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Wednesday 22 October

Quantum Physics · Cross-listed from cond-mat.str-el

Dimensionality-Changing Transition from a Non-Fermi Liquid to a Spin-Solid in a Multichannel Kondo Lattice

Simon Martin, Marcin Raczkowski, Fakher F. Assaad et al. (+1 more)

A multichannel Kondo system, where a single quantum spin couples to multiple channels of an electronic bath, provides one of the simplest examples of a zero-dimensional non-Fermi liquid. It is natural to ask: what happens when an extensive number of such systems are coupled together? A simple renormalization group argument implies that in a chain of SU(N) multichannel quantum systems, where each spin is coupled to its own bath of K channels, the individual spins dynamically decouple at low energy when N>K, resulting in a 'sliding' non-Fermi liquid. Using Quantum Monte Carlo (QMC) simulations, we find evidences of a continuous, 'dimensionality-changing' phase transition out of this non-Fermi liquid into a valence-bond solid phase as the intersite coupling is increased. Remarkably, at the critical point, correlations exhibit a power-law behavior even along the direction in which the spins are coupled, indicating the breakdown of dynamical decoupling at the transition. We also develop an RG scheme to understand the universal aspects of this transition.
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All authors Simon Martin, Marcin Raczkowski, Fakher F. Assaad, Tarun Grover

Quantum Physics

Mind the gaps: The fraught road to quantum advantage

Jens Eisert and John Preskill

Quantum computing is advancing rapidly, yet substantial gaps separate today's noisy intermediate-scale quantum (NISQ) devices from tomorrow's fault-tolerant application-scale quantum (FASQ) machines. We identify four related hurdles along the road ahead: (i) from error mitigation to active error detection and correction, (ii) from rudimentary error correction to scalable fault tolerance, (iii) from early heuristics to mature, verifiable algorithms, and (iv) from exploratory simulators to credible advantage in quantum simulation. Targeting these transitions will accelerate progress toward broadly useful quantum computing.
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Quantum Physics · Cross-listed from hep-th

Probing the Hierarchy of Genuine Multipartite Entanglement with Generalized Latent Entropy

Byoungjoon Ahn, Jaydeep Kumar Basak, Keun-Young Kim et al. (+3 more)

We introduce generalization of the recently proposed Latent Entropy (L-entropy) \cite{Basak:2024uwc} as a refined measure of genuine multipartite entanglement (GME) in pure states of $n$-party quantum systems. Generalized L-entropy provides a natural ordering among $k$-uniform states, maximising for absolutely maximally entangled states (AME), effectively capturing the hierarchical structure of multipartite entanglement. We analyze the behavior of this measure for $n$-party Haar-random states and demonstrate that, in the large local-dimension limit, the maximal L-entropy saturates its upper bound for odd $n$, while for even $n$ it approaches the bound asymptotically. Furthermore, we apply this framework to examine multipartite entanglement properties of quantum states in several variants of the Sachdev-Ye-Kitaev (SYK) model, including SYK$_4$, SYK$_2$, mass-deformed SYK, sparse SYK, and $\mathcal{N}=2$ supersymmetric SYK model. The results demonstrate that the generalized L-entropy serves as a sensitive probe of multipartite entanglement, revealing how deformations influence quantum entanglement structure in such strongly interacting systems.
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All authors Byoungjoon Ahn, Jaydeep Kumar Basak, Keun-Young Kim, Gwon Bin Koo, Vinay Malvimat, Junggi Yoon

Quantum Physics · Cross-listed from hep-ph

Beyond Qubits: Multilevel Quantum Sensing for Dark Matter

Xiaolin Ma, Volodymyr Takhistov, Norikazu Mizuochi et al. (+1 more)

Quantum sensing with qubits has advanced fundamental physics searches, but higher dimensional systems offer untapped potential. We present a universal qutrit framework that yields a sequence-independent fourfold increase in quantum Fisher information and a twofold gain in sensitivity. In ultralight dark matter searches, spin-1 NV-center qutrits can enhance the axion-electron coupling reach by an order of magnitude beyond qubits. This principle applies broadly to multilevel quantum systems including superconducting, neutral atom and trapped-ion qutrits, establishing higher dimensional sensing as a powerful tool for probing new physics.
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All authors Xiaolin Ma, Volodymyr Takhistov, Norikazu Mizuochi, Ernst David Herbschleb

Quantum Physics · Cross-listed from cond-mat.mtrl-sci

A unified picture of phonon anomalies in crystals and glasses

Alessio Zaccone

Phonon spectra in solids often display anomalies that defy the simple Debye law, most prominently the van Hove singularity in crystals and the boson peak in glasses. Although traditionally regarded as distinct, both features are increasingly recognized as sharing a common physical origin. In a recent work, G. Ding et al. (Nat. Phys. 2025) propose a resonant-damping model that unifies these anomalies within a single framework. By coupling phonon damping to vibrational softening, their theory explains why some materials exhibit van Hove peaks, others boson peaks, and many show both. This advance extends earlier ideas and theories of Baggioli and Zaccone on the competition between phonon propagation and damping, while also connecting to microscopic mechanisms such as nonaffine motions in glasses. The resonant-damping paradigm thus offers a promising step toward a unified understanding of vibrational anomalies across ordered and disordered solids.
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Quantum Physics

