Quantum Physics
Phase-Flag Access and No-Go Constraints on Quotient-Space Real Quantum Mechanics
Jeongho Bang, Kyoungho Cho and Kyunghyun Baek
Barrios Hita et al. [Phys. Rev. Lett. 136, 240202 (2026)] proposed a quotient-space formulation of
quantum mechanics over the real numbers and concluded that complex numbers are only a convenience. A preceding analysis [arXiv:2607.05865] showed that, when the construction is empirically equivalent to complex
quantum mechanics, its real flag is protected by a hidden complex structure $\hat{J}$ and its composition rule is the balanced tensor product over that structure. Here we ask what would follow from the stronger operational reading that the flag is an accessible real degree of freedom. The answer is a direct clash with overlap-determinability (OD), introduced in [arXiv:2601.14638], and with standard no-go
theorems. A readable or coherently controllable flag fixes a representative of each ray, and hence supplies precisely the phase convention that OD identifies as the missing resource for universal superposition of unknown states. Once promoted to a generic phase-access primitive, this resource enables probabilistic cloning of a linearly dependent set, steering-based signaling, and logarithmic-query unstructured search. Thus the quotient construction has a sharp follow-up interpretation: as a protected gauge representation it is standard complex
quantum mechanics in real notation; as an accessible real theory it leaves the
quantum operational framework.
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Quantum Physics
Fast Unitary Preparation of Surface-Code Logical States on Neutral-Atom Hardware
Ludwig Schmid and Robert Wille
Preparing logical states is a constantly recurring primitive at the start of any surface-code-based
quantum computation. The standard measurement-based protocol is expensive on neutral atoms, since measurements are orders of magnitude slower than gates and, on single-zone architectures, additionally require shuttling the atoms to a readout zone. In this work, we present a measurement-free,
unitary preparation of surface-code Pauli eigenstates tailored to neutral atoms. A bidirectional stabilizer-expanding CNOT cascade grows the patch from its middle line outward in depth $(d{+}3)/2$, while retaining fault distance $d$ in the protected error direction. Each layer maps to one collective atom move followed by a single global Rydberg pulse, with about one row pickup per two layers. We compile and simulate the resulting schedules with the open-source bloqade toolchain under a hardware-calibrated,
circuit-level neutral-atom noise model. The bidirectional cascade achieves the lowest logical error rate at every distance, and is the only construction tested that operates below threshold for both logical states at the considered device-level noise.
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Quantum Physics
QECirc: A Community-Driven, FAIR Library of Quantum Error Correction Circuits
Ludwig Schmid, Tom Peham and Robert Wille
Quantum error correction (QEC) underpins fault-tolerant
quantum computing, but the
circuits that realize QEC codes (encoders, logical state-preparation and gate routines, and syndrome-extraction schedules) are scattered across papers and one-off repositories in incompatible formats, often documented too sparsely to reconstruct or reuse. We present QECirc (https://qecirc.com/), a community-driven, open library that makes QEC
circuits Findable, Accessible, Interoperable, and Reusable (FAIR). Each entry links to its underlying code, ships in Stim and OpenQASM with interactive visualizations, and is checked for correctness before publication. In short, where a catalog such as the Error Correction Zoo describes the codes, QECirc provides the concrete
circuits that realize them.
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Quantum Physics
Symmetry-initialized quantum Gibbs sampling: a non-Abelian asymmetry cascade
Soo-Jong Rey
For a
quantum Gibbs sampler whose mixing bottleneck is a weakly broken
symmetry, I show that the correct initialization is determined by representation theory. I first prove a general speedup-versus-prefactor dichotomy: by exactly eliminating slow-mode overlap, I convert a nominal prefactor reduction into a fundamental, asymptotic acceleration of the system's mixing time. I then show that when the bottleneck is a weakly broken
symmetry, the otherwise exponentially expensive bottleneck eigenvector is the
symmetry charge.For a non-Abelian group $G$, I prove that the correct initialization target is the group-averaging asymmetry. Projecting this asymmetry out requires a $G$-invariant input. Partial, subgroup-invariant inputs produce a cascade of mixing-time speedups indexed by the subgroup
lattice. Conversely, matching first moments alone is provably insufficient.The asymmetry is a directly measurable initialization target. I verify these results analytically and numerically in an $SU(2)$ Davies sampler where total-
spin multiplets constitute the slow modes. The predicted speedup cascade emerges with relaxation rates scaling linearly with the
symmetry-breaking parameter. Finally, the model maps the boundaries of the asymmetry target, illustrating where the separate matching of conserved logical data becomes necessary.
