Theoretical physics articles within Nature Communications

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  • Article
    | Open Access

    R.-J. Slager et al. extend the theory of multigap topology from static to non-equilibrium systems. They identify Floquet-induced non-Abelian braiding, resulting in a phase characterized by anomalous Euler class, a multi-gap topological invariant. They also find a gapped anomalous Dirac string phase. Both phases have no static counterparts and exhibit distinct boundary signatures.

    • Robert-Jan Slager
    • , Adrien Bouhon
    •  & F. Nur Ünal

  • Article
    | Open Access

    Periodically driven quantum systems have been extensively studied but with a predominant focus on long-time dynamics. Here, the authors study short-to-intermediate-time dynamics of an isolated many-body system, showing that its response to driving is supressed for the initial state close to thermal equilibrium.

    • Lennart Dabelow
    •  & Peter Reimann

  • Article
    | Open Access

    Consistent theories have been proposed in which spacetime is treated classically while matter remains quantum. Here, the authors prove that such theories are constrained by a trade-off between the decoherence induced in the quantum system, and stochasticity in the classical one, providing a way to experimentally test the quantum nature of gravity.

    • Jonathan Oppenheim
    • , Carlo Sparaciari
    •  & Zachary Weller-Davies

  • Article
    | Open Access

    The physics of confinement manifested in quantum spin chain models has been recently studied in quantum simulators. Here the authors report a numerical study of confinement of soliton excitations in a nonintegrable bosonic quantum field theory realized with a superconducting quantum electronic circuit.

    • Ananda Roy
    •  & Sergei L. Lukyanov

  • Article
    | Open Access

    The quantum error-correcting codes formed by tensor network models of holography have so far failed to produce the expected correlation functions in the boundary states. Here, the authors fill this gap by modifying a previously proposed model of hyperinvariant tensor networks.

    • Matthew Steinberg
    • , Sebastian Feld
    •  & Alexander Jahn

  • Article
    | Open Access

    Quantum theory allows for indefinite causal order, but experimental demonstrations of such scenarios have so far required trust in the internal functioning of the apparatus. Here, the authors point out a scenario where indefinite causal order could be certified in a device-independent way, if one excludes superluminal and retrocausal influences.

    • Tein van der Lugt
    • , Jonathan Barrett
    •  & Giulio Chiribella

  • Article
    | Open Access

    In order to be practical, schemes for characterizing quantum operations should require the simplest possible gate sequences and measurements. Here, the authors show how random gate sequences and native measurements (followed by classical post-processing) are sufficient for estimating several gate set properties.

    • J. Helsen
    • , M. Ioannou
    •  & I. Roth

  • Article
    | Open Access

    Machine learning methods in condensed matter physics are an emerging tool for providing powerful analytical methods. Here, the authors demonstrate that convolutional neural networks can identify nematic electronic order from STM data of twisted double-layer graphene—even in the presence of heterostrain.

    • João Augusto Sobral
    • , Stefan Obernauer
    •  & Mathias S. Scheurer

  • Article
    | Open Access

    Variational approaches combined with machine learning are promising for solving quantum many-body problems, but they often suffer from scaling and optimization issues. Here the authors demonstrate that a stochastic representation of wavefunctions enables reducing the ground state search to standard regression.

    • Hristiana Atanasova
    • , Liam Bernheimer
    •  & Guy Cohen

  • Article
    | Open Access

    Understanding the structure of the Kondo cloud formed by conduction electrons screening the impurity spin is a long-standing problem in many-body physics. Shim et al. propose the spatial and energy structure of the multichannel Kondo cloud, by studying quantum entanglement between the impurity and the channels.

    • Jeongmin Shim
    • , Donghoon Kim
    •  & H.-S. Sim

  • Article
    | Open Access

    The properties of edge states at the boundary between a quantum Hall insulator and a superconductor have recently been under scrutiny. Here, the authors find theoretically that Andreev reflection of an edge state is possible only if the superconductor is in the disordered limit, leading to stochastic edge state conductance and providing an explanation of a recent experiment.

    • Vladislav D. Kurilovich
    • , Zachary M. Raines
    •  & Leonid I. Glazman

  • Article
    | Open Access

    A measure of symmetry breaking in a quantum many-body system could provide insight into its dynamics. Ares et al. introduce a subsystem measure of symmetry breaking dubbed entanglement asymmetry and apply it to quantum quench dynamics in spin chains, revealing a quantum analogue of the Mpemba effect.

    • Filiberto Ares
    • , Sara Murciano
    •  & Pasquale Calabrese

  • Article
    | Open Access

    Quantum theory can describe scenarios with an indefinite causal order, but whether such processes could be witnessed in real scenarios by violating causal inequalities is still subject to debate. Here, the authors give an affirmative answer, showing that noncausal processes admit a description using the framework of time-delocalised subsystems.

    • Julian Wechs
    • , Cyril Branciard
    •  & Ognyan Oreshkov

  • Article
    | Open Access

    Implementations of shallow quantum machine learning models are a promising application of near-term quantum computers, but rigorous results on their trainability are sparse. Here, the authors demonstrate settings where such models are untrainable.