Quantum walks as a tool to design robust quantum batteries: the role of topology and chirality

Simone Cavazzoni, Giovanni Ragazzi, Paolo Bordone et al. (+1 more)

The maximum work that can be extracted from a quantum battery is bounded by the ergotropy of the system, which is determined by the spectral properties of the Hamiltonian. In this paper, we employ the formalism of quantum walks to investigate how the topology of the battery and the chirality of the Hamiltonian influence its performance as an energy storage unit. We analyze architectures of battery cells based on ring, complete, and wheel graph structures and analyze their behavior in the presence of noise. Our results show that these structures exhibit distinct ergotropy scaling, with the interplay between chirality and topology providing a tunable mechanism to optimize work extraction and enhance robustness against decoherence. In particular, chirality enhances ergotropy for complete quantum cells, without altering the linear scaling with size, whereas in ring cells, it bridges the performance gap between configurations with odd and even number of units. Additionaly, chirality may be exploited to force degeneracies in the Hamiltonian, a condition that can spare the ergotropy to vanish in the presence of pure dephasing. We conclude that topology and chirality are key resources for improving ergotropy, offering guidelines to optimize quantum energy devices and protocols.
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All authors Simone Cavazzoni, Giovanni Ragazzi, Paolo Bordone, Matteo G. A. Paris

Quantum Physics

Atomic-superfluid heat engines controlled by twisted light

Aritra Ghosh, Nilamoni Daloi and M. Bhattacharya

We theoretically propose a quantum heat engine using a setup consisting of a ring-trapped Bose-Einstein condensate placed in a Fabry-Pérot cavity where the optical field carries orbital angular momentum. We first show that the cavity-enhanced light-atom coupling leads to the emergence of polaritonic modes whose character can be reversibly switched between photonlike and phononlike by detuning sweeps, allowing work extraction governed by distinct reservoirs. We investigate the dependence of the engine efficiency on the orbital angular momentum. Beyond ideality, we discuss finite-time scenarios based on shortcuts to adiabaticity such that the efficiency retains its ideal-operation value, despite finite-time operation. Our analysis identifies orbital angular momentum as a control knob that can reconfigure the performance of such quantum heat engines.
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Quantum Physics · Cross-listed from physics.atom-ph

Single Sr Atoms in Optical Tweezer Arrays for Quantum Simulation

Veronica Giardini, Luca Guariento, Andrea Fantini et al. (+6 more)

We report on the realization of a platform for trapping and manipulating individual $^{88}$Sr atoms in optical tweezers. A first cooling stage based on a blue shielded magneto-optical trap (MOT) operating on the $^1S_0$ -> $^1P_1$ transition at 461 nm enables us to trap approximately $4\times 10^6$ atoms at a temperature of 6.8 mK. Further cooling is achieved in a narrow-line red MOT using the $^1S_0$ -> $^3P_1$ intercombination transition at 689 nm, bringing $4\times 10^5$ atoms down to 5 $\mu$K and reaching a density of $\approx 10^{10}$ cm$^{-3}$. Atoms are then loaded into 813 nm tweezer arrays generated by crossed acousto-optic deflectors and tightly focused onto the atoms with a high-numerical-aperture objective. Through light-assisted collision processes we achieve the collisional blockade, which leads to single-atom occupancy with a probability of about $50\%$. The trapped atoms are detected via fluorescence imaging with a fidelity of $99.986(6)\%$, while maintaining a survival probability of $97(2)\%$. The release-and-recapture measurement provides a temperature of $12.92(5)$ $\mu$K for the atoms in the tweezers, and the ultra-high-vacuum environment ensures a vacuum lifetime higher than 7 min. These results demonstrate a robust alkaline-earth tweezer platform that combines efficient loading, cooling, and high-fidelity detection, providing the essential building blocks for scalable quantum simulation and quantum information processing with Sr atoms.
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All authors Veronica Giardini, Luca Guariento, Andrea Fantini, Shawn Storm, Massimo Inguscio, Jacopo Catani, Giacomo Cappellini, Vladislav Gavryusev, Leonardo Fallani

Quantum Physics

Good quantum codes with addressable and parallelizable transversal non-Clifford gates

Virgile Guémard

In this work, we prove that for any $m>1$, there exists a family of good qudit quantum codes supporting transversal logical $\mathsf{C}^{m-1}\mathsf{Z}$ gates that can address specified logical qudits and be largely executed in parallel. Building on the family of good quantum error-correcting codes presented in He et al. (2025), which support addressable and transversal logical $\mathsf{CCZ}$ gates, we extend their framework and show how to perform large sets of gates in parallel. The construction relies on the classical algebraic geometry codes of Stichtenoth (IEEE Trans. Inf. Theory, 2006). Our results lead to a substantial reduction in the depth overhead of multi-control-$Z$ circuits. In particular, we show that the minimal depth of any logical $\mathsf{C}^{m-1}\mathsf{Z}$ circuit involving qudits from $m$ distinct code blocks is upper bounded by $O(k^{m-1})$, where $k$ is the code dimension. While this overhead is optimal for dense $\mathsf{C}^{m-1}\mathsf{Z}$ circuits, for sparse circuits we discuss how the depth overhead can be significantly reduced by exploiting the structure of the quantum code.
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Quantum Physics

