Quantum Physics · Cross-listed from hep-th
Rethinking quantum information in gravity and fields
Kanato Goto, Yuta Hamada, Kohtaro Kato et al. (+6 more)
This paper presents a curated selection of research questions at the intersection of
quantum gravity and
quantum information, chosen to highlight issues that we regard as particularly important for researchers in both fields. We organize the discussion into four main themes: the operational characterization of observables, the role of observers,
quantum error correction, and the infinite-dimensionality of
Hilbert spaces. We hope that addressing these questions will engage researchers across both communities and further strengthen the profound interplay between the two disciplines.
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All authors Kanato Goto, Yuta Hamada, Kohtaro Kato, Takato Mori, Yoshifumi Nakata, Masataka Watanabe, Hayata Yamasaki, Masahito Yamazaki, Takuya Yoda
Quantum Physics · Cross-listed from hep-ph
Regularized Compton double scattering via unitarity
Shanmuka Shivashankara and Isra Gashi
When two initially entangled
photons each undergo Compton scattering, the scattered electrons become correlated. However, the final reduced density
matrix of one scattered pair is not influenced by the other scattered pair due to unitarity. Herein, we keep unitarity up to tree level for Compton double scattering and obtain different results than recent literature. The initial four particles, where the initial
photons are entangled, are written as a superposition of two states with a relative phase. The final density
matrix has two area divergences that are regularized with unitarity. The regularization procedure, i.e. solving for the roots of a
polynomial that represents the probability for no scattering, suggests a novel definition of the scattering cross-section. Vieta's formulas relate these divergences to finite cross-sections. For an initial pure state, the formulas for the final density
matrix and the correlation of final electronic polarizations are given. The correlation implies double scattering is analogous to Young's diffraction experiment. The two initial superposed states are the circular apertures while the Feynman amplitudes are the interfering complex light fields.
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Quantum Physics · Cross-listed from cond-mat.supr-con
Magnetic Dipole in a Cuboidal Superconducting Trap
Francis J. Headley
We derive the exact image-dipole potential of a point dipole inside a closed cuboidal superconducting trap. The construction generalises the parallel-plate result to a geometry that confines every translational degree of freedom, and we prove that the image
lattice satisfies the Meissner boundary condition on all six walls. For a centred dipole the orientational energy reduces to a diagonal quadratic form whose three coefficients are Epstein-
zeta-type
lattice sums. We show that in both the infinite and finite rectangular traps the dipole orientation aligns with the short cross-sectional axis over a finite range of aspect ratios. The
equilibrium orientation in both cases is described by a phase diagram whose degeneracies we classify. Every prediction is verified against finite-element solutions of the same boundary-value problem, with agreement better than $0.16\%$.
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Quantum Physics
CryoZip: An Efficient Cryogenic Compressor for Quantum Error Correction Syndromes
Guanchen Tao, Alexander Knapen, Jacob Mack et al. (+4 more)
Scaling fault tolerant
quantum computing is increasingly constrained by the limited bandwidth and power budget across the 4 K to room temperature (RT) interface. We present CryoZip, a cross stack cryogenic compression framework that cooperates with a lightweight cryogenic
quantum error correction (QEC) predecoder to reduce 4 K to RT syndrome transmission under realistic,
circuit level noise. CryoZip targets sparse syndrome vectors with a sliding window compression architecture sized under strict decoding latency constraints to maximize energy efficiency. We implement and evaluate the design in 22 nm FDSOI characterized at 4 K, using vector based power, performance, and area analysis to obtain realistic hardware data. CryoZip achieves up to 48x compression, 1.8x higher than state of the art compressors, across various QEC codes while delivering 4 to 26x energy savings. When paired with a QEC predecoder, it yields over 14,238x bandwidth reduction, while energy savings rise to 42x when accounting for realistic QEC interface overheads.
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All authors Guanchen Tao, Alexander Knapen, Jacob Mack, Gokul Subramanian Ravi, Qirui Zhang, Mehdi Saligane, Dennis Sylvester
Quantum Physics · Cross-listed from hep-ph
A Levitated-Magnet Vector Force Sensor for Spin-Dependent Exotic Interactions
Dorian W. P. Amaral, Lei Cong, Tim M. Fuchs et al. (+2 more)
We present a magnetically levitated
ferromagnetic vector force sensor that enables selective searches for
spin-dependent exotic interactions mediated by beyond-Standard-Model
bosons. A defining feature of
spin-dependent exotic interactions is that they can generate forces with distinct directional signatures set by the relative
spin configuration of the interacting bodies. We show that our sensor resolves these signatures by mapping forces along different axes onto distinct translational modes with different resonance frequencies, thereby separating interaction channels within the same coupling class. As a representative example, we study parity-violating axial-vector–vector interactions mediated by a
spin-1 $Z'$
boson between a sensing and a driving levitated ferromagnet. Using a matched-filter likelihood analysis, we show that a setup based on an already demonstrated experiment can probe the pure electron–electron coupling $g_A^e g_V^e$ in the previously inaccessible force range $\lambda \lesssim 1\,\mathrm{cm}$, corresponding to mediator masses $M_{Z'} \gtrsim 10^{-5}\,\mathrm{eV}/c^2$. Our results establish levitated ferromagnets as a promising platform for millimeter-scale searches for
spin-dependent fifth forces and for resolving the multiple effective potentials associated with a given coupling class.