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Quantum Physics · Cross-listed from physics.class-ph
Multibranched parametric resonance and swallowtail catastrophe in electromechanical oscillators with nonlinear friction
P. Y. Chan, L. Huang, X. Dong et al. (+2 more)
Parametric resonance underpins the operation of a wide range of physical systems, from nanomechanical resonators to
quantum-information systems and Ising machines. As an archetypal class of driven-dissipative systems, parametric oscillators are generally expected to exhibit a single pair of stable period-two states with opposite phases. This bistable behavior enables both the simulation of
spin Hamiltonians and the preparation of superconducting cat states. Whether multiple pairs of such states can coexist in a single oscillator, however, remains an open question. Here, we show experimentally and theoretically that conventional controlled nonlinear friction can induce the coexistence of two distinct pairs of period-two states in a micromechanical oscillator. The friction is implemented via a canonical approach, utilizing a drive-induced resonant coupling that transfers two vibrational quanta from the oscillatory mode to a faster decaying mode. We demonstrate that the onset of multistability is governed by a swallowtail catastrophe and quantitatively map the associated bifurcation structure. Our results broaden the understanding of parametric resonance and establish micro- and nano-mechanical oscillators as a versatile platform for studying catastrophe theory and multistable nonequilibrium dynamics.
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All authors P. Y. Chan, L. Huang, X. Dong, M. I. Dykman, H. B. Chan
Quantum Physics
Malleability of transformations on the ciphertext in noisy Quantum public key encryption
Pete Rigas
We characterize a noisy variant of a
Quantum public encryption protocol recently introduced by Malavolta and Walter which demonstrated that the notion of everlasting security can be rigorously formulated for
Quantum key distribution after two rounds of interaction between Alice and Bob. To address one possible direction of research that is related to injecting noise in the cryptographic protocol related to
Quantum key distribution we formulate arguments for further examining the notion of everlasting security through malleability assumptions on transformations of the ciphertext. Assumptions surrounding malleability were introduced by Maurer and Tackmann for the purposes of comparing how authenticate then encrypt, and encrypt then authenticate, protocols behave through a variety of expressions for the forwarding error, deleting error, and reconstruction probabilities. Such probabilities are put to further use for obtaining connections between the indistinguishability and security threshold for a cryptographic protocol of interests. To further build upon such associations we demonstrate, through an adaptation of the Gentle Measurement Lemma from
Quantum information theory, how upper bounds on the trace distance can be used to generalize the negligibility function obtained by Malavolta and Walter in the noiseless setting. Besides the fact that the negligibility function in the noisy setting is related to a higher security threshold it continues to remain of interest to determine whether computations provided in this work for upper bounding the trace distance can be related to other settings that are centered more on game-theoretic approaches.
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Quantum Physics
Agentic AI for Scientific Reasoning in Autonomous Quantum Sensing Experiments
Takuya Isogawa, Ryotaro Okabe, Nutdech Phadetsuwannukun et al. (+2 more)
We implement an agentic AI workflow built around a large
language model (LLM)
agent for autonomous experiments with nitrogen-vacancy (NV) centers in diamond. NV centers are a widely used platform for
quantum sensing, and the ability to control many measurements from a computer makes NV experiments a natural setting for autonomous workflows. We make two main contributions. First, we demonstrate an autonomous NV experiment workflow that combines persistent project records, quantitative calculation and data analysis tools, and deterministic experiment control. In one autonomous experiment, the
agent selected a single NV center, calibrated its resonant frequency, measured \(T_2^\ast\) with Ramsey measurements, and added a Carr–Purcell–Meiboom–Gill (CPMG) measurement to check a weak feature that could be related to nearby \(^{13}\mathrm{C}\). Second, we introduce two offline
benchmarks that evaluate the
agent's
reasoning separately from laboratory execution. We evaluated both
benchmarks with GPT-5.4, GPT-5.5, and GPT-5.6 Sol. In the Ramsey checkpoint
benchmark, greater
reasoning effort generally improved recognition of a residual resonance calibration offset. By contrast, in the pulsed optically detected
magnetic resonance (pODMR) data evaluation
benchmark, pulse sequence information alone produced more false positive resonance judgments at higher
reasoning effort. Requiring an expected signal calculation kept false positive rates low across all three models and
reasoning settings. The results suggest a clear division of labor for autonomous experiments. The
agent forms scientific hypotheses and uses quantitative tools to evaluate data, while deterministic code controls the hardware and enforces safety constraints.