Passive quantum error correction of photon loss at breakeven

Shruti Shirol, Sean van Geldern, Hanzhe Xi et al. (+1 more)

Physical qubits in a quantum computer are often represented by superposition states of single particles or excitations. Decay of the excitation itself is a fundamental error channel that is difficult to overcome via external drive or control techniques. Quantum error correcting codes, which encode information in superpositions involving multiple excitations, provide a path to preserve information beyond the capacity of individual excitations, but typically require exquisite active operations on the system. Here, we demonstrate a steady-state driven dissipative quantum system, composed of a superconducting cavity and a transmon ancilla, that preserves a logical qubit beyond the photon-lifetime limit by about 5% using a binomial encoding. This realization of continuous quantum error correction at the breakeven point highlights the quantitative competitiveness of passive correction strategies while circumventing some demanding hardware requirements of its active counterparts.
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All authors Shruti Shirol, Sean van Geldern, Hanzhe Xi, Chen Wang

Quantum Physics

Quantum Coherence in Superconducting Vortex States

Ameya Nambisan, Simon Günzler, Dennis Rieger et al. (+13 more)

Abrikosov vortices, where the superconducting gap is completely suppressed in the core, are dissipative, semi-classical entities that impact applications from high-current-density wires to superconducting quantum devices. In contrast, we present evidence that vortices trapped in granular superconducting films can behave as two-level systems, exhibiting microsecond-range quantum coherence and energy relaxation times that reach fractions of a millisecond. These findings support recent theoretical modeling of superconductors with granularity on the scale of the coherence length as tunnel junction networks, resulting in gapped vortices. Using the tools of circuit quantum electrodynamics, we perform coherent manipulation and quantum non-demolition readout of vortex states in granular aluminum microwave resonators, heralding new directions for quantum information processing, materials characterization, and sensing.
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All authors Ameya Nambisan, Simon Günzler, Dennis Rieger, Nicolas Gosling, Simon Geisert, Victor Carpentier, Nicolas Zapata, Mitchell Field, Milorad V. Milošević, Carlos A. Diaz Lopez, Ciprian Padurariu, Björn Kubala, Joachim Ankerhold, Wolfgang Wernsdorfer, Martin Spiecker, Ioan M. Pop

Quantum Physics

A simplified version of the quantum OTOC$^{(2)}$ problem

Robbie King, Robin Kothari, Ryan Babbush et al. (+4 more)

This note presents a simplified version of the OTOC$^{(2)}$ problem that was recently experimentally implemented by Google Quantum AI and collaborators. We present a formulation of the problem for growing input size and hope this spurs further theoretical work on the problem.
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All authors Robbie King, Robin Kothari, Ryan Babbush, Sergio Boixo, Kostyantyn Kechedzhi, Thomas E. O'Brien, Vadim Smelyanskiy

Quantum Physics

Explicitly Quantum-parallel Computation by Displacements

Uchenna Chukwu, Mohammad-Ali Miri and Nicholas Chancellor

We introduce an encoding of information in the relative displacement or photon number of different optical modes. Since the loss rate to interference is insensitive to squeezing and many non-Gaussian fluctuations, such a space is relatively protected from imperfections. We show that photon subtraction protocols can be used to create high-quality quantum superpositions of squeezed states with much higher fidelity than when the protocol is restricted to producing only cat states (superpositions of coherent states). We also show that the amount of squeezing and anti-squeezing introduced is moderate, and unlikely to dominate the photon number. This parallel processing allows for explicit use of non-Gaussian interference as opposed to the more incidental role played by non-Gaussianity in all-optical coherent Ising machines. A key observation we make is that displacements of optical states provide a convenient degree of freedom to encode information for quantum parallel processing. Furthermore, we discuss important considerations for realizing an optical quantum annealer based on differential photon number encoding. In particular, we discuss the need to perform quantum erasure on loss channels from interference, as well as the ability to correct degrees of freedom not used for the encoding without disrupting the processed quantum information.
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Quantum Physics

Entanglement production in the decay of a metastable state

Sergei Khlebnikov

When a metastable state decays into radiation, there must be entanglement between the radiation and the decaying system, as well as between radiation collected at late and early times. We study the interplay between these two types of entanglement in simple Gaussian models in the Markov approximation. We define, via a windowed Fourier transform, multimode quantum states associated with radiation fragments produced at different times and compute the corresponding entanglement entropy increments. On the basis of these results, we argue that such entropy increments are useful entanglement measures, especially in cases, such as Hawking radiation, where one wishes to separate the radiation into “old” and “new.”
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Quantum Physics