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All authors Dorian W. P. Amaral, Lei Cong, Tim M. Fuchs, Hendrik Ulbricht, Dmitry Budker
Quantum Physics
Diffusion-warm sampling of the XY model enables fast thermalization at scale
Sehmimul Hoque, Roger Melko and Pooya Ronagh
We introduce a novel technique for scalable sampling of
spin-system states with continuous
symmetries using
diffusion models. By applying our approach to the XY model, a fundamental continuous-
spin model in condensed matter physics, we show that our technique addresses the shortfalls of the Markov chain Monte Carlo (MCMC) in generalization to varying system sizes. More specifically, we show that training a temperature-conditioned
diffusion model on smaller-size XY model
lattices enables the generation of accurate samples in larger
lattice sizes. By tracking physically important observables of the model, such as
spin correlations, our experiments demonstrate that
diffusion sampling followed by a few MCMC steps reduces the thermalization time by an order of magnitude relative to the standard MCMC with random initialization. Our study provides valuable insight as to how generative models can be used to study continuous-state condensed matter systems at scale.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Density Wave Ordering with Disordered Ultracold Fermions in Optical Cavities
Óscar Rios Alves, Filippo Ferrari, Lorenzo Fioroni et al. (+2 more)
We investigate the interplay between cavity-induced density-wave ordering and controllable disorder in a trapped two-dimensional gas of ultracold
fermions. The atoms are dispersively coupled to an optical cavity and transversely driven by a pump beam, while an additional speckle beam spatially modulates the atom-light coupling through an AC-Stark shift of the atomic transition. In momentum space, this disorder converts the usual coupling between the cavity mode and a discrete set of density-wave
Fourier components into a coupling to a continuum of fermionic density modes, weighted by the spectrum of the speckle pattern. Using linear response theory, we derive the superradiant threshold and show that the disordered interaction renormalizes the effective light-matter coupling, lowering the critical pump strength on average, with the threshold becoming self-averaging for short speckle correlation lengths. We complement this analysis with a numerical mean-field treatment that gives access to the intracavity
photon number and to the real-space
fermion density across the transition. These results confirm that the disorder shifts the photonic phase boundary and, above threshold, distorts the density-wave
crystal by populating
Fourier components beyond those selected by the clean cavity geometry. Our findings identify both the emitted cavity light and in situ density images as probes of engineered disorder in fermionic matter coupled to optical cavities.
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All authors Óscar Rios Alves, Filippo Ferrari, Lorenzo Fioroni, Alberto Mercurio, Vincenzo Savona
Quantum Physics · Cross-listed from hep-ph
Toponium effects on quantum steering and Bell nonlocality of top quarks
J. A. Aguilar-Saavedra and S. Rodríguez-Benítez
We investigate
quantum steering and Bell nonlocality in top-
quark pair production at the LHC near threshold. The toponium contribution strengthens the
spin-singlet component and substantially enhances the
entanglement between the two
quark spins. With current LHC data,
quantum steering appears observable with a statistical significance around $10\sigma$. For Bell nonlocality, the statistical significance can also be high close to threshold, reaching about $9\sigma$, although the feasibility of such a measurement will depend crucially on the control of systematic uncertainties.
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Quantum Physics
Deep Reinforcement Learning for Individual Atomic Control and Cooling
Matthew L. Peters, Guoqing Wang, David C. Spierings et al. (+5 more)
Real-time feedback control of
quantum systems is often limited by partial observations, nonlinear dynamics and measurement noise, which make accurate model-based controllers difficult to design. Here we show that deep reinforcement learning can cool the motion of a single neutral atom coupled to a high-finesse optical cavity using only the continuously monitored cavity transmission. We first train the controller in simulation and then transfer it to the experiment, where online fine-tuning adapts it to unmodeled experimental dynamics. The learned policy damps the atom's motion in real time and achieves a cooling time constant of 388 +/- 14 microseconds, corresponding to only two motional periods in the trap. It also outperforms a standard linear differentiator controller in cooling speed while maintaining comparable atom retention over a broad range of operating conditions. These results establish reinforcement learning as a practical strategy for feedback control in
quantum-limited experiments where compact analytical models are incomplete.
