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Saturday 21 February

Quantum Physics

Predicting Magic from Very Few Measurements

J. M. Varela, L. L. Keller, A. de Oliveira Junior et al. (+3 more)

The nonstabilizerness of quantum states is a necessary resource for universal quantum computation, yet its characterization is notoriously demanding. Quantifying nonstabilizerness typically requires an exponential number of measurements and a doubly exponential classical post-processing cost to evaluate its standard monotones. In this work, we show that nonstabilizerness is, to a large extent, in the eyes of the beholder: it can be witnessed and quantified using any set of $m$ $n$-qubit Pauli measurements, provided the set contains anti-commuting pairs. We introduce a general framework that projects the stabilizer polytope onto the subspace defined by these observables and provide an algorithm that estimates magic from Pauli expectation values with runtime exponential in the number of measurements $m$ and polynomial in the number of qubits $n$. By relating the problem to a stabilizer-restricted variant of the quantum marginal problem, we also prove that deciding membership in the corresponding reduced stabilizer polytope is NP-hard. In particular, unless $\mathrm{P} = \mathrm{NP}$, no algorithm polynomial in $m$ can solve the problem in full generality, thus establishing fundamental complexity-theoretic limitations. Finally, we employ our framework to compute nonstabilizerness in different Hamiltonian ground states, demonstrating the practical performance of our method in regimes beyond the reach of existing techniques.
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All authors J. M. Varela, L. L. Keller, A. de Oliveira Junior, D. A. Moreira, R. Chaves, R. A. Macêdo

Quantum Physics

Time-Rescaled STIRAP Enables Compact Cascaded Frequency Conversion

J. L. Montenegro Ferreira

In this letter we explore how full frequency conversion can be performed in shorter optical devices by using a STIRAP-like protocol modified by the time rescaling shortcut to adiabaticity. We first show a detailed, step-by-step process of how the coupled equations for two simultaneous three-wave mixing processes can be written in terms of a STIRAP-like system. From the properties of the STIRAP protocol, we know that the ensuing process will result in robust conversion, but require long propagation distances inside the bulk crystal or waveguide. We then apply the time rescaling shortcut to adiabaticty in the conversion process to create a TR-STIRAP protocol, showing that full and robust conversion is also obtained, but at a fraction of the propagation distance. We also show how the original shaping of the coupling coefficients required by the TR-STIRAP can be approximated by Gaussian functions with high conversion fidelity, thus simplifying the experimental implementation. Finally, a possible application of the new protocol is discussed in the context of single-photon detection in the MIR.
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Quantum Physics

Trotter Error and Orbital Transformations in Quantum Phase Estimation

Marvin Kronenberger, Mihael Erakovic and Markus Reiher

Quantum computation with Trotter product formulae is straightforward and requires little overhead in terms of logical qubits. The choice of the orbital basis significantly affects circuit depth, with localised orbitals yielding lowest circuit depths. However, literature results point to large Trotter errors incurred by localised orbitals. Here, we therefore investigate the effect of orbital transformations on Trotter error. We consider three strategies to reduce Trotter error by orbital transformation: (i) The a priori selection of an orbital basis that produces low Trotter error. (ii) The derivation of an orbital basis that produces a ground state energy free of Trotter error (as we observed that the Trotter error is a continuous function in the Givens-rotation parameter, from which continuity of this error upon orbital transformation can be deduced). (iii) Application of propagators that change the computational basis between Trotter steps. Our numerical results show that reliably reducing Trotter error by orbital transformations is challenging. General recipes to produce low Trotter errors cannot be easily derived, despite analytical expressions which suggest ways to decrease Trotter error. Importantly, we found that localised orbital bases do not produce large Trotter errors in molecular calculations, which is an important result for efficient QPE set-ups.
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Quantum Physics

Why measurements are made of effects

Tobias Fritz

Both in quantum theory and in general probabilistic theories, measurements with $n$ outcomes are modelled as $n$-tuples of effects summing up to the unit effect. Why is this the case, and can this assumption be meaningfully relaxed? Here we develop generalized measurement theories (GMTs) as a mathematical framework for physical theories that is complementary to general probabilistic theories, and where this kind of question can be made precise and answered. We then give a definition of probabilistic state on a GMT, prove that measurements are made of effects in every GMT in which the probabilistic states separate the measurements, and also argue that this separation condition is physically well-motivated. Finally, we also discuss when a GMT should be considered classical and characterize GMTs corresponding to Boolean algebras as those that are strongly classical and projective.
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Quantum Physics

Frozen and Growing Quantum Work under Noise: Coherence and Correlations as Key Resources