Classical theories of gravity produce entanglement

Joseph Aziz and Richard Howl

The unification of gravity and quantum mechanics remains one of the most profound open questions in science. With recent advances in quantum technology, an experimental idea first proposed by Richard Feynman is now regarded as a promising route to testing this unification for the first time. The experiment involves placing a massive object in a quantum superposition of two locations and letting it gravitationally interact with another mass. In modern versions of the experiment, if the two objects subsequently become entangled, this is considered unambiguous evidence that gravity obeys the laws of quantum mechanics. This conclusion derives from theorems that treat a classical gravitational interaction as a local interaction capable of only transmitting classical, not quantum, information. Here, we argue that the classical gravitational interaction can transmit quantum information, and thus generate entanglement through physically local processes. The effects are found to scale differently to the considered quantum gravity effect, providing information on the form of the experiment required to evidence the quantum nature of gravity.
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Quantum Physics

Addressing spins at the clock transitions with a frequency- and bandwidth-tunable superconducting resonator

Yutian Wen, V. Ranjan, T. Lorriaux et al. (+5 more)

Solid-state spin ensembles addressed via superconducting circuits are promising candidates for quantum memory applications, offering multimodal storage capability and second-long coherence times at their clock transition. Implementing practical memory schemes requires dynamic control over both the resonator frequency and bandwidth. In this letter, we report measurements of a superconducting resonator whose frequency can be tuned by passing a DC current through the high-kinetic-inductance thin film, and whose bandwidth can be tuned by parametric coupling to a low-Q buffer resonator. Using this resonator, we address an ensemble of bismuth donors at their clock transition, measuring a Hahn-echo coherence time of 450 ms. We demonstrate RF driving of the bismuth donor hyperfine transitions, as well as dynamic bandwidth control of the resonator.
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All authors Yutian Wen, V. Ranjan, T. Lorriaux, D. Vion, B. Huard, A. Bienfait, E. Flurin, P. Bertet

Quantum Physics

Universal bound on the Lyapunov spectrum of quantum master equations

Paolo Muratore-Ginanneschi, Gen Kimura, Frederik vom Ende et al. (+1 more)

The spectral properties of positive maps are pivotal for understanding the dynamics of quantum systems interacting with their environment. Furthermore, central problems in quantum information such as the characterization of entanglement can be reformulated in terms of spectral properties of positive maps. The present work aims to contribute to a better understanding of the spectrum of positive maps. Specifically, our main result is a new proof of a universal bound on the $d^{2}-1$ generically non vanishing decay rates $\Gamma_{i}$ of time-autonomous quantum master equations on a $d$-dimensional Hilbert space: $$\Gamma_{\mathrm{max}}\,\leq\,\varkappa_{d}\,\sum_{i=1}^{d^{2}-1}\Gamma_{i}$$ The prefactor $\varkappa_{d}$ %, which we explicitly determine, depends only on the dimension $d$ and varies depending on the sub-class of positive maps to which the semigroup solution of the master equation belongs. We provide a brief but self-consistent survey of these concepts. We obtain our main result by resorting to the theory of Lyapunov exponents, a central concept in the study of dynamical systems, control theory, and out-of-equilibrium statistical mechanics. We thus show that progress in understanding positive maps in quantum mechanics may require ideas at the crossroads between different disciplines. For this reason, we adopt a notation and presentation style aimed at reaching readers with diverse backgrounds.
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All authors Paolo Muratore-Ginanneschi, Gen Kimura, Frederik vom Ende, Dariusz Chruściński

Quantum Physics

Heisenberg-Limited Quantum Eigenvalue Estimation for Non-normal Matrices

Yukun Zhang, Yusen Wu and Xiao Yuan

Estimating the eigenvalues of non-normal matrices is a foundational problem with far-reaching implications, from modeling non-Hermitian quantum systems to analyzing complex fluid dynamics. Yet, this task remains beyond the reach of standard quantum algorithms, which are predominantly tailored for Hermitian matrices. Here we introduce a new class of quantum algorithms that directly address this challenge. The central idea is to construct eigenvalue signals through customized quantum simulation protocols and extract them using advanced classical signal-processing techniques, thereby enabling accurate and efficient eigenvalue estimation for general non-normal matrices. Crucially, when supplied with purified quantum state inputs, our algorithms attain Heisenberg-limited precision–achieving optimal performance. These results extend the powerful guided local Hamiltonian framework into the non-Hermitian regime, significantly broadening the frontier of quantum computational advantage. Our work lays the foundation for a rigorous and scalable quantum computing approach to one of the most demanding problems in linear algebra.
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Quantum Physics · Cross-listed from cond-mat.mes-hall

Single-Scale Magnetoelastic Landau Quantization: Thermodynamics, Quantum Oscillations, and Metrology