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All authors Takuya Isogawa, Ryotaro Okabe, Nutdech Phadetsuwannukun, Mingda Li, Paola Cappellaro
Quantum Physics
Exceptional-point braiding with native controls
Vishnu Chavva, Nero Osmancevic and Hugo Ribeiro
Exceptional points define branch-exchange state transfers through holomorphic continuation of non-Hermitian eigenmodes, but realizing these transfers dynamically remains difficult. Slow encircling does not generally transport the full set of instantaneous eigenstates, while shortcuts to adiabaticity can require controls outside the native experimental
manifold. Here, we introduce a constrained shortcut-to-adiabaticity principle for exceptional-point braiding using native controls. In a dressed instantaneous-eigenstate frame, the available controls cancel the accessible transition channels locally, while the residual channels remain active during the
evolution but are constrained to have no net accumulated effect over the closed loop. The protocol therefore targets the endpoint state transfer selected by ideal adiabatic branch exchange, rather than enforcing complete local cancellation or adiabatic following throughout the trajectory. We demonstrate the construction in a minimal two-mode non-Hermitian model equivalent, up to a trace shift and a basis convention, to the effective
Hamiltonian used in dissipative-transmon exceptional-point experiments, where detuning and drive amplitude provide real controls and the relative loss imbalance fixes the non-Hermitian scale. Smooth real waveforms reshape only these controls, reproduce the branch-exchange transfer, and remain accurate under calibration errors, exceptional-point uncertainty and finite-bandwidth filtering with modest overhead.
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Quantum Physics
Covariant Quantum Measurements and Stochastic Dynamics on Representation Space
Naeem Shahid
We develop a framework for group-covariant
quantum measurements in which measurement-induced transitions between irreducible representation sectors are described by a
stochastic process on representation space. Starting from the Peter-Weyl decomposition, we construct covariant measurement operators from irreducible tensor operators and show that, for
symmetry-invariant states, the measurement channel reduces to a Markov process on the representation graph. We further show that analyticity of the measurement operator constrains the detector spectrum through Sugiura's
theorem, motivating a class of exponentially decaying detector models. Specializing to SU(2), we obtain the transition kernel in closed form, establish reversibility and the associated invariant measure, and derive a continuum Fokker-Planck description of the induced dynamics. Analytical predictions for the drift and
diffusion coefficients are found to agree with numerical simulations. Our results provide a
stochastic description of repeated covariant
quantum measurements on representation space.
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Quantum Physics
A broadband, individually addressing two- and three-dimensional photonic integrated circuit for trapped-ion qubit control
Daniel Klawson, Yiyang Zhi, Bingran You et al. (+21 more)
Trapped ions provide a high-fidelity platform for
quantum information processing, yet delivery of multiple, distinct wavelengths across large networks of interaction zones remains a bottleneck. Conventional free-space light delivery lacks scalability, while on-chip grating couplers suffer from narrow operational bandwidth that increases
circuit footprint and optical interfacing complexity. Here we show a broadband photonic integrated
circuit capable of addressing individual ions. The
circuit combines a planar waveguide lens with a micromirror fabricated using two-
photon polymerization at wafer scale. This implementation can address three individual ions from $\lambda$ = 405 - 880 nm with -27 dB average intensity crosstalk at $5\,\mu\mathrm{m}$ pitch. We trap $^{40}\mathrm{Ca}^{+}$ and $^{138}\mathrm{Ba}^{+}$ ions above such devices, characterize optical crosstalk with barium ions, and demonstrate individual repumping of calcium ions. This monolithic photonic architecture brings broadband addressing in an on-chip modality to trapped-ion technology. More generally, integrating additive manufacturing into
quantum devices is poised to unlock expanded design space for implementing novel
quantum architectures.
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All authors Daniel Klawson, Yiyang Zhi, Bingran You, Michael Bareian, Elijah Mossman, Chun-Yuan Fan, Arkadev Roy, Ke Sun, Jason Lee, Sung Cheol Yoon, Qiming Wu, Lai Jiang, Wenjun Ke, Weiwei Wu, Sirui Tang, Zachary Wall, Jiaxiang Wang, Louis Paul Romero, Sam Vizvary, Steven Diaz, Eric R. Hudson, Wesley C. Campbell, Hartmut Haeffner, Ming C. Wu
Quantum Physics · Cross-listed from hep-ph
Quantum detection of CP violation in the $t\bar{t}$ system: tomography
Priyanka Lamba, Fabio Maltoni, Olimpia Miniati et al. (+1 more)
We develop a
quantum-tomographic framework for determining whether possible CP-odd effects in $t\bar t$ events originate in production, in decay, or in both. In the narrow-width approximation, the process $I\to t\bar t\to b\ell^+\nu\,\bar b\ell^-\bar\nu$ factorises into a production density
matrix and top and antitop decay density
matrices. We extend the standard
tomography procedure to a general anomalous $Wtb$ vertex and derive the corresponding angular distributions. Polar-angle distributions retain their usual tomographic form, up to modifications of the
spin-analysing powers, and can therefore be used to reconstruct the production density
matrix and test its CP properties. By contrast, dedicated azimuthal observables involving the $b$–
lepton decay planes contain characteristic sine modulations that provide linear probes of possible new CP-violating interactions in the decay vertex. Combining the two classes of observables gives a systematic strategy for separating sources of CP violation in production and in decay. We illustrate the resulting angular signatures for representative $t\bar t$ production scenarios at
hadron and
lepton colliders.