    • Eric R. Anschuetz
    •  & Bobak T. Kiani

  • Article
    | Open Access

    Renormalisation group methods serve for finding analytic solutions, critical points and computing phase diagrams of many-body systems. Here the authors demonstrate that renormalisation group schemes can be constructed for undecidable many-body systems, giving rise to the types of renormalisation group flow which are strictly more unpredictable than chaotic flows.

    • James D. Watson
    • , Emilio Onorati
    •  & Toby S. Cubitt

  • Article
    | Open Access

    Getting a grip on the chaotic properties of quantum systems is difficult. Now, the effect of translational invariance in space in time in an ensemble of random quantum circuits is shown to lead to largely universal scaling laws describing the system without the need of knowing microscopic details.

    • Amos Chan
    • , Saumya Shivam
    •  & Andrea De Luca

  • Article
    | Open Access

    Quantum simulators allow for experimental studies of many-body systems in complex geometries, which has rarely been addressed by theory. Here the authors study many-body Hamiltonians on generic random graphs and show that many-body effects emerge only in a small class of exceptional, highly structured graphs.

    • Joseph Tindall
    • , Amy Searle
    •  & Dieter Jaksch

  • Article
    | Open Access

    The effects of detection noise on quantum metrology performances have not been rigorously investigated yet. Here, the authors fill this gap by generalising the quantum Fisher information to the case of noisy readout, and showing the consequences the imperfect measurements bring.

    • Yink Loong Len
    • , Tuvia Gefen
    •  & Jan Kołodyński

  • Article
    | Open Access

    Here the authors provide a theoretical description of non-Hermitian topological phenomena in an atomic mirror. They find out diverse and unexpected phenomena by constructing an ad-hoc theoretical model. In particular, exceptional points, dispersive bulk Fermi arcs, and non-Hermitian geometry-dependent skin effect.

    • Yi-Cheng Wang
    • , Jhih-Shih You
    •  & H. H. Jen

  • Article
    | Open Access

    Quantum trajectory frameworks describe systems weakly coupled to their environment. Here, by including an extra 1D variable in the dynamics, the authors introduce a quantum trajectory framework for time local master equations derived at strong coupling while keeping the computational complexity under control.

    • Brecht Donvil
    •  & Paolo Muratore-Ginanneschi

  • Article
    | Open Access

    The computational capabilities of quantum annealing in the accessible regimes of operation are still subject to debate. Here, the authors study a model admitting an analytical solution far from the adiabatic regime, and show evidences of better convergence and energy relaxation rates over classical annealing.

    • Bin Yan
    •  & Nikolai A. Sinitsyn

  • Article
    | Open Access

    Multi-gap topology is a new avenue in topological phases of matter but it remains difficult to verify in real materials. Here, the authors predict multi-gap topologies and associated phase transitions driven by braiding processes in the phonon spectra of monolayer silicates, providing clear signatures for experimental verification.

    • Bo Peng
    • , Adrien Bouhon
    •  & Robert-Jan Slager

  • Article
    | Open Access

    Aumann’s agreement theorem states that observers of classical systems can’t “agree to disagree." Here, the authors show that the same epistemic consistency holds for observers of quantum states, but not for observers of post-quantum no-signalling boxes, hinting at its potential status as a physical principle.

    • Patricia Contreras-Tejada
    • , Giannicola Scarpa
    •  & Pierfrancesco La Mura

  • Article
    | Open Access

    Three-dimensional topological insulators have become a research focal point on topological quantum matter. Here, the authors propose the non-Hermitian analogue, the exceptional topological insulator, with anomalous surface states only existing within the topological bulk embedding.

    • M. Michael Denner
    • , Anastasiia Skurativska
    •  & Titus Neupert

  • Article
    | Open Access

    Current hypotheses towards quantisation of gravity imply the presence of a minimal length scale, which may have a role in explaining quantum-to-classical transition. Here, the authors show how assuming the minimal length scale to be a fluctuating quantity leads to a possible universal decoherence mechanism.

    • Luciano Petruzziello
    •  & Fabrizio Illuminati

  • Article
    | Open Access

    Tensor network simulations of lattice gauge theories may overcome the limitations of the Monte Carlo approach, but results have been limited to 1+1 and 2+1 dimensions so far. Here, the authors report a tree-tensor-based numerical study of a 3+1d truncated U(1) lattice gauge theory with fermionic matter.

    • Giuseppe Magnifico
    • , Timo Felser
    •  & Simone Montangero

  • Article
    | Open Access

    Device-independent quantum key distribution aims at the ultimate quantum-based unconditional security, but current protocols’ rates are quite far from anything practical. The authors’ protocol narrows this gap by using two randomly chosen key generating bases instead of one.

    • René Schwonnek
    • , Koon Tong Goh
    •  & Charles C.-W. Lim

  • Article
    | Open Access

    Discussions about the role of time in quantum mechanics are often detached from the corresponding classical limit. Here, the authors provide the Page and Wootters mechanism with a classical limit using the large-N approach based on generalized coherent states, without the need to introduce time measurements.