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All authors Matthew L. Peters, Guoqing Wang, David C. Spierings, Niv Drucker, Meng-Wei Chen, Audrey Bartlett, Isaac Chuang, Vladan Vuletić
Quantum Physics · Cross-listed from cond-mat.stat-mech
Athermality of generalized Gibbs ensembles
Riccardo Senese, Bruno Bertini, Katja Klobas et al. (+1 more)
Integrable
quantum systems evolving from non-
equilibrium initial states do not thermalize to conventional Gibbs ensembles (GE). Instead, at long times they relax to generalized Gibbs ensembles (GGEs), which incorporate the full set of local and quasi-local conserved quantities. While GGEs have been extensively studied in the literature, a quantitative analytic characterization of how different they are from ordinary GEs is still lacking. In this work, we address this question by employing the concept of athermality, which we define within
quantum resource theory as the relative
entropy between a given state and the closest thermal state. By means of integrability techniques we compute the athermality for several
quantum quenches in paradigmatic integrable models, including the free XY
spin chain, the interacting Lieb-Liniger model, the XXZ
spin chain, and the harmonic chain. We find that often the athermality becomes anomalously small when the post-quench
Hamiltonian is critical in its ground state, despite probing physics at a finite energy density. We also prove that it systematically develops a singularity at criticality, which is inherited from the
entropy of the GGE.
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All authors Riccardo Senese, Bruno Bertini, Katja Klobas, Pasquale Calabrese
Quantum Physics
Improving the loss threshold for quantum advantage in photonic sensors by complete photon counting
Gerard J. Machado, Yazeed K. Alwehaibi, Guillaume Thekkadath et al. (+9 more)
Tolerance to imperfections is a defining performance criterion for
quantum sensors. The threshold for achieving a
quantum advantage depends on the input state, sensor configuration, detection scheme, and, critically for optical platforms,
photon loss. We consider a nonlinear interferometer in which two gain-optimized parametric nonlinear optical processes couple the state to the internal sensor and subsequently mix the reference and sensor beams. We demonstrate that measuring the full
photon-number output statistics of this setup yields marked improvements in the loss threshold. Using
photon-number-resolving detection (PNRD) based on transition-edge sensors (TESs), we experimentally reconstruct the joint
photon-number statistics at the interferometer output. Subject to internal and external losses of approximately 25 % and 45 %, respectively – and without any post-selection or loss correction – we observe an unconditional violation of the shot-noise limit by $2.37 \pm 0.11$ dB. This translates to a 44 % enhancement in estimation precision over conventional click-detection strategies. We verify this performance by evaluating the classical Fisher information against both an analytical model of the joint
photon-number distribution and the raw measured statistics. Ultimately, our results demonstrate that combining nonlinear interferometry with PNRD unlocks metrological information fundamentally inaccessible to click detectors, establishing a clear path toward practical,
quantum-enhanced sensing under realistic loss conditions.
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All authors Gerard J. Machado, Yazeed K. Alwehaibi, Guillaume Thekkadath, Zhenghao Li, Aonan Zhang, Shang Yu, Adriana E. Lita, Richard P. Mirin, Martin J. Stevens, Juan P. Torres, Raj B. Patel, Ian A. Walmsley
Quantum Physics
Schrödinger and Heisenberg non-Markovianity in quantum information tasks
Federico Settimo, Kimmo Luoma, Jyrki Piilo et al. (+3 more)
Quantum non-Markovianity has been widely studied and connected to the existence of
memory effects in the dynamics of open systems. Surprisingly, working in the Schrödinger or in the Heisenberg picture can provide inequivalent description non-Markovianity: a process can appear to be memoryless in one picture, while displaying
memory effects in the other. Here, we investigate which kind of
memory is relevant for different
quantum information tasks. Some of them, such as sending information via a noisy channel, require
memory in both pictures in order to exhibit revivals in the task performance. For others, only one type of
memory is sufficient. We also provide necessary conditions for non-Markovianity in both pictures by only considering the dynamics in one picture, showing for instance that the previously considered witness of Schrödinger non-Markovianity in terms of the volume of accessible states does indeed witness non-Markovianity in both pictures at the same time.
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All authors Federico Settimo, Kimmo Luoma, Jyrki Piilo, Bassano Vacchini, Dariusz Chruściński, Andrea Smirne
Quantum Physics
Revivals of Bell nonlocality require Schrödinger and Heisenberg non-Markovianity
Federico Settimo, Kimmo Luoma, Jyrki Piilo et al. (+3 more)
Bell nonlocality is a key resource in
quantum information, demonstrating the nonclassicality of
quantum theory. Noise, however, is in general detrimental to nonlocality, and can cause the loss of the ability to violate any Bell inequality.
Memory effects, on the other hand, can restore this quantumness and, as recently shown, they can be differently characterized in the Schrödinger and in the Heisenberg picture. Here, we show that if
memory effects allow for revivals in time of nonlocality, then the dynamics must be non-Markovian in both pictures. We showcase our findings through a device-independent
quantum key distribution task, for which Bell nonlocality is necessary.
