
Quantum Physics
Bridging continuous control and Floquet driving for charging many-body spin chains
Sebastián V. Romero, Xi Chen and Yue Ban
Recent advances in
quantum information and
quantum thermodynamics have reshaped the understanding of energy storage at the microscopic scale, paving the way toward protocols for storing and transferring energy in
quantum devices. These systems, known as
quantum batteries, offer a conceptual alternative to conventional macroscopic chemical batteries by exploiting
quantum coherence, correlations, and many-body dynamics. By navigating the landscape of established
spin-based
quantum batteries, we review existing charging and work-extraction protocols, as well as the impact of external factors on their performance. Motivated by Floquet-engineered proposals, we further establish a connection between continuously and periodically driven
spin chains as platforms for
quantum energy storage. Finally, we survey experimental realizations and proposals, highlighting their implementability and scalability in current and near-term
quantum technologies.
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Quantum Physics
Towards Quantum Networks: Characterizing Raman Noise over Metropolitan-scale Fiber Network
Marcello Caleffi, Laura d'Avossa, Italo Ignacio Machuca Flores et al. (+3 more)
The coexistence of
quantum and classical signals in optical fiber infrastructures represents a major challenge for large-scale
quantum networks, as noise sources such as Raman scattering can significantly impact
entanglement distribution, and the
quantum protocols based on it. In this work, we analyze Raman scattering in the C-band, used for
entanglement distribution, generated by a classical the O-band signal. The main contribution of this study lays in investigating these effects in a real metropolitan-fiber network, moving beyond controlled laboratory experiments to deployed telecommunication environments. Measurements are performed over a 7 km metropolitan fiber link using commercial sources and narrowband
lasers. The experimental results shows good agreement between the measurements taken under laboratory conditions, although, within the metropolitan-scale loop, localized spectral anomalies are observed in the deployed fibers. Therefore, our results show that, whenever a
quantum signal propagates in the C-band alongside an O-band classical channel within the same fiber, careful selection of the operating frequency is required, as Raman scattering and other real-world noise sources can significantly affect the quality and stability of the
quantum transmission. In particular, we identify spectral regions that are less affected by Raman noise, thereby providing practical guidelines for optimal
quantum channel allocation. We demonstrate that Raman-induced noise constitutes a dominant contribution to the
quantum signal-to-noise ratio (SNR) in realistic deployments, beyond background and detector noise. Overall, our findings offer practical insights for deploying
quantum communication systems over existing fiber networks, supporting the development of robust and scalable
quantum infrastructures.
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All authors Marcello Caleffi, Laura d'Avossa, Italo Ignacio Machuca Flores, Marco Grillo, Elena Montella, Angela Sara Cacciapuoti
Quantum Physics
Exponential Advantage of Multipartite Entanglement over Quantum Communication with Applications to Bounded-Storage Cryptography
Ananya Chakraborty, Manik Banik and Ronald de Wolf
We establish an exponential communication advantage enabled by multipartite
quantum entanglement. Building on the bipartite Hidden Matching problem, we introduce a communication task involving multiple spatially separated senders and a single receiver. We show that a shared Greenberger-Horne-Zeilinger state enables completion of this task using only logarithmically many bits of classical communication from each sender. In contrast, without preshared
entanglement, any protocol achieving high success probability requires
polynomial communication from at least one sender, even when
quantum communication is allowed. Thus, classical communication assisted by multipartite
entanglement can be exponentially more powerful than
quantum communication without preshared
entanglement. As a cryptographic application, we construct a seeded two-source randomness extractor and establish an exponential separation between entangled and unentangled
quantum side-information. Specifically, compromising the extractor with two unentangled
quantum states storing information about the two sources, respectively, requires
polynomial-size
memory, whereas exponentially smaller
quantum memory suffices in the presence of a small amount of shared
entanglement.
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Quantum Physics
A CPU+DCU Heterogeneous Parallel Framework for Post-Processing Reconstruction in Quantum Circuit Cutting
Qingqing Jiang, Weidong Liu, Yufu Liu et al. (+4 more)
In the NISQ era, limited
qubit resources make it difficult to execute large
quantum circuits directly on real hardware.