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All authors Priyanka Lamba, Fabio Maltoni, Olimpia Miniati, Eleni Vryonidou
Quantum Physics · Cross-listed from hep-ph
Quantum detection of CP violation in the $t\bar{t}$ system: production
Priyanka Lamba, Fabio Maltoni, Olimpia Miniati et al. (+1 more)
We investigate how possible new CP-violating top-
quark interactions are encoded in the
quantum state of a produced $t\bar t$ pair. We derive analytic expressions for the production density
matrix in several
benchmark channels relevant to
hadron,
lepton and
photon colliders. In a common
spin basis, we identify two characteristic CP-odd structures in the Fano–Bloch decomposition: a difference between the top and antitop polarisation vectors and an antisymmetric component of the
spin-correlation
matrix. We construct observables that directly probe these structures and study how
quantum information measures, including discord, concurrence, magic and trace distance, respond to CP-even and CP-odd SMEFT contributions. Finally, using current measurements and future
collider projections, we assess the sensitivity of these observables to possible new sources of CP violation in top-
quark production. This establishes the production-level framework whose experimental reconstruction is developed in a companion paper.
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All authors Priyanka Lamba, Fabio Maltoni, Olimpia Miniati, Eleni Vryonidou
Quantum Physics
Invariant-based master equation applied to driven qutrit coupled to a bath and a leaky cavity
Sagarika Basak, A. Javadi and D. Blume
We employ a generalized approach to the master equation for driven open $N$-level ($N>2$)
quantum systems using Lewis-Riesenfeld invariants, which avoids the driving-strength restrictions inherent to conventional approaches. We show that the invariant-based master equation provides a unifying generalized framework, which reduces to the frequently employed laboratory-frame master equations and the less frequently employed rotating-frame master equation framework under appropriate simplifications. Extending the prototypical two-level system, we show that the inclusion of another state coupled to the ground state via reservoir-induced dephasing gives rise to qualitatively new dissipative behaviors that are, in general, not captured by standard approximations. We also apply the invariant-based master equation framework to a driven
quantum dot coupled to a leaky cavity, demonstrating the framework's ability to capture relevant dissipative dynamics without additional assumptions. Our work paves the way for
quantum-control applications in the presence of dissipation.
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Quantum Physics
A Model Predictive Control-Inspired Quantum Algorithm
Dominic Messina, Alicia B. Magann, Mohan Sarovar et al. (+1 more)
We introduce a new hybrid
quantum-classical
algorithm inspired by an advanced control strategy known as model predictive control (MPC). This
algorithm unifies the optimization-based design of variational
quantum algorithms (VQAs) with the feedback-based design of feedback-based
quantum algorithms (FQAs). Variational
circuit parameters are optimized using a layer-wise receding horizon strategy, where observable measurements after every layer initialize a classically simulated dynamic model used to predict
quantum state
evolution and optimize over future parameterized gates. This hybrid
algorithm can be used for applications such as ground state preparation and approximate combinatorial optimization, and presents an ideal use case for the integration of
quantum computers with high-performance computing, where the latter resource can be used to increase the scale and efficiency of the predictions critical to MPC. We show through mathematical proof and numerical evidence that the MPC-based
algorithm can be guaranteed to at least match the performance of FQAs. Through simulations on Max-Cut problems and a two-dimensional transverse-field Ising model, we demonstrate that relaxed implementations of the MPC-based
algorithm can also provide improved performance in practice compared to an FQA.
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All authors Dominic Messina, Alicia B. Magann, Mohan Sarovar, Helen Durand
Quantum Physics
Maximal complementarity in the n-qubit Pauli group
Markus Frembs, Giovanni Natale, Christopher S.P. Wever et al. (+1 more)
Observables in
quantum mechanics are generally complementary, that is, they reveal mutually incompatible pieces of information about a given system. This property is not only a fundamental tenet of the
quantum formalism, but also a key component in
quantum cryptographic protocols. As such it has fuelled much research into finding sets of highly complementary observables. In its strongest form – the one we consider in this work – the information between complementary observables is not merely incompatible but mutually exclusive: maximal information about one observable implies no information about the other, and vice versa. Maximal sets of non-degenerate complementary observables are known to exist in systems of
prime power dimension such as n-
qubit systems. Here, we study complementarity of degenerate observables, specifically we prove that observables associated with the n-
qubit Pauli group also exhibit complementarity under this restriction: first, we obtain a criterion for two such observables to be complementary and, second, we relate maximal sets of complementary observables with informational pure state complementarity equalities, further studied in two companion papers. Equivalently, these results can be formulated in terms of maximal complementary sets of (not necessarily maximal) Abelian subgroups of the Pauli group linking with (possibly coarse-grained) mutually unbiased bases. Finally, we prove that these complementarity sets entail strong entropic uncertainty relations.