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All authors Federico Settimo, Kimmo Luoma, Jyrki Piilo, Andrea Smirne, Bassano Vacchini, Dariusz Chruściński
Quantum Physics
Theory and practice of Trotter product formulas for quantum chemistry
Pablo A. M. Casares, William Maxwell, Danial Motlagh et al. (+7 more)
Trotter product formulas are a fundamental class of methods for
Hamiltonian simulation, particularly attractive due to their low
qubit requirements. However, they are often overlooked for use with fault-tolerant
quantum algorithms, because of their perceived higher gate counts and the difficulty of estimating Trotter error. Here, we introduce
Symmetry-Protected Randomized near-Integrable Trotter (SPRINT) formulas, a framework for building optimized product formulas for electronic structure
Hamiltonians widely used in
quantum chemistry. SPRINT integrates a generalization of classical near-integrability, randomization,
symmetry protection, use of QROM, and other techniques into a thoroughly optimized methodology for
Hamiltonian simulation. When applied to concrete simulation tasks, we find SPRINT leads to substantial reduction in gate count compared to previous approaches. Alongside SPRINT, we introduce and analyze a Generalized Rank Decomposition (GRADE) of electronic
Hamiltonians that generalizes previous factorization methods. We apply these techniques to the task of simulating the X-ray absorption spectrum of Li$_4$Mn$_2$O, a candidate battery cathode material, leveraging recent advances in tight Trotter error estimation to carefully identify the best version of SPRINT for this problem. Using a Trotter error estimation tool developed in the PennyLane software platform, we show that SPRINT reduces the Toffoli gate cost by a factor of $4.5$ relative to the previous state of the art for this problem, with a gate cost only $\times 2.5$ higher than qubitization, while requiring a dramatic $\times 5.5$ fewer logical
qubits. These results establish well-designed Trotter product formulas as an attractive
Hamiltonian simulation method for industrially relevant problems in chemistry and materials science.
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All authors Pablo A. M. Casares, William Maxwell, Danial Motlagh, Hitarth Choubisa, Zy Niu, Ignacio Loaiza, Jonathan E. Mueller, Arne-Christian Voigt, Juan Miguel Arrazola, Stepan Fomichev
Quantum Physics
Practical Estimation of Trotter Error for Hamiltonian Simulation
William Maxwell, Pablo A. M. Casares, Robert A. Lang et al. (+7 more)
Trotter product formulas are a leading approach for
Hamiltonian simulation on
quantum computers, yet their practical performance has remained difficult to assess due to the challenge of accurately estimating the Trotter error. In this work, we develop new theoretical results,
algorithms, and software tools that advance the state-of-the-art in Trotter error estimation by orders of magnitude in both scale and accuracy. On the theoretical side, we prove that in the asymptotic limit the error of a product formula depends on the diagonal elements of the Baker-Campbell-Hausdorff (BCH) error operator in the eigenbasis of the
Hamiltonian, rather than its full spectral norm – yielding an improved scaling for
Hamiltonian simulation using product formulas. On the algorithmic side, we introduce a compact representation of the BCH expansion that reduces the number of commutators from $\mathcal{O}(n^3)$ to $\mathcal{O}(n)$ for second-order, and from $\mathcal{O}(n^5)$ to $\mathcal{O}(n^2)$ for fourth-order formulas on $n$ fragments, complemented by an importance sampling scheme to further reduce the computational cost. We provide implementations of these techniques in software and demonstrate their power on two applications: (i) X-ray absorption spectroscopy of an electronic
Hamiltonian (Li$_4$Mn$_2$O) at up to 56
qubits using tensor networks; and (ii) vibronic dynamics of naphthalene at over 100
qubits using ML-MCTDH, where we find that naive analytical bounds overestimate the required number of Trotter steps by nearly five orders of magnitude. Our framework enables, for the first time, the accurate estimation of Trotter error at practically relevant system sizes, providing a foundation for fair algorithmic comparisons and rational design of product formulas.
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All authors William Maxwell, Pablo A. M. Casares, Robert A. Lang, Stepan Fomichev, Juan Miguel Arrazola, Soran Jahangiri, Ali Asadi, Luis Alfredo Nunez Meneses, Thomas Germain, Danial Motlagh
Quantum Physics · Cross-listed from hep-ph
$\Lambda \bar \Lambda$ spin correlations in high-energy collisions from quantum channels: an open quantum system view of hadronization
João Barata, Iván Cuntín, Enrique Rico et al. (+1 more)
We construct a
quantum information-centered approach to describe the experimentally observed behavior of hyperon
spin-pair correlations in high-energy
collider experiments. The
evolution of the
spin density
matrix of the hyperon pair is treated in the
language of
quantum channels, accounting both for the
spin dynamics in $\mathbb{C}^2\otimes\mathbb{C}^2$ and for the pair's angular separation $\Delta R$. We show that the experimental data are consistent with an
evolution under a two-
qubit depolarizing channel, from which a Lindblad master equation is derived. This provides an open
quantum system picture of
spin dynamics during the hadronization transition, which is not naturally captured by other
quantum channels, and we discuss its microscopic origins. These results show that
quantum information science can offer new insights into confinement dynamics beyond the classification of
entanglement in the final particle states.