Mohammad B. Arjmandi

We investigate the decomposition of ergotropy into incoherent and coherent contributions for quantum systems subject to typical Markovian noise channels. The incoherent part originates from population inversion in the energy eigenbasis after dephasing, while the coherent part captures the role of quantum coherence in work extraction. For single-qubit systems, we derive explicit conditions for freezing and enhancement of coherent ergotropy and obtain an analytical upper bound, showing that it cannot exceed one half of the state's quantum coherence. We then study two classes of separable two-qubit states under local noise. For Bell-diagonal states, which are locally completely passive and possess no local coherence, we prove that the total extractable work equals the average of geometric quantum and classical correlations. In this case, coherent ergotropy cannot be enhanced, although freezing occurs under specific noise conditions. By contrast, for separable states with local coherence, coherent ergotropy can increase under all considered noise channels, including phase-flip and depolarizing noise. Extending the analysis to multipartite systems, we show that both the magnitude and range of noise-induced enhancement grow with the number of qubits, indicating collective reinforcement. Finally, we demonstrate through an explicit example that entanglement does not prevent this enhancement: coherent ergotropy may increase under noise even for entangled states. Our results reveal that noise can assist energy storage, challenging the conventional view of noise as purely detrimental and suggesting compatibility between noise-assisted enhancement and fast entanglement-based charging mechanisms in quantum batteries.
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Quantum Physics

Hierarchies of Gaussian multimode entanglement from thermodynamic quantifiers

Mrinmoy Samanta, Sudipta Mondal, Ayan Patra et al. (+2 more)

We develop a thermodynamic characterization of multimode entanglement in pure continuous-variable systems by quantifying the gap between globally and locally extractable work (ergotropy). For arbitrary pure multimode Gaussian states, we prove that the $2$-local ergotropic gap is a faithful entanglement monotone across any bipartition and constitutes a functionally independent upper bound to the Renyi-2 entanglement entropy. We further introduce the $k$-ergotropic score, the minimum $k$-local ergotropic gap, and show that it faithfully quantifies multimode entanglement across $k$ partitions. For pure three-mode Gaussian states, we derive its closed-form relation with the geometric measure for genuine multimode entanglement $(k=2)$, and total Gaussian multimode entanglement $(k=3)$. For systems with more than three modes, the $k$-ergotropic score becomes a functionally independent measure of multimode entanglement to the standard geometric measures. Our results reveal a direct operational hierarchy linking Gaussian multimode entanglement to work extraction under locality constraints, and provide a computable and experimentally accessible thermodynamic framework for characterizing quantum correlations.
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All authors Mrinmoy Samanta, Sudipta Mondal, Ayan Patra, Saptarshi Roy, Aditi Sen De

Quantum Physics · Cross-listed from cond-mat.mes-hall

FQH Liquids with Flux-Expulsion Islands allow Nonabelian Anyons

Hisham Sati and Urs Schreiber

The idea that topologically protected quantum states, such as anyons, may be attached to super/semiconductor heterostructures has received enormous attention, but experimental signatures in 1D systems remain elusive. Here we revisit theoretical underpinnings of anyons in 2D fractional quantum Hall (FQH) systems, whose signatures have been experimentally observed by independent groups. Invoking novel theorems about the Hopfion or $\mathbb{C}P^1$-model understood as flux quantization in 2-Cohomotopy, which retrodicts abelian FQH anyon properties in fine detail. Examining this theoretical model further, we find that islands of expelled (surplus) flux enlarge the ground-state monodromy from an abelian group to a (framed spherical) braid group, nonabelian for two or more islands, so that nonabelian anyonic states become topologically admissible; superconducting islands are one candidate realization of such flux expulsion.
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2602.18748 · 21 Feb · 7 pages, 2 figures; v2 (published version): wording adjusted to clarify, first, that (surplus) flux-expulsion is the required condition, while superconductivity is just the canonical candidate realization, and, second, that the result is lifting of the topological obstruction for nonabelian anyons and classification of their reps, not prediction of a specific braiding rep

Abstract PDF $QNTM ×2$ATTN ×1$HALL ×1$HTRO ×1 +3

Quantum Physics · Cross-listed from cs.DC

What Distributed Computing Got Wrong: The Category Mistake That Turned Design Choices into Laws of Nature