Quantum circuit cutting mitigates this limitation by decomposing a large
circuit into smaller subcircuits, but it shifts substantial overhead to classical post-processing. As
circuit size, complexity, and cut count increase, reconstruction becomes a major computational and storage bottleneck. This paper presents a CPU+DCU heterogeneous parallel framework for
circuit-cutting post-processing reconstruction. Instead of constructing a dense $2^n$-dimensional probability vector or returning only high-probability states, the framework reconstructs the nonzero-probability states in the original output distribution from subcircuit measurement results. It combines heterogeneous CPU+DCU execution with a high/low-word integer representation for global basis-state indices beyond 64 bits and a three-level cooperative storage mechanism spanning device
memory, host
memory, and out-of-core storage. Experiments on the Songshan supercomputer show that the framework maintains high reconstruction fidelity while achieving up to $259\times$ speedup over an optimized serial baseline on linear-cluster states and up to $4\times$ speedup over a homogeneous CPU-parallel method on random
circuits. The framework can also complete reconstruction tasks at the hundred-
qubit scale. These results demonstrate that HPC-oriented heterogeneous reconstruction can effectively alleviate the classical post-processing bottleneck and improve reconstruction scalability.
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All authors Qingqing Jiang, Weidong Liu, Yufu Liu, Ruiqing He, Jiandong Shang, Hengliang Guo, Qiang Chen
Quantum Physics
Complementary Matrix-Gated QKAN Fast-Weight Programmers for Quantum Dynamics Forecasting
Kuo-Chung Peng, Samuel Yen-Chi Chen, Jiun-Cheng Jiang et al. (+14 more)
Sequence models must decide what to write into
memory and what to retain. In
quantum and
quantum-inspired sequence learning, nonlinear recurrent updates often require repeated
circuit evaluations and sequential backpropagation through time, making long contexts costly. Gated fast-weight programmers (FWPs) based on
quantum-inspired Kolmogorov-Arnold networks (QKANs) alleviate this bottleneck by storing context in time-varying fast parameters. However, their scalar gate applies one retention-write balance to every fast-state coordinate, forcing all parameters to share a
memory timescale. We introduce Self-Modulating QKAN-based FWPs, which replace this broadcast gate with low-rank-generated element-wise modulation of the new-proposal branch, a bounded old-state branch, or both. We further propose Complementary
Matrix Gating (CMG), which uses one sigmoid
matrix gate to retain the old state and its complement to write the new proposal. CMG provides coordinate-wise
memory control while preserving the bounded convex update and affine prefix-scan structure of scalar gating, at the modulation-head cost of a single-branch rule. We compare four self-modulating rules with scalar gating across four FWP architectures combining classical and QKAN-based slow and fast programmers. Across seven single-step
forecasting benchmarks and five sequence lengths, CMG gives the most consistent improvements for architectures whose fast programmer incorporates a QKAN-based module. In direct multi-step
forecasting of Jaynes-Cummings and transmon-resonator dynamics simulated with CUDA-Q Dynamics, CMG models maintain mean-squared errors on the order of 0.001 or lower across
forecasting horizons of 4, 8, and 16 steps, while improving on their scalar-gated counterparts by at least 91.2%. These results establish coordinate-wise complementary modulation as a stable and effective update for QKAN-based FWPs.
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All authors Kuo-Chung Peng, Samuel Yen-Chi Chen, Jiun-Cheng Jiang, Chen-Yu Liu, En-Jui Kuo, Yun-Yuan Wang, Tzung-Chi Huang, Prayag Tiwari, Chi-Sheng Chen, Chun-Hua Lin, Yu-Chao Hsu, Tai-Yue Li, Saif Al-Kuwari, Simon See, Kuan-Cheng Chen, Nan-Yow Chen, Hsi-Sheng Goan
Quantum Physics
Design of a Quantum Error Correction Decoder Exploiting Temporal Parallelism
Leo Itoh, Takuya Kasamura, Hidetsugu Irie et al. (+1 more)
In the pursuit of fault-tolerant
quantum computing, low-latency
quantum error correction (QEC) is essential to prevent rapid error accumulation within the syndrome measurement cycle. In this work, we propose a microarchitecture that implements the sandwich decoding method using the Union-Find
algorithm by exploiting temporal parallelism, and we present an ASIC implementation. Through logic synthesis and simulation, we show that the proposed decoder achieves an average latency reduction of 35% at code distance d = 21 compared with a conventional batch Union-Find decoder, while maintaining a comparable logical error threshold of approximately 1.5% under phenomenological noise.