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All authors Markus Frembs, Giovanni Natale, Christopher S.P. Wever, Philipp A. Hoehn
Quantum Physics
An algebraically closed family of informational n-qubit purity invariants
Markus Frembs, Giovanni Natale, Christopher S.P. Wever et al. (+1 more)
We present a family of quadratics in Pauli expectation values, and prove that they constitute state-independent invariants for all n-
qubit pure states. This family generalises the two-
qubit `pentagon identities', discovered in the reconstruction programme of [P. A. Höhn,
Quantum 1, 38 (2017), P. A. Höhn and C. S. P. Wever, Phys. Rev. A 95, 012102 (2017)], where they characterise the space of pure states, as well as the
unitary group, and are interpreted as complementarity equalities in the Brukner-Zeilinger information measure. The generalisation to arbitrarily many
qubits is nontrivial as it requires new tools which in turn reveal novel structural properties that are absent in the two-
qubit case. A thorough analysis of these properties, and their relation with mutual unbiasedness and complementarity in the n-
qubit Pauli group, can be found in two companion papers.
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All authors Markus Frembs, Giovanni Natale, Christopher S.P. Wever, Philipp A. Hoehn
Quantum Physics
Quantum reference frames beyond subsystems: a reconstruction and generalization of the perspective-neutral framework
Stefan L. Ludescher, Manuel Mekonnen, Thomas D. Galley et al. (+1 more)
We generalize the notion of
quantum reference frames (QRFs) to cases where the frame does not necessarily correspond to a tensor factor subsystem, but to a covariant
quantum instrument. This unlocks a variety of physical applications: "frames of labeling" for indistinguishable particles, suggesting explanations for the symmetrization postulate and the absence of parastatistics, and yielding a transparent description of the
entanglement of
bosons and
fermions; and relational clocks reproducing the Schrödinger equation exactly even when all subsystems are interacting or when there are frequency superselection sectors. Our work generalizes the perspective-neutral approach to QRFs pioneered by Höhn and co-authors, which we reconstruct from a simple operational scenario. We give a resource-theoretic grounding of this framework, and show how the notion of completely covariant operations explains the relevance of the charge-zero sector and the pure-state transformation behavior across perspectives. This also suggests operational clarifications of some aspects of constraint quantization, e.g. of the meaning of constraint equations such as $C|\psi\rangle=0$. Some of our results, such as our generalization of relationalization maps to instruments, apply more broadly to other QRF frameworks too, and they contribute to bridging the gap between operational
quantum information theory and the internal QRF research program.
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All authors Stefan L. Ludescher, Manuel Mekonnen, Thomas D. Galley, Markus P. Mueller
Quantum Physics
A Kernel-Based Density of States Estimator for Quantum Computing
Julio Cesar Siqueira Rocha
The density of states (DoS) encodes the thermodynamic and spectral properties of
quantum many-body systems, yet its reconstruction becomes intractable for
Hilbert spaces too large to diagonalize. Classically, the kernel
polynomial method (KPM) addresses this by combining
stochastic trace estimation with a smoothing kernel. Here we show that the Rodeo
algorithm—one of the simplest
eigenvalue-location protocols for near-term
quantum hardware—provides a direct
quantum analogue of this approach. Averaging the Rodeo response over Haar-random input states yields the DoS convolved with a spectral kernel fixed entirely by the distribution of
evolution times: the random states play the role of
stochastic trace estimation, and the temporal sampling distribution that of the damping kernel. The construction requires only the standard single-ancilla
circuit, and
quantum typicality suppresses the statistical error as the
Hilbert-space dimension grows. We derive the
estimator and its uncertainties, establish an explicit dictionary between signal-processing window functions and
quantum reconstruction kernels, and validate the method on the one-dimensional transverse-field Ising and
spin-1 models.
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Quantum Physics
Universal Optimization and Tighter Fidelity Bounds for Approximate Quantum Error Correction
Jing Wu, Michele Grossi, Doga Kurkcuoglu et al. (+1 more)
Approximate
quantum error correction (AQEC) extends the framework of discrete- and continuous-variable
quantum error correction beyond the Knill-Laflamme (KL) conditions, where the recovery performance is quantified by
entanglement fidelity. Recent studies have enabled efficient evaluation of near-optimal
entanglement fidelity using transpose-channel recovery. Yet, determining the global optimal recovery map and its
entanglement fidelity for general codes beyond the KL conditions remains a major computational challenge. Direct optimization becomes prohibitive as the number of noise Kraus operators grows rapidly with system size, and existing approaches lack rigorous guarantees for reducing this optimization to a tractable dimension. Here, we derive an explicit characterization of the optimal environmental state of complement channel, which transforms the optimization over recovery channels into an equivalent optimization over quotient
unitaries. For a broader class of codes that satisfy only the orthogonality part of the KL conditions, we show that the optimal recovery map admits an explicit analytical form. Building on this form, we derive novel rigorous lower bounds of
entanglement fidelity that strictly improve upon the transpose-recovery bound. We further develop a novel recovery strategy based on principle components, and derive a rigorous bound on the error introduced by noise truncation. Our approach enables efficient searches for approximate recovery maps for AQEC codes, avoiding the need to optimize over the full Kraus-operator space.