Paul Borrill

The foundational impossibility results of distributed computing – the Fischer-Lynch-Paterson theorem, the Two Generals Problem, the CAP theorem – are widely understood as discoveries about the physical limits of coordination. This paper argues that they are nothing of the sort. They are consequences of a category mistake: treating Forward-In-Time-Only (FITO) information flow as a law of nature rather than recognizing it as a design choice inherited from Shannon's channel model and Lamport's happened-before relation. We develop this argument in six steps. First, we introduce the category mistake framework from Ryle through Spekkens' ontic/epistemic distinction in quantum foundations. Second, we identify FITO as the hidden axiom that unifies the classical impossibility results. Third, we apply Spekkens' Leibnizian principle to show that FITO-based models contain surplus ontological structure. Fourth, we develop the counterfactual: what changes when FITO is dropped. Fifth, we demonstrate that the impossibility theorems are theorems about FITO systems, not about physics. Sixth, we sketch the transactional alternative – bilateral interactions that dissolve the apparent impossibilities by replacing unidirectional message passing with atomic bilateral transactions. The implication is that distributed computing has spent fifty years optimizing within the wrong design space.
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Quantum Physics

Higher-order circuits

Matt Wilson

We write down a series of basic laws for (strict) higher-order circuit diagrams. More precisely, we define higher-order circuit theories in terms of: (a) nesting, (b) temporal and spatial composition, and (c) equivalence between lower-order bipartite processes and higher-order bipartite states. In category-theoretic terms, these laws are expressed using enrichment and cotensors in symmetric polycategories, along with a frobenius-like coherence between them. We describe how these laws capture the salient features of higher-order quantum theory, and discover an upper bound for higher-order circuits: any higher-order circuit theory embeds into the theory of strong profunctors.
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Friday 20 February

Quantum Physics

Auto Quantum Machine Learning for Multisource Classification

Tomasz Rybotycki, Sebastian Dziura and Piotr Gawron

With fault-tolerant quantum computing on the horizon, there is growing interest in applying quantum computational methods to data-intensive scientific fields like remote sensing. Quantum machine learning (QML) has already demonstrated potential for such demanding tasks. One area of particular focus is quantum data fusion – a complex data analysis problem that has attracted significant recent attention. In this work, we introduce an automated QML (AQML) approach for addressing data fusion challenges. We evaluate how AQML-generated quantum circuits perform compared to classical multilayer perceptrons (MLPs) and manually designed QML models when processing multisource inputs. Furthermore, we apply our method to change detection using the multispectral ONERA dataset, achieving improved accuracy over previously reported QML-based change detection results.
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Quantum Physics · Cross-listed from cs.DC

The Category Mistake of Cislunar Time: Why NASA Cannot Synchronize What Doesn't Exist

Paul Borrill

In April 2024, the White House directed NASA to establish Coordinated Lunar Time (LTC) by December 2026. The programme assumes that a unified time standard can be constructed by deploying atomic clocks on the lunar surface, computing relativistic corrections, and distributing synchronized time via LunaNet. This paper argues that the entire enterprise rests on a category mistake in the sense introduced by Ryle and developed by Spekkens in quantum foundations: it treats "synchronized time" as an ontic entity – something that exists independently and can be transmitted from authoritative sources to dependent receivers – when it is in fact an epistemic construct: a model-dependent representation of observer-relative clock relationships. We analyze the cislunar time programme through the lens of Forward-In-Time-Only (FITO) assumptions, Spekkens' Leibnizian operationalism, the Wood-Spekkens fine-tuning argument, and the distinction between ontic and epistemic interpretations that has dissolved long-standing puzzles in quantum mechanics. We show that the same conceptual move that dissolves quantum "mysteries" – recognizing what is epistemic versus what is ontic – dissolves the apparent coherence of the cislunar time programme and reveals it as an engineering project built on a philosophical confusion. We sketch a transactional alternative grounded in bilateral atomic interactions rather than unidirectional time distribution.
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Quantum Physics

Four- and six-photon stimulated Raman transitions for coherent qubit and qudit operations

Gabriel J. Gregory, Evan R. Ritchie, Alex Quinn et al. (+4 more)

Quantum computers are typically composed of an array of two-level systems, or qubits, encoded in some information carrier, such as an electron, photon, or quantized circuit. The size of this array is restricted by finite access to resources like laser power, cooling capacity, and control lines for trapping and manipulation. Under these constraints, the system's processing power can be increased by using more energy levels per information carrier, but common techniques for qubit control provide only limited connectivity between these additional states. We experimentally demonstrate transitions between electronic angular momentum states with a difference in magnetic quantum numbers $\Delta \mathrm{m_J} = $ 3, 4, and 5 via resonant four- and six-photon stimulated Raman transitions in a single trapped atom. Derivation of the corresponding Rabi frequencies, which are verified experimentally, follows the standard treatment of two-photon transitions including the adiabatic elimination of intermediate states. Finally, we discuss pathways to increase the observed multi-photon transition fidelities to $>99.99\%$, providing a tool for efficient, high-fidelity control of qudits and single-atom logical qubits.
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All authors Gabriel J. Gregory, Evan R. Ritchie, Alex Quinn, Sean Brudney, David J. Wineland, David T. C. Allcock, Jameson O'Reilly