Denise Assafrão, Faizuddin Ahmed and Edilberto O. Silva

We develop a unified, single-scale description of thermodynamics and quantum oscillations in electronic systems with a uniform areal density of screw dislocations under a uniform magnetic field. A single tunable gap, $\hbar|\omega_{eff}|$ with $\omega_{eff}=\omega_{c}+\omega_{cl}$, organizes all equilibrium observables obtained from a compact harmonic-oscillator partition function: free energy, internal energy, entropy, heat capacity, magnetization, magnetic susceptibility, and magnetocaloric responses collapse onto universal hyperbolic kernels in $x=\hbar|\omega_{eff}|/(2k_{B}T)$. We identify a compensated-field regime where the transverse gap closes and the heat capacity reaches an equipartition plateau, providing a sharp signature of magnetoelastic interference. In transport and torque, the same scale rigidly shifts the Hall fan and compresses the $1/B$ period of de Haas-van Alphen and Shubnikov-de Haas oscillations when expressed in $1/B_{eff}$, enabling a phase-unwarping protocol that metrologizes the dislocation density from a single field sweep. In mesoscopic samples, boundary corrections to the Landau degeneracy generate finite-size calorimetric oscillations that diagnose the effective magnetic length. Moderate disorder and weak interactions preserve the kernel structure while smoothing amplitudes. We outline an experimental roadmap combining on-chip calorimetry, torque magnetometry, and transport, and discuss device-level opportunities in caloritronics and strain engineering, magnetocaloric microcooling, magnetoelastic heat switching, and dilatometric transduction, where the single scale $\hbar|\omega_{eff}|$ enables rational design and optimization.
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Quantum Physics

Twisted Superconducting Quantum Diodes for High Fidelity Anharmonic Qubits

Han Zhong, Denis Kochan, Igor Zutic et al. (+1 more)

In superconducting circuits, bidirectional signal propagation can cause back-reflections and back-action that couple environmental noise back to coherent elements, degrading fidelity. Nonreciprocal superconducting diodes provide an intrinsic route to suppress such backscattering at the circuit level. However, their diode efficiency and quantum integration remain limited. Here, we realize a quantum diode in twisted NbSe2 under in-plane and out-of-plane magnetic fields. A mere 1 degree twist yields an efficiency enhancement over pristine devices, reaching 27.6 percent. Quantum simulations reveal that this intermediate efficiency, well below 100 percent ideal, is both experimentally practical and optimal for preserving qubit anharmonicity and stabilizing two-level systems. These findings show that maximal rectification is not always beneficial for quantum information, establishing a new principle for designing the fundamental properties of twisted superconductors towards low-power, high-fidelity quantum circuits.
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All authors Han Zhong, Denis Kochan, Igor Zutic, Yingying Wu

Quantum Physics

Hyperfine spectroscopy and nuclear spin control of dark defects in diamond

Alexander Ungar, Hao Tang, Andrew Stasiuk et al. (+5 more)

Spin defects in diamond serve as powerful building blocks for quantum technologies, especially for quantum sensing and networking. Electron-nuclear defects formed near optically active spins such as the nitrogen-vacancy center provide a resource for multi-qubit quantum registers. However, many remain uncharacterized, limiting their control and integration in quantum devices. Here, we identify the hyperfine components, including their relative sign, and nuclear spin species of two unknown dark spin defects using zero-field double electron-electron resonance spectroscopy and a protocol based on electron-nuclear double resonance. New ab initio calculations assign one defect to a known nitrogen interstitial, while the hydrogen-related defect matches no calculated or previously reported structure. We further use this protocol to demonstrate initialization, unitary control, and long-lived coherence of the hydrogen nuclear spin qubit, with a coherence time of 1.0(3) ms. These characterization and control tools establish a framework for harnessing unknown electron-nuclear defects for quantum registers.
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All authors Alexander Ungar, Hao Tang, Andrew Stasiuk, Bo Xing, Boning Li, Ju Li, Alexandre Cooper, Paola Cappellaro

Quantum Physics

Upper bounds on charging power and tangible advantage in quantum batteries

Sreeram PG, J. Bharathi Kannan and M. S. Santhanam

Quantum battery is expected to outperform its classical counterpart due to quantum effects. Usually, in a quantum battery made of $N$ cells, quantum advantage is demonstrated through super-extensive scaling of the upper bound to the charging power with $N$. In this work, we show that potential quantum advantage as measured by the power bounds need not translate to tangible advantage in practice. We demonstrate this by considering an all-to-all coupled spin-chain model of a quantum battery with 2-local interactions. It exhibits super-extensive charging when analyzed using the upper bound derived from the uncertainty principle. Unlike the previously studied models, the contribution to this apparent quantum advantage is two-fold – arising from both the battery and the charger. The model is also experimentally friendly, as it does not require global couplings and yet generates genuine multipartite entanglement. However, we demonstrate that the potential quantum advantage in this scenario is not tangible by employing a tighter upper bound on power. Additionally, we show that even this tighter bound can fail in a range of physical situations and indicate a quantum enhancement that is intangible in practice. Hence, we argue that actual power transferred must be evaluated along with proper characterization of the resources before claiming quantum advantage.
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Quantum Physics