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All authors Jing Wu, Michele Grossi, Doga Kurkcuoglu, Silvia Zorzetti
Quantum Physics · Cross-listed from cond-mat.mes-hall
A.-F. Kalo, E. A. Rodríguez-Mena, J. C. Abadillo-Uriel et al. (+2 more)
Unstrained bulk germanium is a particularly attractive material for
circuit quantum electrodynamics with
spins (
spin-cQED). We show, through systematic modeling and comparison with state-of-the-art strained germanium
heterostructures, that hole
spins in bulk germanium double
quantum dots readily reach the strong-coupling regime with superconducting microwave resonators, achieving
spin-
photon coupling strengths $g_s/2\pi\gtrsim100$\,MHz. This enhancement originates from large
spin-orbit interactions beyond the perturbative regime. In addition, the coupling is much less sensitive to the orientation of the applied
magnetic field, which shall ease operation and limit the impact of device-to-device variability. Our results establish bulk germanium as a compelling platform for scalable
spin-cQED.
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All authors A.-F. Kalo, E. A. Rodríguez-Mena, J. C. Abadillo-Uriel, M. Filippone, Y.-M. Niquet
Quantum Physics
Non-Hermitian entropy production from fluctuation theorems
Vasco Cavina, Frank Ernesto Quintela Rodríguez and Donato Farina
We develop a first-principles thermodynamic framework for non-Hermitian dynamics based on a post-selected version of the fluctuation
theorem. This allows us to identify a quantity that remains positive throughout the non-Hermitian
evolution and can be interpreted as the
entropy production of the post-selected dynamics. We relate this quantity to previously proposed notions of non-Hermitian
entropy and derive an associated second law. Furthermore, we establish a connection with information-theoretic quantities, in particular the Petz-Rényi divergences, and leverage this connection to derive upper and lower bounds. Finally, we decompose the
entropy production into incoherent and coherent contributions, identifying distinctive features of the coherent term in the vicinity of exceptional points. We illustrate our results using a paradigmatic model of non-Hermitian
evolution based on a two-level system.
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Quantum Physics
Realizing Error Suppression in Partially Fault-Tolerant Quantum Simulations with IBM Quantum Computers
Henry Froland, Dorota M. Grabowska, Sebastian Grieninger et al. (+8 more)
Quantum error-detecting codes offer a near-term path for improving the performance of
quantum simulations on noisy hardware. Using IBM's superconducting
quantum computer ibm_boston, we show that partially fault-tolerant encoded
quantum simulations of the Ising model in 1+1D and 2+1D outperform their unencoded counterparts in estimating local observables. To represent 42 logical
qubits on the heavy-hex
quantum processor, 21 blocks of the [[4, 2, 2]] Iceberg code and up to 136 physical
qubits are used. By pairing fault-tolerant syndrome extraction with non-fault-tolerant logical operations, this scheme preserves many of the benefits of error detection while avoiding the overhead typically required for a fully fault-tolerant logical gate set. The encoding's square logical connectivity, together with the freedom to place logical
qubits within each block, enables simulations of a 2D spatial
lattice with lower
circuit depth than the unencoded implementation requires. We introduce Observable-Ranked Postselection, a selective-filtering technique based on syndrome correlations that recovers reliable results without the prohibitive shot loss of full syndrome postselection. Under the cumulative effect of device errors, this encoding improves local-observable accuracy over the unencoded baseline by 2-6% at intermediate times in 1+1D simulations, growing with
circuit depth to over 200% in 2+1D at the latest times studied.
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All authors Henry Froland, Dorota M. Grabowska, Sebastian Grieninger, Jeremy Hartse, Anne L. Lashbrook, Zhiyao Li, Ziyuan Li, Sarah J. M. Powell, Martin J. Savage, Xiaojun Yao, Nikita A. Zemlevskiy
Quantum Physics
Numerical Modeling of Quasiparticle-Induced Dissipation in Fluxonium Qubits
Kate Azar, Max Hays and Kyle Serniak
Nonequilibrium quasiparticles (QPs) generated by stray infrared and ionizing radiation can limit the performance of superconducting
quantum processors and present challenges for
quantum error correction schemes. Models of QP-induced energy relaxation commonly assume that the characteristic energy of the QPs and the
qubit transition energy are both small relative to the superconducting gap. Under these assumptions, certain
qubits such as the fluxonium would exhibit protection against QP-induced dissipation at specific bias points. Here, we show that this is not necessarily the case, numerically analyzing the predicted rate of QP-induced dissipation in fluxonium
qubits for different QP energy distributions and for QPs created via
photon-assisted tunneling processes. We find that accounting for small numerical factors, existing theoretical models predict sensitivity to QP-induced errors at bias points previously thought to be protected. We find that inclusion of asymmetry in the superconducting gap energy across the junction can reintroduce suppression of QP-induced relaxation, as expected. Additionally, for QPs created by
photon-assisted tunneling, we predict that $T_1$ protection will only occur for a specific energy of pair-breaking radiation. This understanding of fluxonium sensitivity to QP-induced dissipation informs the development of fluxonium-based processors and future QP-mitigation strategies.