Quantum Physics

Controlling emergent dynamical behavior via phase-engineered strong symmetries

Marc Nairn, Beatriz Olmos and Parvinder Solanki

Symmetry constraints provide a powerful means to control the dynamics of open quantum systems. However, the set of accessible control parameters is often limited. Here, we show that a tunable phase in the collective light-matter coupling of a cavity QED system induces a phase-dependent strong symmetry of the Liouvillian, enabling dynamical control of the open quantum system evolution. We demonstrate that tuning this phase substantially reduces the critical driving strength for dissipative phase transitions between stationary and non-stationary phases. We illustrate this mechanism in two experimentally relevant cavity QED settings: a two-species ensemble of two-level atoms and a single-species ensemble of three-level atoms. Our results establish phase control as a versatile tool for engineering dissipative phase transitions, with implications for quantum state preparation.
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Quantum Physics

Engineering quantum criticality and dynamics on an analog-digital simulator

Alexandra A. Geim, Nazli Ugur Koyluoglu, Simon J. Evered et al. (+14 more)

Understanding emergent phenomena in out-of-equilibrium interacting many-body systems is an exciting frontier in physical science. While quantum simulators represent a promising approach to this long-standing problem, in practice it can be challenging to directly realize the required interactions, measure arbitrary observables, and mitigate errors. Here we use coherent mapping between the Rydberg and hyperfine qubits in a neutral atom array simulator to engineer and probe complex quantum dynamics. We combine efficient analog dynamics with fully programmable state preparation and measurement, leverage non-destructive readout for loss information and atomic qubit reuse, and use an atom reservoir for replacing lost atoms. With this analog-digital approach, we first demonstrate dynamical engineering of ring-exchange and particle hopping dynamics via Floquet driving and measure the spectral function of single excitations by evolving initial superposition states. Extending these techniques to a 271-site kagome lattice, we employ closed-loop optimization to target an out-of-equilibrium critical quantum spin liquid of the Rokhsar-Kivelson type. We observe the key features of such a state, including the absence of local order, many-body coherences between nearly equal-amplitude dimer configurations over up to 18 sites, and universal correlations consistent with predictions from field theory. Together, these results pave the way for using dynamical control in analog-digital quantum simulators to study complex quantum many-body systems.
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All authors Alexandra A. Geim, Nazli Ugur Koyluoglu, Simon J. Evered, Rahul Sahay, Sophie H. Li, Muqing Xu, Dolev Bluvstein, Nik O. Gjonbalaj, Nishad Maskara, Marcin Kalinowski, Tom Manovitz, Ruben Verresen, Susanne F. Yelin, Johannes Feldmeier, Markus Greiner, Vladan Vuletic, Mikhail D. Lukin

Quantum Physics

Time uncertainty and fundamental sensitivity limits in quantum sensing: application to optomechanical gravimetry

Salman Sajad Wani, Saif Al-Kuwari, Arshid Shabir et al. (+3 more)

High-sensitivity accelerometers and gravimeters, achieving the ultimate limits of measurement sensitivity are key tools for advancing both fundamental and applied physics. While numerous platforms have been proposed to achieve this goal, from atom interferometers to optomechanical systems, all of these studies neglect the effects of intrinsic quantum uncertainty in time estimation. Starting from the Hamiltonian of a generic linear quantum sensor, we derive the two-parameter quantum Fisher information matrix and establish the corresponding Cram'er-Rao bound, treating time as an uncertain (nuisance) parameter. Our analysis reveals a fundamental coupling between time and signal estimation that inherently degrades measurement sensitivity, with the standard single-parameter quantum limit recovered only at specific interrogation times or under special decoupling conditions. We then apply these results to an optomechanical gravimeter and explicitly derive an optimal decoupling condition under which the effects of time uncertainty are averaged out in a continuous measurement scheme. Our approach is general and can be readily extended to a broad class of quantum sensors.
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All authors Salman Sajad Wani, Saif Al-Kuwari, Arshid Shabir, Paolo Vezio, Francesco Marino, Mir Faizal

Quantum Physics

Participation Ratio as a Quantum Probe of Hierarchical Stickiness

Ariel A. Galindo Duque, Miguel A. Prado Reynoso, Miguel Gonzalez et al. (+1 more)