Quantum computation of molecular geometry via many-body nuclear spin echoes

C. Zhang, R. G. Cortiñas, A. H. Karamlou et al. (+320 more)

Quantum-information-inspired experiments in nuclear magnetic resonance spectroscopy may yield a pathway towards determining molecular structure and properties that are otherwise challenging to learn. We measure out-of-time-ordered correlators (OTOCs) [1-4] on two organic molecules suspended in a nematic liquid crystal, and investigate the utility of this data in performing structural learning tasks. We use OTOC measurements to augment molecular dynamics models, and to correct for known approximations in the underlying force fields. We demonstrate the utility of OTOCs in these models by estimating the mean ortho-meta H-H distance of toluene and the mean dihedral angle of 3',5'-dimethylbiphenyl, achieving similar accuracy and precision to independent spectroscopic measurements of both quantities. To ameliorate the apparent exponential classical cost of interpreting the above OTOC data, we simulate the molecular OTOCs on a Willow superconducting quantum processor, using AlphaEvolve-optimized [5] quantum circuits and arbitrary-angle fermionic simulation gates. We implement novel zero-noise extrapolation techniques based on the Pauli pathing model of operator dynamics [6], to repeat the learning experiments with root-mean-square error $0.05$ over all circuits used. Our work highlights a computational protocol to interpret many-body echoes from nuclear magnetic systems using low resource quantum computation.
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All authors C. Zhang, R. G. Cortiñas, A. H. Karamlou, N. Noll, J. Provazza, J. Bausch, S. Shirobokov, A. White, M. Claassen, S. H. Kang, A. W. Senior, N. Tomašev, J. Gross, K. Lee, T. Schuster, W. J. Huggins, H. Celik, A. Greene, B. Kozlovskii, F. J. H. Heras, A. Bengtsson, A. Grajales Dau, I. Drozdov, B. Ying, W. Livingstone, V. Sivak, N. Yosri, C. Quintana, D. Abanin, A. Abbas, R. Acharya, L. Aghababaie Beni, G. Aigeldinger, R. Alcaraz, S. Alcaraz, T. I. Andersen, M. Ansmann, F. Arute, K. Arya, W. Askew, N. Astrakhantsev, J. Atalaya, B. Ballard, J. C. Bardin, H. Bates, M. Bigdeli Karimi, A. Bilmes, S. Bilodeau, F. Borjans, A. Bourassa, J. Bovaird, D. Bowers, L. Brill, P. Brooks, M. Broughton, D. A. Browne, B. Buchea, B. B. Buckley, T. Burger, B. Burkett, J. Busnaina, N. Bushnell, A. Cabrera, J. Campero, H.-S. Chang, S. Chen, Z. Chen, B. Chiaro, L.-Y. Chih, A. Y. Cleland, B. Cochrane, M. Cockrell, J. Cogan, R. Collins, P. Conner, H. Cook, W. Courtney, A. L. Crook, B. Curtin, S. Das, M. Damyanov, D. M. Debroy, L. De Lorenzo, S. Demura, L. B. De Rose, A. Di Paolo, P. Donohoe, A. Dunsworth, V. Ehimhen, A. Eickbusch, A. M. Elbag, L. Ella, M. Elzouka, D. Enriquez, C. Erickson, V. S. Ferreira, M. Flores, L. Flores Burgos, E. Forati, J. Ford, A. G. Fowler, B. Foxen, M. Fukami, A. W. L. Fung, L. Fuste, S. Ganjam, G. Garcia, C. Garrick, R. Gasca, H. Gehring, R. Geiger, É. Genois, W. Giang, C. Gidney, D. Gilboa, J. E. Goeders, E. C. Gonzales, R. Gosula, S. J. de Graaf, D. Graumann, J. Grebel, J. Guerrero, J. D. Guimarães, T. Ha, S. Habegger, T. Hadick, A. Hadjikhani, M. P. Harrigan, S. D. Harrington, J. Hartshorn, S. Heslin, P. Heu, O. Higgott, R. Hiltermann, J. Hilton, H.-Y. Huang, M. Hucka, C. Hudspeth, A. Huff, E. Jeffrey, S. Jevons, Z. Jiang, X. Jin, C. Joshi, P. Juhas, A. Kabel, H. Kang, K. Kang, R. Kaufman, K. Kechedzhi, T. Khattar, M. Khezri, S. Kim, R. King, O. Kiss, P. V. Klimov, C. M. Knaut, B. Kobrin, F. Kostritsa, J. M. Kreikebaum, R. Kudo, B. Kueffler, A. Kumar, V. D. Kurilovich, V. Kutsko, N. Lacroix, D. Landhuis, T. Lange-Dei, B. W. Langley, P. Laptev, K.-M. Lau, L. Le Guevel, J. Ledford, J. Lee, B. J. Lester, W. Leung, L. Li, W. Y. Li, M. Li, A. T. Lill, M. T. Lloyd, A. Locharla, D. Lundahl, A. Lunt, S. Madhuk, A. Maiti, A. Maloney, S. Mandra, L. S. Martin, O. Martin, E. Mascot, P. Masih Das, D. Maslov, M. Mathews, C. Maxfield, J. R. McClean, M. McEwen, S. Meeks, K. C. Miao, R. Molavi, S. Molina, S. Montazeri, C. Neill, M. Newman, A. Nguyen, M. Nguyen, C.-H. Ni, M. Y. Niu, L. Oas, R. Orosco, K. Ottosson, A. Pagano, S. Peek, D. Peterson, A. Pizzuto, E. Portoles, R. Potter, O. Pritchard, M. Qian, A. Ranadive, M. J. Reagor, R. Resnick, D. M. Rhodes, D. Riley, G. Roberts, R. Rodriguez, E. Ropes, E. Rosenberg, E. Rosenfeld, D. Rosenstock, E. Rossi, D. A. Rower, M. S. Rudolph, R. Salazar, K. Sankaragomathi, M. C. Sarihan, K. J. Satzinger, M. Schaefer, S. Schroeder, H. F. Schurkus, A. Shahingohar, M. J. Shearn, A. Shorter, N. Shutty, V. Shvarts, S. Small, W. C. Smith, D. A. Sobel, R. D. Somma, B. Spells, S. Springer, G. Sterling, J. Suchard, A. Szasz, A. Sztein, M. Taylor, J. P. Thiruraman, D. Thor, D. Timucin, E. Tomita, A. Torres, M. M. Torunbalci, H. Tran, A. Vaishnav, J. Vargas, S. Vdovichev, G. Vidal, C. Vollgraff Heidweiller, M. Voorhees, S. Waltman, J. Waltz, S. X. Wang, B. Ware, J. D. Watson, Y. Wei, T. Weidel, T. White, K. Wong, B. W. K. Woo, C. J. Wood, M. Woodson, C. Xing, Z. J. Yao, P. Yeh, J. Yoo, E. Young, G. Young, A. Zalcman, R. Zhang, Y. Zhang, N. Zhu, N. Zobrist, Z. Zou, G. Bortoli, S. Boixo, J. Chen, Y. Chen, M. Devoret, M. Hansen, C. Jones, J. Kelly, P. Kohli, A. Korotkov, E. Lucero, J. Manyika, Y. Matias, A. Megrant, H. Neven, W. D. Oliver, G. Ramachandran, R. Babbush, V. Smelyanskiy, P. Roushan, D. Kafri, R. Sarpong, D. W. Berry, C. Ramanathan, X. Mi, C. Bengs, A. Ajoy, Z. K. Minev, N. C. Rubin, T. E. O'Brien