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Quantum Physics
Resolving Structure in Prethermal Floquet Dynamics with Precision Quantum Computation
Eyal Leviatan, Tasneem Watad, Roy Perry et al. (+38 more)
Periodically driven interacting
quantum many-body systems can exhibit long-lived prethermal dynamics, where local observables retain coherent structure even as
entanglement and operator complexity grow. Accessing this regime at the system sizes and times needed to determine physical properties of the prethermal state remains a central challenge: state-of-the-art classical methods become unreliable, while noise in
quantum hardware degrades observable expectation values. Here we overcome these limitations for a Floquet Ising magnet realized on a heavy-hex
lattice. Using the advanced error mitigation software QESEM on an IBM Heron r3 superconducting
quantum processor, we measure magnetization dynamics with percent-level precision and resolve long-lived subharmonic prethermal oscillations in systems of up to 74
qubits. These experiments reach regimes for which leading tensor-network simulations fail to converge, while sparse Pauli-path simulations remain strongly truncation dependent despite extensive computations on advanced GPUs and the Fugaku supercomputer. Leveraging this
quantum-accessible regime, we extend finite-size scaling to larger systems and find an unexpectedly slow decrease of the oscillation amplitude with system size, providing strong evidence that this oscillatory response persists in the thermodynamic limit of heavy-hex ladders. A hierarchy of mitigation and validation tests, including unbiased error mitigation, agreement between independent mitigation
estimators, noise-model validation on the superconducting hardware, and cross-platform corroboration at selected Floquet cycles on Quantinuum System Model H2 and Quantinuum Helios trapped-ion hardware, supports the reliability of these findings. Our work establishes error-mitigated
quantum processors as quantitative scientific instruments for discovering new physics in non-
equilibrium quantum matter.
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All authors Eyal Leviatan, Tasneem Watad, Roy Perry, Lukas Broers, Mohammed Zuhair Mullath, Ori Alberton, Itai Arad, Yosi Atia, Eyal Bairey, Shaul Barkan, Matan Ben Dov, Asaf Berkovitch, Ewout van den Berg, Itsik Cohen, Omri Golan, Ilya Gurwich, Avieli Haber, Barak A. Katzir, Oded Kenneth, Roei Levi, Yotam Y. Lifshitz, Yaron Lukovsky, Ron Melcer, Adiel Meyer, Boris Muratov, Aviad Panahi, Gili Schul, Tali Shnaider, Maor Shutman, Alireza Seif, Tomonori Shirakawa, Asif Sinay, Vincent P. Su, Hayk Tepanyan, Omri Trebitch, Assaf Zubida, Dorit Aharonov, Hrant Gharibyan, Abhinav Kandala, Seiji Yunoki, Netanel H. Lindner
Quantum Physics · Cross-listed from cond-mat.stat-mech
Effective Field Theory of Operator Scrambling from Strong-to-Weak Symmetry Breaking
Bai-Lin Cheng, Shao-Kai Jian and Zhi-Cheng Yang
Operator scrambling is commonly diagnosed by the growth of out-of-time-ordered correlators (OTOCs), yet a general
symmetry principle underlying their effective dynamics has remained elusive. For Brownian or short-time-correlated large-$N$ Majorana systems, we develop a
symmetry-based effective field theory for operator scrambling, organized by a strong-to-weak U(1)
symmetry breaking in operator space. The key observation is that, in the noninteracting
fermion limit, the four-fold Keldysh contour representation of an OTOC admits an emergent strong U(1)
symmetry in a doubled
Hilbert-space description, even when the original system has no ordinary conserved quantity. The associated slow mode is the phase of the strong-charge creation operator, whose conjugate density is identified with the local operator size. Generic interactions explicitly break the strong
symmetry and generate a mass term at lowest order for the would-be Goldstone mode, thereby converting diffusive operator spreading into chaotic growth. We further show that higher-order
symmetry breaking terms are tightly constrained by an emergent duality that combines time reversal with contour permutation. This duality fixes the effective action up to quadratic order in the response field, relates the multiplicative noise strength directly to the Lyapunov exponent, and makes the positivity of the Lyapunov exponent a consequence of convergence of the real-time path integral. The resulting OTOC dynamics is governed by a noisy FKPP equation, which captures within a unified framework the early-time exponential growth, ballistic propagation, nonlinear saturation, and
stochastic front broadening of operator scrambling. We verify this construction in a Brownian SYK chain, where a direct saddle-point expansion reproduces the
symmetry-based effective action. Our results reveal a
symmetry origin of operator-size hydrodynamics and scrambling.