We investigate how quantum localization encodes the hierarchical stickiness that governs transport in mixed classical phase spaces. Using the periodically driven kicked top, we show that the participation ratio (PR) of coherent states in the Floquet eigenbasis resolves the same layered structure that appears classically as a multimodal distribution of finite-time Lyapunov exponents (FTLEs). To establish a quantitative correspondence, we introduce a Gaussian coarse graining of the FTLE matched to the intrinsic semiclassical resolution of coherent states. Both local correlations and global comparisons of probability distributions demonstrate that quantum and classical indicators agree optimally within a finite window of evolution times, where sticky structures are most clearly resolved. Our results promote the participation ratio from a global measure of chaos to a sensitive probe of hierarchical transport and provide a practical route for diagnosing anomalous localization in driven quantum systems.
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All authors Ariel A. Galindo Duque, Miguel A. Prado Reynoso, Miguel Gonzalez, Jorge G. Hirsch

Quantum Physics

Qubit error bursts in superconducting quantum processors of Quantum Inspire: quasiparticle pumping and anomalous time dependence

G. R. Di Carlo, M. Samiotis, A. Kamlapure et al. (+9 more)

We investigate qubit error bursts in 5- and 7-transmon processors of similar design, fabrication and packaging, but with different types of qubit Josephson junctions. Measurements for each are performed in two refrigerators to discern device-specific from refrigerator-dependent characteristics. The duration and rate of bursts are device specific but within the range of prior experiments and consistent with ionizing radiation. We observe two unforeseen signatures specifically in the processor with Dolan junctions. First, increasing the rate of $\pi$ pulsing in the detection scheme shortens the recovery time to equilibrium, which is explained by a quasiparticle pumping mechanism. The second signature is an anomalous time dependence in the burst rate: a surge happens days or weeks after cooldown, followed by a strong suppression that persists until thermal cycling.
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All authors G. R. Di Carlo, M. Samiotis, A. Kamlapure, M. Finkel, N. Muthusubramanian, M. W. Beekman, N. Haider, M. S. Moreira, J. F. Marques, B. Segers, S. Vallés-Sanclemente, L. DiCarlo

Quantum Physics

Multipartite Bell-Greenberger-Horne-Zeilinger nonclassicality from interwoven frustrated down-conversion

Marek Żukowski, Paweł Cieśliński, Marcin Markiewicz et al. (+1 more)

We present a theory of an interference process that starts with $N$ coherently pumped two-mode parametric down-conversion (PDC) sources, whose output modes are directed to $N$ observers such that each observer receives modes from two different source crystals. Each observation station is equipped with a locally controlled PDC crystal, coherently pumped with the source crystals, whose output modes are perfectly aligned with the input modes from the source PDCs. By varying the local phases of the input modes, perfect $2N$-photon interference can be observed in $2N$ single-photon registrations, one in each output mode of these $N$ local PDCs. The interference results from the indistinguishability of the origins of the detected $2N$ photons: either they all originate from the source PDCs or from the local PDCs. Bell-GHZ nonclassicality of the process emerges when one also considers situations in which at least one of the local PDC processes is blocked. In such cases, the $2N$-photon interference disappears. A ”lifted“ Clauser-Horne inequality is violated when its sole negative term, involving all observers with all local pumps active, is tuned to maximal destructive interference, while all other terms involve settings in which one of the local pumps is off.
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All authors Marek Żukowski, Paweł Cieśliński, Marcin Markiewicz, Konrad Schlichtholz

Quantum Physics

Theory and interpretability of Quantum Extreme Learning Machines: a Pauli transfer matrix approach

Markus Gross and Hans-Martin Rieser

Quantum reservoir computers (QRCs) have emerged as a promising approach to quantum machine learning, since they utilize the natural dynamics of quantum systems for data processing and are simple to train. Here, we consider $n$-qubit quantum extreme learning machines (QELMs) with initial-state encoding and continuous-time reservoir dynamics. Applying the Pauli transfer matrix (PTM) formalism reveals the encoding-generated features and their linear transformation by quantum channels, and thereby provides a nonlinear vector autoregression model as an interpretable classical representation of a QELM. We show that the nonlinear information processing capacity of a QELM factorizes into the ability of the encoding to represent a given target function and the decodability of the relevant Pauli features after the effective PTM. Comparing geometric and statistical decodability reveals features that are algebraically hidden yet recoverable through data correlations. We relate the decodability of Pauli features to the spreading of measurement operators under Heisenberg evolution (Krylov growth). Focusing on the unitary channel, we discuss how structured Hamiltonians, when paired with certain observables, can impair model performance, as reflected in a low rank of the readout and the effective PTMs. We trace this effect to Hamiltonian symmetries and derive asymptotic rank estimates for symmetry-resolved observable families. Our decodability analysis disentangles the contributions of encoding, reservoir dynamics, and measurement observables, and thus provides interpretable diagnostics that can inform the design of QELMs and reduce reliance on exhaustive hyperparameter search. As a specific application, we consider forecasting nonlinear dynamical systems and show that the readout of a trained QELM closely matches the expression of the underlying flow map at leading orders.
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Quantum Physics