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All authors João Barata, Iván Cuntín, Enrique Rico, Bin Wu
Quantum Physics
Disorder-Induced Enhancement of Fermionic Superradiance
David Pascual Solis, Andrea Legramandi, Soumik Bandyopadhyay et al. (+1 more)
Collective light-matter phenomena such as Dicke superradiance are often described as a collection of effective
spins coupled homogeneously to a bosonic mode, giving rise to a collective bright mode with enhanced light-matter coupling. In fermionic systems, Pauli exclusion and
Fermi-surface structure can significantly modify this picture, while randomness in the atom-light couplings raises the question of whether disorder promotes or suppresses collective behavior. Here, we study a cavity model in which fermionic particles couple to a photonic mode through a random all-to-all interaction
matrix with tunable mean and variance. Combining numerical mean-field methods, analytic stability analysis and random-
matrix predictions, and
benchmarks against exact diagonalization, we characterize both the onset and structure of the superradiant phase. While mean coupling and disorder variance contribute in the same way to the onset, they lead to drastically different behavior within the condensed phase. Uniform coupling supports a single bright collective fermionic mode with conventional Dicke-like scaling of the cavity field. Disorder, instead, gives rise to a qualitatively different collective regime in which many grey fermionic states participate coherently, producing a parametrically enhanced scaling of the condensate with system size. Our results reveal a mechanism through which disorder can, perhaps counterintuitively, promote collective light-matter phenomena.
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All authors David Pascual Solis, Andrea Legramandi, Soumik Bandyopadhyay, Philipp Hauke
Quantum Physics
Controlling the non-Markovianity of quantum Brownian motion
Guglielmo Pellitteri, Vittorio Giovannetti and Vasco Cavina
We analyze the exact dynamics of a generalized
quantum Brownian motion model, employing Gaussian master equation methods. We demonstrate that, by modulating the relative weights of specific interaction channels, we can control the degree of non-Markovianity of the system, and induce a transition from non-Markovian to Markovian regimes. The non-Markovianity of the
evolution is formally characterized by leveraging the Gorini–Kossakowski–Sudarshan–Lindblad
theorem and by employing quantitative measures of information backflow. Finally, we clarify the physical mechanism behind the phenomenology of this model, thereby providing a systematic platform for environment engineering through the strategic tuning of dissipation channels.
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Quantum Physics
Authentication in Quantum Networks
Christopher Battarbee, Suchetana Goswami, Elham Kashefi et al. (+1 more)
In this review, we survey the cryptographic task of authentication from the perspective of
quantum communication. We review three main flavours of authentication that are often conflated in the literature: authentication of classical messages, authentication of
quantum messages, and entity authentication, also covering recent hardware-assisted approaches. We compare representative protocols for each functionality in terms of their security assumptions, set-up requirements, composability, and scalability in large or dynamic networks, and use these criteria to identify and recommend suitable candidates. Finally, applications are surveyed: we provide a detailed case study of authentication and
quantum key distribution (QKD), then extend the discussion to protocols beyond QKD, where the role of authentication is more complex. Our take-home message is that an authentication requirement is not an intrinsic limitation of
quantum networks: as with all secure communication, each protocol relies on a particular authentication resource, and the security claim of that protocol is meaningful only once the authentication resource and its deployment assumptions are made explicit. At the same time, the existing classical and
quantum literature already offers a range of
quantum-secure authentication schemes, which can support different applications when carefully matched to the required functionality, assumptions, and security guarantees.
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All authors Christopher Battarbee, Suchetana Goswami, Elham Kashefi, Mina Doosti
Quantum Physics
Provable random-matrix spectral ramp in a static, geometrically local Hamiltonian
Matteo Ippoliti
Quantum chaos is commonly associated with the emergence of random-
matrix statistics in the spectra of
quantum systems. A useful diagnostic is provided by the spectral form factor (SFF), which for random
matrix ensembles displays a universal linear-growth regime (`ramp'). In the last decade, a landmark result by Bertini, Kos and Prosen (BKP) identified for the first time a class of geometrically local
quantum dynamics of finite-dimensional particles where the SFF provably exhibits a random-
matrix ramp: periodically driven (Floquet) qudit chains whose
evolution is described by `dual-
unitary'
circuits. Here, building on the BKP result and on a recently proposed variant of the Feynman-Kitaev clock construction, we obtain a spectral ramp in a class of static, geometrically local
Hamiltonians. Our strategy is to embed the Floquet quasienergy spectrum of a dual-
unitary circuit into the energy spectrum of a static local
Hamiltonian, and to prove that the latter's connected SFF inherits the BKP ramp within a
symmetry sector. This is to our knowledge the first proof of a spectral ramp in a time-independent, geometrically local many-body system with finite local
Hilbert space dimension.