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Quantum Physics
Efficient computation of real-time correlators using Pauli Propagation
Alexander F. Kemper, Raghav G. Jha, Arnab Bachhar et al. (+3 more)
Pauli propagation has shown promise for classically simulating
quantum dynamics by evolving observables directly in the Heisenberg picture. In this work, we investigate its use for computing real-time two-point time-ordered correlators in one- and two-dimensional
quantum systems. One major limitation of Pauli propagation is the rapid growth in the number of Pauli strings beyond short times. We overcome this limitation by combining accurate short-time Pauli-propagation data with time extension methods based on a positivity condition and the observation that the dynamics are often dominated by a small number of characteristic frequencies. This combined approach extends correlation functions far beyond the directly accessible time window while avoiding the exponential proliferation of Pauli operators. We demonstrate that the resulting correlators retain the relevant dynamical and spectral information. Our results broaden the regime in which classical methods can reliably probe the real-time dynamics of interacting
quantum many-body systems.
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All authors Alexander F. Kemper, Raghav G. Jha, Arnab Bachhar, Goksu C. Toga, Mariano Guerrero Perez, Nicholas J. Mayhall
Quantum Physics
Fault-tolerant distributed quantum computing with a single nucleus per node
Yotam Vaknin, Shoham Jacoby, Roi Nevo et al. (+2 more)
Distributed
quantum computing interconnects small, high-quality nodes through optical links, but this architecture carries a pronounced asymmetry: in-node gates and measurements are cheap and high-fidelity, whereas inter-node communication relies on a low-coherence communication
qubit and faulty photonics. Previous approaches overcame the noisy link by placing several high-quality data
qubits in each node and consuming them for Bell pair and GHZ state
distillation. Here we show that
distillation can be avoided altogether. The key observation is that we can engineer a communication error bias, where photonic Bell pairs suffer frequent phase errors but only rare bit-flip errors. We design the syndrome-extraction
circuits so that this phase noise appears solely as a measurement error that does not propagate to the data
qubits, and is therefore suppressed by simply repeating the measurement; letting the error-correcting code itself, rather than a dedicated
distillation subroutine, to purify the link. This dramatically reduces the need for ancillary
nuclei: Floquet codes require only a single data
qubit per node, while general stabilizer codes require just one additional ancilla. We demonstrate high error-correction thresholds throughout this regime, and we identify
lattice surgery as inherently robust for this setting, enabling logical operations at a threshold close to that of
quantum memory. As a result, the performance of the
quantum computer is limited by the high-quality data
qubits, while the requirements on
photon indistinguishability and coherence of the communication
qubit are substantially relaxed.
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All authors Yotam Vaknin, Shoham Jacoby, Roi Nevo, Aleksander Kubica, Alex Retzker
Quantum Physics · Cross-listed from cond-mat.supr-con
Probing nonlocal superconducting fluctuations with covariance noise magnetometry
Gustav Romare, Ilya Esterlis, Shimon Kolkowitz et al. (+1 more)
The nonlocal superconducting fluctuation corrections to the conductivity tensor $\sigma_{ij}(\mathbf{q},\omega)$ are calculated within the time-dependent Ginzburg-Landau framework, and their observable consequences for
quantum noise magnetometry are worked out. For a single nitrogen-vacancy (NV) sensor we obtain the relaxation rate $1/T_1$ as a function of temperature, sample-sensor distance, and probe frequency, identifying the scales at which the nonlocality and the dynamics of the pair fluctuations cut off the critical enhancement near $T_c$. For two-sensor covariance magnetometry we show that the two-point field correlator develops additional spatial structure whose range directly measures the fluctuation correlation length $\xi(T)$. We further analyze two channels that accompany the paraconductivity: the Maki-Thompson correction to the
spin susceptibility, and the fluctuation diamagnetism. Finally, we solve exactly, to all orders in a dc electric field and at all wave vectors, for the nonequilibrium current noise of the fluctuating film: the noise decouples from the nonlinear paraconductivity, violating the fluctuation-dissipation
theorem by universal factors at criticality and acquiring a bias-induced spatial anisotropy directly measurable by covariance magnetometry. The results are connected to a recent experiment measuring current noise near a thin film of BSCCO.
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All authors Gustav Romare, Ilya Esterlis, Shimon Kolkowitz, Alex Levchenko