Interaction-resolved decomposition of multi-qubit unitaries via computational-basis phases

Bora Baran, Tommaso Calarco, Matthias M. Mueller et al. (+1 more)

In multi-qubit quantum control, target unitary operations are commonly specified through full-unitary target descriptions and assessed through global comparison measures. In this work, we introduce an interaction-resolved decomposition of n-qubit unitaries that provides explicit access to their many-body interaction structure through computational-basis phases collected in a diagonalizing frame. Such a frame is conveniently given by local rotations for many operationally relevant operations, including gates generated by single Pauli strings or commuting sets of Pauli strings, such as stabilizer operations, controlled-phase gates, Toffoli-type operations, and Ising interactions. We derive parity-weighted sums of these computational-basis phases that exactly and uniquely resolve k-body interaction terms supported on arbitrary qubit subsets, which we term support-selective phase invariants. These invariants provide an interaction-resolved coordinate system that organizes unitary operations according to their multipartite interaction structure, giving direct access to local, pairwise, tripartite, and general k-partite interaction content underlying entangling operations. This enables the formulation of selective quantum optimal control targets for synthesizing desired combinations of many-body interactions. We supplement this with numerical demonstrations for a representative hardware model, a realistic nitrogen-vacancy spin register, where we synthesized isolated tripartite interactions up to local equivalence within a single control pulse, guided by these invariants, for both diagonal (ZZZ) and non-diagonal (XZZ) terms.
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All authors Bora Baran, Tommaso Calarco, Matthias M. Mueller, Felix Motzoi

Quantum Physics · Cross-listed from cs.IT

Quantum Maximum Likelihood Prediction via Hilbert Space Embeddings

Sreejith Sreekumar and Nir Weinberger

Maximum likelihood prediction (MLP) is a core task at the heart of modern large language models. Here, we study a quantum version of this task for a simplified data model consisting of independent and identically distributed samples, as a first step. The quantum maximum likelihood predictor (QMLP) is obtained by embedding of empirical probability distributions into quantum states and performing a minimization of quantum relative entropy over a given class of states. We derive non-asymptotic performance guarantees for QMLP in terms of convergence rates and concentration inequalities, both in trace norm and quantum relative entropy. Our approach provides a unified framework to handle MLP within both classical and quantum LLMs. We also consider the related problem of quantum information projection and generalize the quantum Pythagorean theorem to mixture families specified by possibly non-self-adjoint linear constraints. We further show that the Pythagorean inequality continues to hold in the infinite-dimensional setting whenever the convex information-projection problem attains a finite minimum.
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Quantum Physics

Improving Single Excitation Fidelity in Rydberg Superatoms for Efficient Single Photon Emission

Vidisha Aggarwal, Boxi Li, Eloisa Cuestas et al. (+4 more)

Deterministic single photon emission from a Rydberg ensemble coupled to an optical cavity requires high-fidelity preparation of collective single excitations. In such a setup imperfect Rydberg blockade can lead to unwanted double excitations, which degrade photon indistinguishability. In this work we adapt the Derivative Removal by Adiabatic Gate (DRAG) technique, originally developed for superconducting qubits, to shape optical pulses that suppress double excitations in this atomic platform. By combining analytical modeling with numerical optimization, DRAG provides an improvement over conventional sine-squared pulses. Further optimization of pulse duration and atomic ensemble size identifies a parameter regime, distinct from that used in [Nature Photonics 17, 688 (2023)], that enhances the single excitation probability from the previous theoretical benchmark of 77% to 91.9%, approaching the fundamental limits set by decoherence in the system. Benchmarking against GRAPE (Gradient Ascent Pulse Engineering) confirms that DRAG operates close to the optimal control limit, while maintaining smooth, experimentally feasible pulse shapes. These results demonstrate the effectiveness and cross platform adaptability of DRAG for a high-fidelity single photon source.
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All authors Vidisha Aggarwal, Boxi Li, Eloisa Cuestas, Tommaso Calarco, Robert Zeier, Alexei Ourjoumtsev, Felix Motzoi