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Quantum Physics
Cavity-mediated probabilistic magic $T$-gate injection
Sofia Cocciaretto, Roberto Menta and Vittorio Giovannetti
Non-Clifford gates are a necessary resource for universal
quantum computation, yet their fault-tolerant implementation typically relies on magic-state
distillation, which incurs significant overhead in
qubit count and
circuit depth. In this work, we propose a probabilistic cavity-based magic-state injection protocol. Our scheme exploits controlled atom-cavity interactions and conditional measurements to probabilistically prepare an effective magic state encoded in the first two level Fock subspace of a single cavity mode, achieving a success probability of $0.74$ per attempt, independent of the target magic phase. The cavity-encoded magic state is subsequently injected into a computational atom via a teleportation-based protocol mediated by dressed-state transitions, requiring only Clifford operations and a single auxiliary atom for readout. We show that all required operations – state preparation, two-
qubit exchange gates, and projective measurement – can be implemented with experimentally available techniques in Rydberg atom-cavity platforms. We further discuss how the scheme can in principle be adapted to operate at the logical level, where collective Rydberg interactions and optical nonlinearities provide a route toward cavity-mediated $T$-gate injection directly into code-encoded
qubits.
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Quantum Physics
Repetition-code-based readout error detection and correction across hardware platforms and generations
Csaba Czabán, Orsolya Kálmán, Sergey N. Filippov et al. (+1 more)
Readout errors are one of the dominant sources of noise in current
quantum processors, limiting both expectation-value estimation and sampling-based applications. Since they affect only the classical measurement outcomes, they can be addressed using classical coding techniques: immediately before measurement, each data
qubit is redundantly encoded with ancilla
qubits, and the resulting bit string is decoded either by post-selection or by majority voting. Unlike conventional readout error mitigation, which corrects only aggregate quantities such as expectation values, this approach operates on individual measurement shots and can therefore produce approximately corrected samples. We present a systematic cross-platform and cross-generation experimental evaluation of repetition-code readout error detection and correction. We
benchmark the same protocol on IBM Heron r1-r3 superconducting processors and Quantinuum H1 and H2 trapped-ion processors while independently varying the code distance, hardware generation, and encoding layout. We find that both error detection and correction improve readout fidelity on every device and generation tested, even as the unencoded baseline improves substantially across successive hardware releases. At the same time, the value of additional redundancy depends strongly on the underlying hardware. On superconducting processors, the extra gate errors introduced by the encoding rapidly offset its benefits, whereas on trapped-ion processors the much lower gate error rates allow larger code distances to remain advantageous.
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All authors Csaba Czabán, Orsolya Kálmán, Sergey N. Filippov, Zoltán Zimborás
Quantum Physics
Provably Efficient Learning of Fermionic Correlations under Particle-Number Symmetry
Yuki Koizumi, Kaito Wada, Toshinori P. Takama et al. (+1 more)
Predicting local fermionic correlations is a central task in
quantum many-body physics, as these correlations encode many physically relevant local observables. The ubiquitous particle-number
symmetry imposes strong structural constraints on
quantum states, suggesting that local correlations should be learned with fewer samples than by
symmetry-agnostic approaches. However, it has remained unclear whether such a provable advantage exists in collective learning of local correlations. Here, we develop a framework of number-conserving fermionic-shadow
tomography based on random orbital rotations. We prove that, for every given order $k$, we can simultaneously estimate all $k$-body fermionic correlations of an $N$-mode $\eta$-particle state with a given variance $\varepsilon^2$ using only $O_k(\eta^k/\varepsilon^2)$ samples, which are independent of the system size $N$. We further establish a matching information-theoretic lower bound $\Omega_k(\eta^k/\varepsilon^2)$ for any adaptive protocol based on single-copy measurements, showing that the $(\eta^k,\varepsilon)$-dependence is optimal up to constants depending only on $k$. Furthermore, numerical studies show a 20-fold query reduction for one-body correlation estimation at $N=200$, $\eta=20$, and $\varepsilon=10^{-2}$, compared with the best alternative including Heisenberg-limited estimation methods. Relative to fermionic Gaussian-
unitary shadows, sample costs for the square-
lattice Fermi-Hubbard model at $N=288$ are reduced by factors of $148$ for energy density and $161$ for
spin structure factor estimation. For molecular energy derivatives with respect to atomic coordinates, our method reduces sample requirements by factors of $53$ for $\mathrm{Cr}_2$ and $18.5$ for $[\mathrm{Fe}_2\mathrm{S}_2]^{2-}$ at $N=128$. These results demonstrate the practical advantage of particle-number
symmetry for fermionic observable estimation.