Quantum Physics

Revealing the quantum nature of memory in non-Markovian dynamics on IBM Quantum

Charlotte Bäcker, Krishna Palaparthy and Walter T. Strunz

We investigate memory effects in non-Markovian dynamics on superconducting quantum processors provided by IBM Quantum. We use a collision-model approach to implement suitable single- and two-qubit dynamics with a gate-based quantum circuit. Coupling the system of interest to an ancilla allows for a characterization of the process with respect to non-Markovian memory effects in general, as well as concerning the quantumness of that memory. We demonstrate that current noisy quantum hardware is capable of verifying quantum memory in single-qubit dynamics. We then discuss why a generalization of this dynamics to the two-qubit case cannot directly be simulated in a way that allows quantum memory to be observed. Nevertheless, we present an alternative toy example that demonstrates how quantum memory of two-qubit dynamics can be witnessed using current noisy quantum computers.
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Quantum Physics · Cross-listed from cond-mat.dis-nn

Fock-space fragmentation in quenches of disordered interacting fermions

Ishita Modak, Rajesh Narayanan, Ferdinand Evers et al. (+1 more)

Fock-space fragmentation, as it is currently investigated, primarily originates from specific kinematic constraints or emergent conservation laws in many-body systems with translation invariance. It leads to non-ergodic dynamics and possible breakdown of the eigenstate thermalization hypothesis. Here, we demonstrate that also in disordered systems, such as the XXZ model with random on-site fields, fragmentation appears as a natural concept offering fresh perspectives, for example, on many-body localization (MBL). Specifically, for a given quenched initial state, we identify a potential-energy shell in Fock space, defined by its energy and energy uncertainty, which represents the energetically accessible part of the many-body configuration space. For example, the drastic sample-to-sample fluctuations observed in the weak disorder regime, $W<W_{\mathrm{frag}}$, represent fluctuations of the mass of the shell. Upon increasing disorder, the potential-energy shell becomes increasingly fragmented. Its finite-size evolution exhibits a turnaround flow; extrapolating this flow to the disorder at which the turnaround vanishes provides a heuristic estimate of the strong-fragmentation scale $W_{\mathrm{frag}}$. We conjecture that the fragmentation identified here provides a geometric mechanism contributing to the finite-time slowing down of relaxation observed in studies of disordered interacting systems.
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All authors Ishita Modak, Rajesh Narayanan, Ferdinand Evers, Soumya Bera

Quantum Physics · Cross-listed from math-ph

On the maximum purity of absolutely separable bipartite states

Hoang Phi Dung and Vu The Khoi

In this study, we investigate the problem of determining the maximum purity for absolutely separable and absolutely PPT quantum states. From the geometric viewpoint, this problem is equivalent to asking for the exact Euclidean radius of the smallest ball around the maximally mixed state that encompasses the set of all absolutely separable or absolutely PPT states. Our results provide an analytic solution for two qubit states. Based on numerical computation, we propose a conjectured maximum purity for absolutely separable qubit-qudit states and absolutely PPT qutrit-qudit states.
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Quantum Physics