Quantum Physics

Quantum-enhanced phase sensitivity in an all-fiber Mach-Zehnder interferometer

Romain Dalidet, Sébastien Tanzilli, Gregory Sauder et al. (+3 more)

Recent advances in quantum photonics have enabled increasingly robust protocols in optical phase estimation, achieving precisions beyond the standard quantum limit and approaching the Heisenberg limit. While intrinsic losses hinder the realization of unconditional super-sensitivity, reaching quantum advantage, defined as sensitivity surpassing that of any classical counterpart with identical resources, remains achievable. Here we experimentally demonstrate such an advantage using a fully fibered Mach-Zehnder-type interferometer operating at telecom wavelengths, free of post-selection. The scheme relies on the conversion of polarization-entangled photon pairs, a degree of freedom commonly favored for experimental convenience, into energy-time entanglement, which is particularly well suited for scalable fiber-based sensors. All system imperfections, including asymmetric losses and detector inefficiencies, are accounted for in the Fisher information analysis, yielding a measured quantum advantage of 10%. This result highlights the practicality of compact, alignment-free quantum interferometers for real-world sensing applications.
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All authors Romain Dalidet, Sébastien Tanzilli, Gregory Sauder, Nicolas Fabre, Laurent Labonté, Anthony Martin

Quantum Physics

Quantum-enhanced satellite image classification

Qi Zhang, Anton Simen, Carlos Flores-Garrigós et al. (+7 more)

We demonstrate the application of a quantum feature extraction method to enhance multi-class image classification for space applications. By harnessing the dynamics of many-body spin Hamiltonians, the method generates expressive quantum features that, when combined with classical processing, lead to quantum-enhanced classification accuracy. Using a strong and well-established ResNet50 baseline, we achieved a maximum classical accuracy of 83%, which can be improved to 84% with a transfer learning approach. In contrast, applying our quantum-classical method the performance is increased to 87% accuracy, demonstrating a clear and reproducible improvement over robust classical approaches. Implemented on several of IBM's quantum processors, our hybrid quantum-classical approach delivers consistent gains of 2-3% in absolute accuracy. These results highlight the practical potential of current and near-term quantum processors in high-stakes, data-driven domains such as satellite imaging and remote sensing, while suggesting broader applicability in real-world machine learning tasks.
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All authors Qi Zhang, Anton Simen, Carlos Flores-Garrigós, Gabriel Alvarado Barrios, Paolo A. Erdman, Enrique Solano, Aaron C. Kemp, Vincent Beltrani, Vedangi Pathak, Hamed Mohammadbagherpoor

Quantum Physics

A Fine-Grained and Efficient Reliability Analysis Framework for Noisy Quantum Circuits

Jindi Wu, Tianjie Hu and Qun Li

Evaluating the reliability of noisy quantum circuits is essential for implementing quantum algorithms on noisy quantum devices. However, current quantum hardware exhibits diverse noise mechanisms whose compounded effects make accurate and efficient reliability evaluation challenging. While state fidelity is the most faithful indicator of circuit reliability, it is experimentally and computationally prohibitive to obtain. Alternative metrics, although easier to compute, often fail to accurately reflect circuit reliability, lack universality across circuit types, or offer limited interpretability. To address these challenges, we propose a fine-grained, scalable, and interpretable framework for efficient and accurate reliability evaluation of noisy quantum circuits. Our approach performs a state-independent analysis to model how circuit reliability progressively degrades during execution. We introduce the Noise Proxy Circuit (NPC), which removes all logical operations while preserving the complete sequence of noise channels, thereby providing an abstraction of cumulative noise effects. Based on the NPC, we define Proxy Fidelity, a reliability metric that quantifies both qubit-level and circuit-level reliability. We further develop an analytical algorithm to estimate Proxy Fidelity under depolarizing, thermal relaxation, and readout error channels. The proposed framework achieves fidelity-level reliability estimation while remaining execution-free, scalable, and interpretable. Experimental results show that our method accurately estimates circuit fidelity, with an average absolute difference (AAD) ranging from 0.031 to 0.069 across diverse circuits and devices.
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Quantum Physics

Universal Protection of Quantum States from Decoherence

Francesco Atzori, Salvatore Virzì, Francesco Devecchi et al. (+6 more)