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All authors Yuki Koizumi, Kaito Wada, Toshinori P. Takama, Nobuyuki Yoshioka
Quantum Physics
Untangling QLDPC Codes with Biased Noise Ancilla
Runjiang Bi, Kathleen Chang and Shruti Puri
Remarkable technical progress has made high-rate, high-distance,
quantum low-density parity-check codes (QLDPC) promising candidates for scalable
quantum computing. However, it is hard to design low-depth syndrome extraction
circuits that do not spread errors from ancilla
qubits to multiple data
qubits, also known as hook errors, for general QLDPC codes. Additionally, widely used decoders for these codes based on belief propagation are impaired due to short loops in the Tanner graph. Here, we investigate a hardware-aware approach to avoid these hooks and loops using biased noise ancillas. Using examples of bicycle bivariate codes and a cyclic hypergraph product code, which have been widely considered for practical application, we show that the effective fault-distance of the conventional syndrome extraction
circuit can be significantly higher and the number of short loops can be significantly lower when the ancillas are subject to phase-flip errors only, compared to when they are also subject to bit-flip errors. This can result in almost an order of magnitude improvement in the logical error rate at
circuit noise of $2\times 10^{-3}$ and when bit-flip errors in the ancilla are 50 times less likely than phase-flip errors. Our work demonstrates a significant and practical
quantum error correction advantage with biased noise
qubits in which full-bias cannot be maintained.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Equilibrium and non-equilibrium phases of microwave-dressed polar molecules beyond rotational symmetries
Matteo Ciardi, Andreas Schindewolf, Tim Langen et al. (+1 more)
Recent experiments on molecular droplets have opened a new frontier of self-organization in strongly dipolar
quantum matter. Microwave-dressing of polar molecules permits to tune both the strength and the angular structure of long-range interactions, potentially promoting a rich spectrum of
quantum phases, from superfluid droplets with varying geometry and insulating or supersolid droplet arrays to strongly correlated
crystals of individual molecules. Using path-integral Monte Carlo simulations of large molecular ensembles, we demonstrate that experimentally observed droplet arrays emerge as a metastable non-
equilibrium state from the quenching of a gas-droplet phase transition under entirely broken rotational
symmetry of the microwave-induced interaction potential. We moreover find that a crystalline phase of molecules, predicted for antidipolar interactions, is absent under conditions of recent experiments. This is traced back to the lack of angular
symmetry in currently employed microwave-dressing, which qualitatively reshapes the many-body energy landscape and cannot be captured by effective scalar interaction parameters. Our results provide the first direct comparison of ab initio simulations and experiments and establish interaction anisotropy as a key aspect of molecular
quantum gases.
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All authors Matteo Ciardi, Andreas Schindewolf, Tim Langen, Thomas Pohl
Quantum Physics
Revisiting crossed-correlated baths in open quantum systems simulated by HEOM or T-TEDOPA
Brieuc Le Dé, Etienne Mangaud, Alex W. Chin et al. (+1 more)
Excited-state dynamics of open
quantum systems is analyzed by the hierarchical equations of motion (HEOM) or the thermalized time-evolving density operator with orthogonal
polynomials algorithm (T-TEDOPA) method when a discrete $ab$ $initio$ linear vibronic model is parametrized by continuous temperature-dependent spectral densities leading to crossed correlation functions, i.e. correlated fluctuations of the energy gap collective modes. We focus on a conical intersection involving two collective modes tuning the energy of each excited state and we revisit the transformation of the initial correlated tuning baths to de-correlated shared baths in order to reduce the computational resources. While a completely frequency-dependent transformation poses problems for HEOM, we find that in some particular cases, an optimal approximate frequency-independent transformation may be derived. On the contrary, T-TEDOPA is very efficient and allows to use this frequency-dependent transformation at the price of managing long-range couplings in the tensor chain. An illustrative application is shown by using the linear vibronic coupling model of a planar symmetrical (phenylethynyl)benzene dimer.
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All authors Brieuc Le Dé, Etienne Mangaud, Alex W. Chin, Michèle Desouter-Lecomte
Quantum Physics
Bridging the NISQ and Fault-Tolerant Regimes: Generative-ML-Assisted Quantum Selected CI for Molecular Simulations
Anurag K. S. V.