Quantum Machine Learning methods for Fourier-based distribution estimation with application in option pricing

Fernando Alonso, Álvaro Leitao and Carlos Vázquez

The ongoing progress in quantum technologies has fueled a sustained exploration of their potential applications across various domains. One particularly promising field is quantitative finance, where a central challenge is the pricing of financial derivatives-traditionally addressed through Monte Carlo integration techniques. In this work, we introduce two hybrid classical-quantum methods to address the option pricing problem. These approaches rely on reconstructing Fourier series representations of statistical distributions from the outputs of Quantum Machine Learning (QML) models based on Parametrized Quantum Circuits (PQCs). We analyze the impact of data size and PQC dimensionality on performance. Quantum Accelerated Monte Carlo (QAMC) is employed as a benchmark to quantitatively assess the proposed models in terms of computational cost and accuracy in the extraction of Fourier coefficients. Through the numerical experiments, we show that the proposed methods achieve remarkable accuracy, becoming a competitive quantum alternative for derivatives valuation.
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Quantum Physics

Low overhead circuit cutting with operator backpropagation

Debarthi Pal and Ritajit Majumdar

Current quantum computers suffer from noise due to lack of error correction. Several techniques to mitigate the effect of noise have been studied, in particular to extract the expectation value of observables. One such technique, circuit cutting, partitions large circuits into smaller, less noisy subcircuits, but the exponential increase in the number of circuit executions limits its scalability. Another method, operator backpropagation (OBP) reduces circuit depth by classically simulating parts of it, yet often escalates the number of circuit executions by some factor due to additional non-commuting terms in the updated observable. This paper introduces an optimized approach for minimizing noise in quantum circuits using operator backpropagation (OBP) combined with circuit cutting. We demonstrate that the strategic use of OBP with circuit cutting can mitigate the execution overhead. By employing simulated annealing, our proposed method identifies the optimal backpropagation parameter for specific circuits and observables, maximizing resource reduction in cutting. Results show a 3x and 10x decrease in resource requirements for Variational Quantum Eigensolver and Hamiltonian simulation circuits respectively, while maintaining or even enhancing accuracy. This approach also yields similar savings for other circuits from the Benchpress database and various observable weights, providing an efficient method to lower circuit cutting overhead without compromising performance.
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Quantum Physics

Absence of measurement- and unraveling-induced entanglement transitions in continuously monitored one-dimensional free fermions

Clemens Niederegger, Tatiana Vovk, Elias Starchl et al. (+1 more)

Continuous monitoring of one-dimensional free fermionic systems can generate phenomena reminiscent of quantum criticality, such as logarithmic entanglement growth, algebraic correlations, and emergent conformal invariance, but in a nonequilibrium setting. However, whether these signatures reflect a genuine phase of nonequilibrium quantum matter or persist only over finite length scales is an active area of research. We address this question in a free fermionic chain subject to continuous monitoring of lattice-site occupations. An unraveling phase $\varphi$ interpolates between measurement schemes, corresponding to different stochastic unravelings of the same Lindblad master equation: For $\varphi = 0$, measurements disentangle lattice sites, while for $\varphi = \pi/2$ they act as unitary random noise, yielding volume-law steady-state entanglement. Using replica Keldysh field theory, we obtain a nonlinear sigma model describing the long-wavelength physics. This analysis shows that for $0 \leq \varphi < \pi/2$, entanglement ultimately obeys an area law, but only beyond the exponentially large scale $\ln(l_{\varphi,*}) \sim J/[\gamma \cos(\varphi)]$, where $J$ is the hopping amplitude and $\gamma$ the measurement rate. Resolving $l_{\varphi, *}$ in numerical simulations is difficult for $\gamma/J \to 0$ or $\varphi \to \pi/2$. However, the theory also predicts that critical-like behavior appears below a crossover scale that grows only algebraically in $J/\gamma$, making it numerically accessible. Our simulations confirm these predictions, establishing the absence of measurement- or unraveling-induced entanglement transitions in this model.
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All authors Clemens Niederegger, Tatiana Vovk, Elias Starchl, Lukas M. Sieberer

Quantum Physics

Sequential Semi-Device-Independent Quantum Randomness Certification

Carles Roch I Carceller, Hanwool Lee, Jonatan Bohr Brask et al. (+2 more)

Quantum measurements under realistic conditions reveal only partial information about a system. Yet, by performing sequential measurements on the same system, additional information can be accessed. We investigate this problem in the context of semi-device-independent randomness certification using sequential maximum confidence measurements. We develop a general framework and versatile numerical methods to bound the amount of certifiable randomness in such scenarios. We further introduce a technique to compute min-tradeoff functions via semidefinite programming duality, thus making the framework suitable for bounding the certifiable randomness against adaptive attacking strategies through entropy accumulation. Our results establish sufficient criteria showing that maximum confidence measurements enable the distribution and certification of randomness across a sequential measurement chain.
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All authors Carles Roch I Carceller, Hanwool Lee, Jonatan Bohr Brask, Kieran Flatt, Joonwoo Bae