The fragility of quantum coherence fundamentally limits the scalability of quantum technologies, as unavoidable environmental interactions induce decoherence and rapidly degrade quantum properties. The Quantum Zeno Effect offers a powerful route to suppress quantum evolution and protect coherence through frequent measurements, irrespective of the underlying dynamics. However, existing implementations require prior knowledge of the quantum state, severely restricting their applicability. Here we introduce a state- and dynamics-independent protection protocol embedding the system in a larger Hilbert space, temporarily swapping the quantum information from its original degree of freedom to a decoherence-free ancillary one. We experimentally validate the protocol on a quantum optical platform, demonstrating robust preservation of coherence and purity for arbitrary polarization qubits under decoherence, thereby enabling the universal safeguarding of unknown quantum states.
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All authors Francesco Atzori, Salvatore Virzì, Francesco Devecchi, Domenico Abbondandolo, Alessio Avella, Fabrizio Piacentini, Marco Gramegna, Ivo Pietro Degiovanni, Marco Genovese

Quantum Physics

Instability as a Quantum Resource

Goni Yoeli and Gilad Gour

We consolidate coherence, athermality, and nonuniformity as sub-resources within an underlying quantum resource theory: instability. We formulate instability axiomatically as the transient information within a decaying physical system. Specifying a decay mechanism (e.g., dephasing, thermalization) recovers these familiar resources as specific manifestations of instability. We compute the one-shot distillation yield and dilution cost in various operational paradigms, and use them to pin down the extremal additive monotones. In the asymptotic regime, we show that all conversion rates are governed by a single additive monotone, and thereby establish a universal second law for instability.
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Quantum Physics

Impossibility of Refrigeration and Engine Operation in Minimal Qubit Repeated-Interaction Models

Gabrielle Barsky-Giles, Alessandro Prositto, Matthew Gerry et al. (+1 more)

We investigate the operation of a qubit as a quantum thermal device within the repeated interaction framework, allowing for strong system-bath coupling and finite interaction times. We analyze two minimal models: an alternating-coupling setup, in which the qubit sequentially interacts with hot and cold baths, and a simultaneous-coupling setup, where both baths interact with the qubit during each collision. For the alternating model, we obtain an exact analytical solution for the limit-cycle state, valid for arbitrary coupling strengths and collision durations. Using this solution, we rigorously prove a no-go theorem for quantum refrigeration. We further demonstrate that, although work can be generated locally at individual system-bath contacts, the total work over a cycle is always nonpositive, precluding engine operation. In the absence of work, the model describes pure heat conduction, for which we derive a closed-form expression for the heat current and show that it exhibits a nonmonotonic turnover behavior. The simultaneous-coupling model is analyzed perturbatively. In the short-collision-time limit, it reproduces the same steady-state behavior as the alternating model, reinforcing the generality of the constraints identified. Our results establish fundamental limitations on qubit-based quantum thermal machines operating under Markovian repeated interactions and highlight the need for enriched models to realize functional quantum thermal devices.
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All authors Gabrielle Barsky-Giles, Alessandro Prositto, Matthew Gerry, Dvira Segal

Quantum Physics · Cross-listed from nucl-th

Quantum stress and torsion distributions in the deuteron

Wim Cosyn, Adam Freese and Alan Sosa

Stress distributions in the deuteron are related to form factors of the asymmetric energy-momentum tensor through three-dimensional Fourier transforms. There are eleven such form factors, which we calculate in an impulse approximation. We compare the obtained form factors to prior results for the six form factors that have been previously calculated. We then elaborate on the formalism for relating the form factors to internal distributions of mass, mass flux, momentum, stresses, and forces, and obtain results for all of these distributions. We obtain the principal stresses for the symmetric part of the stress tensor, and show that the antisymmetric part describes reorientation of fermion spin by torsion stress when the nucleon moves between the S- and D-waves. Force distributions in the nucleons depend on the so-called non-conserved form factors through the Cauchy momentum equation, and are non-radial owing to the presence of tensor forces and spin-orbit coupling.
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Quantum Physics

Higher-order spatial photon interference versus dipole blockade effect

Arthur Rotari and Mihai A. Macovei

The steady-state quantum dynamics of three dipole-dipole coupled two-level emitters, fixed at the vertices of an equilateral triangle, and interacting via the environmental thermostat is investigated. We have analytically obtained the populations of the involved three-atom cooperative states as well as of the second- and third-order spatial photon correlation functions of the light scattered by the few-qubit sample. As a consequence, we have demonstrated that this incoherently excited system spontaneously generates streams of single photons possessing sub-Poissonian photon statistics. In analogy to the dipole-dipole blockade, one may expect that at smaller inter particle distances, compared to the photon emission wavelength, the reported phenomenon has the same origin. However, we have shown that the quantum photon features are due to the interaction's nature of the few symmetrically arranged two-level emitters with the surrounding thermal reservoir. Respectively, at larger atomic intervals the effect occurs because of high-order spatial interference phenomena. Sub-wavelength interference fringes can be observed too, via measurements of spatial higher-order photon correlation functions.
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