Calculation of binding energies for
protein-ligand molecular systems requires accurate treatment of the electronic structure, a
quantum chemistry problem that scales exponentially on classical hardware, while current
quantum hardware remains too noisy for the required
circuit depths. This report presents a hybrid
quantum-classical workflow performed on the Fujitsu FX700 ideal state-vector simulator using QARP that addresses two structural inefficiencies in
quantum-sampling-based diagonalization workflows. First, we integrate the Linear Scaling CNOT UCCSD (LCNot-UCCSD) ansatz into the QSCI framework, replacing the $\mathcal{O}(N^6)$ CCSD parameter initialization of the competing LUCJ ansatz approach with $\mathcal{O}(N^4)$ MP2-amplitude initialization. Second, we introduce QSCI-RBM, a variant that replaces the configuration recovery of the SQD framework with a Restricted Boltzmann Machine (RBM) acting as a compact generative subspace expansion model. Both are evaluated on eight different molecules in STO-3G across 14 controlled artificial error levels with 100 independent runs each, validated on potential energy surface scans of the N$_2$ molecule in cc-pVDZ, and embedded within DMET to treat the FDA-approved antiviral Amantadine (C$_{10}$H$_{17}$N, 11 DMET fragments) and the active region of the SARS-CoV-2 main protease complexed with its covalent inhibitor Carmofur (PDB: 7BUY, C$_{15}$H$_{28}$N$_4$O$_5$S, 10 fragments). To our knowledge, this is the first deployment of LCNot-UCCSD within QSCI on a
quantum computing simulator, and the first DMET-QSCI(LCNot-UCCSD)-RBM application to an industry-relevant
protein-ligand system. By utilizing a fraction of the classical computing resources required by the current state-of-the-art work by Cleveland Clinic, RIKEN, and IBM
Quantum, this approach enables more efficient and economical drug discovery simulations for the industry.
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Quantum Physics · Cross-listed from cond-mat.quant-gas
Finite-size effects in Schulz-Shastry-Luttinger models for determining anyonic signatures in 1d spin chains
B. Perković, M. Bonkhoff and T. Posske
We study finite-size properties of Schulz-Shastry-Luttinger
liquids to reveal anyonic signatures, realized as low-energy excitations on top of the helical ground state in saturated
spin-1/2 zigzag chains. The model features asymmetric and marginal couplings of density and phase gradients and belongs to the Schulz-Shastry class. We investigate periodic and Dirichlet boundary conditions and discuss its diagonalization as well as its stability. Although Dirichlet boundary conditions require a fine-tuning of coupling constants and universal parameters, only their magnitude is restricted for cyclic systems. We derive boundary characteristic quantities like Friedel oscillations and persistent currents. Finally, we discuss the bulk and boundary behavior of the longitudinal
spin correlations including subleading corrections.
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Quantum Physics
Working with measurement-based computations on qudits
Piotr Mitosek and Miriam Backens
Measurement-based
quantum computing is a universal model of
quantum computation in which successive product measurements of an entangled resource state drive the computation. The non-deterministic nature of measurements necessitates adaptivity to ensure an overall deterministic computation. Flow structures characterise cases in which such an adaptive correction procedure is possible. Recently, flow has been defined in a setting where the resource states are
prime-dimensional qudit graph states rather than the usual
qubit graph states. Yet, this qudit flow definition is more burdensome to work with than analogous definitions for
qubits. Here, we give a simpler definition of qudit flow and consider various useful properties of this flow, drawing on results for the
qubit case. In particular, we show how to focus qudit flow and argue that focused flow is canonical. We improve the previous algebraic formulation to capture focused flow and use it to obtain an $O(n^3)$ flow-finding
algorithm (where $n$ is the number of qudits), matching the best known complexity for
qubit flows and improving on the previous $O(n^4)$ result for qudits. Furthermore, we explore multiple flow-preserving transformations, thus opening a pathway to using flow for optimisation. These transformations include pivoting, removal and insertion of certain types of vertices, and reversibility of flow. Lastly, we propose an algorithmic approach to generating large qudit computations with flow, for testing or machine learning.
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Quantum Physics
Staged Hybridisation for Visual Quantum Reinforcement Learning via Knowledge Distillation
Javier Lazaro, Juan-Ignacio Vazquez and Pablo Garcia-Bringas
Visual environments are a demanding setting for
quantum reinforcement learning (QRL): high-dimensional observations, unstable RL optimisation, and constrained variational
quantum circuits (VQCs) are difficult to train jointly. This paper studies knowledge
distillation (KD) as a staged hybridisation strategy for visual QRL. Instead of training a hybrid visual
agent end-to-end from pixels, we first train a classical visual teacher, freeze its encoder as a feature interface, and distil the teacher's policy behaviour into compact downstream heads. These heads can be classical or VQC-based, enabling small
quantum-compatible students to be evaluated under the same frozen representation as compact classical controls. We evaluate the pipeline on CartPole Pixels and Acrobot Pixels. The results show that staged KD enables shallow VQC heads to acquire non-trivial visual-control behaviour in settings where direct pixel-based training would be substantially more difficult. Angle-encoded VQC heads retain near-teacher performance, while amplitude-encoded heads push compactness to an extreme regime, at the cost of greater fragility, stronger budget sensitivity, and higher simulation time. Overall, staged KD reframes visual QRL as a compact-head learning problem, opening a practical route for training small
quantum-compatible policies outside the standard end-to-end RL loop.
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