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All authors Leo Itoh, Takuya Kasamura, Hidetsugu Irie, Junichiro Kadomoto
Quantum Physics
Iterative quantum algorithms for the minimum vertex cover problem based on continuous-time quantum walks
Ruben Pariente Bassa, Finley A. Quinton, Franz G. Fuchs et al. (+1 more)
We introduce a constraint-preserving hybrid
quantum-classical greedy framework for the minimum vertex cover problem, which extends directly to maximum independent set by bitwise complementation. The framework uses projected Pauli-X terms whose sum preserves the feasible subspace and acts within it exactly as the adjacency
matrix of a layered graph of feasible covers. This graph is connected, so every feasible cover is linked to the configuration containing all vertices by a sequence of allowed single-vertex flips. Starting from this configuration, the corresponding continuous-time
quantum walk propagates amplitude into layers containing progressively smaller covers. We rank vertices using either their marginal cover probabilities or the expected cover size obtained after fixing each candidate vertex in the cover, and use these rankings to guide recursive greedy reductions. Across several random-graph families, with walk times fixed using independent calibration ensembles, the
quantum-informed
algorithms achieve lower mean approximation ratios and solve a larger fraction of instances optimally than their corresponding classical greedy baselines. The conditioned-energy strategy performs best on the tested instances and retains algorithmic performance close to the exact continuous-time limit under low-depth Trotterisation. For bounded-degree graphs, each Trotter layer has
circuit depth independent of system size, and the framework requires neither penalty terms nor variational training.
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All authors Ruben Pariente Bassa, Finley A. Quinton, Franz G. Fuchs, Pascal Halffmann
Quantum Physics · Cross-listed from physics.chem-ph
A Quantum Mechanical Approach to the Computation of Rovibrational Spectra of Diatomic Molecules in Strong Magnetic Fields
Nikhil Yenugu, Ashwani K Tiwari and Sangita Sen
In the absence of experimental data for molecular spectra in strong
magnetic fields, high resolution and reliable computational spectra are required for the interpretation of spectra collected from highly
magnetic astrophysical objects. In this paper, we extend the Wilson-
Hamiltonian framework, recently implemented and benchmarked by us (J. Chem. Theory Comput., 21, 9753 (2025) ), to a general three-dimensional framework suitable for computing the rovibrational spectra of diatomic molecules in strong uniform
magnetic fields. The field-dependent electronic and
nuclear Hamiltonians capture full non-perturbative coupling between particle motion and the field making the method applicable to all field strengths. The electric quadrupole transition moment integrals for rovibrational transitions in external static
magnetic fields are formulated, implemented, and computed to yield spectra which respect the selection rules of the molecule-field system. The spectral changes with increasing field strength such as shifting, splitting, merging, appearance and disappearance of peaks are noted. Contributions from electrons and
nuclei are studied individually, as well as in unison to reveal the underlying physics such as stiffening of the bond, emergence of a rotational barrier, field-induced coupling/decoupling of states, and
symmetry-breaking in rotational and vibrational states. These results provide the first fully
quantum mechanical computational results for the rovibrational signature of $^1\mathrm{H}_{2}$ in extreme
magnetic field environments with accuracy suitable for experimental interpretation. The methodology developed herein has direct relevance for high-field spectroscopy and astrochemical modeling, both for providing computational data as well as for understanding the spectral impact of strong
magnetic fields on electronic structure and
nuclear motion in molecules.
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