    • Caterina Foti
    • , Alessandro Coppo
    •  & Paola Verrucchi

  • Article
    | Open Access

    Charge transport in strongly correlated electron systems is not fully understood. Here, the authors show that resilient quasiparticles at finite frequency persist into the bad-metal regime near a Mott insulator, where dynamical localization results in a ‘displaced Drude peak’ and strongly enhanced dc resistivity.

    • Andrej Pustogow
    • , Yohei Saito
    •  & Simone Fratini

  • Article
    | Open Access

    The Gibbs paradox stems from the entropy change upon mixing two gases. Here, by considering bosonic and fermionic statistics, the authors show that an observer unable to distinguish the particles’ spins assigns a greater entropy increase to the mixing process than is possible in classical physics.

    • Benjamin Yadin
    • , Benjamin Morris
    •  & Gerardo Adesso

  • Article
    | Open Access

    While unusual processes allowing indefinite causal order are gaining attention in quantum physics, formalisms describing definite causal structures have so far been limited to acyclic ones. Here the authors extend to the cyclic case, offering a causal perspective on causally indefinite processes.

    • Jonathan Barrett
    • , Robin Lorenz
    •  & Ognyan Oreshkov

  • Article
    | Open Access

    The usual definition of external time is unlikely to survive if we want to unite quantum mechanics and relativity. Here the authors consider two quantum clocks moving in curved spacetime and formulate the probability distribution that relates their proper times, allowing them to explore quantum time dilation.

    • Alexander R. H. Smith
    •  & Mehdi Ahmadi

  • Article
    | Open Access

    The use of multi-particle systems in quantum-gravity phenomenology should take into account the expected suppression with increasing number of constituent particles N. Here, the authors analyse the case of polynomial scaling with N, and give bounds from previous experiments with macroscopic pendula.

    • Shreya P. Kumar
    •  & Martin B. Plenio

  • Article
    | Open Access

    The existence of nonlocal correlations which cannot be attained exactly by finite-dimensional systems, but can be attained by infinite-dimensional ones, has been the subject of several theoretical efforts. Here, Coladangelo and Stark exhibit such a correlation, in a form that requires only two players.

    • Andrea Coladangelo
    •  & Jalex Stark

  • Article
    | Open Access

    In information processing applications, directional amplifiers are key components which can be realized in very different systems. Here, the authors present a theoretical framework based on the introduction of a topological invariant that helps to understand directional amplification in coupled cavity arrays.

    • Clara C. Wanjura
    • , Matteo Brunelli
    •  & Andreas Nunnenkamp

  • Article
    | Open Access

    Supersymmetric quantum mechanics enables the description of phenomena exhibiting a supersymmetry only in the space domain. Here, the authors show an underlying time-domain supersymmetry exists in optics, acoustics, and elasticity, and study its properties and potential applicability.

    • Carlos García-Meca
    • , Andrés Macho Ortiz
    •  & Roberto Llorente Sáez

  • Article
    | Open Access

    The theoretical understanding of quantum many-body systems often involves properties of energy eigenstates but these are difficult to probe experimentally. Yang et al. propose an experiment that supports preparation and measurement of single eigenstates, enabling detailed studies of statistical physics.

    • Dayou Yang
    • , Andrey Grankin
    •  & Peter Zoller

  • Article
    | Open Access

    The amount of information that a quantum channel can transmit is fundamentally bounded by the amount of noise in the channel. Here, the authors consider the realistic case with loss and thermal noise errors and prove that correlated multi-mode thermal states can achieve higher rates than single-mode ones.

    • Kyungjoo Noh
    • , Stefano Pirandola
    •  & Liang Jiang

  • Article
    | Open Access

    The lately reported Higgs modes in unconventional superconductors require a classification and characterization allowed by nontrivial symmetry of the gap and the quench pulses. Here, the authors provide a classification scheme of Higgs oscillations with their excitation processes allowing them to distinguish between different symmetries of the superconducting condensate.

    • L. Schwarz
    • , B. Fauseweh
    •  & D. Manske

  • Article
    | Open Access

    Driven quantum many-body systems can host finite densities of quasiparticles with the potential to realise emergent behaviour that is distinct from the equilibrium state. Werner et al. propose a method to cool holes in a correlated system so that more exotic low-entropy phases can be reached.

    • Philipp Werner
    • , Martin Eckstein
    •  & Gil Refael

  • Article
    | Open Access

    There are a number of non-trivial integrable models in one-dimension, making them an attractive starting point for studying quantum dynamics. Biella et al. study transport between two semi-infinite solvable models and show that a slowly-relaxing region forms around the integrability-breaking junction.

    • Alberto Biella
    • , Mario Collura
    •  & Leonardo Mazza

  • Article
    | Open Access

    The presence of processes that cannot be simulated classically in open quantum system dynamics is acknowledged, but an exact quantifier for this non-classical character is still missing. Here, the authors provide a quantitative measure of non-classicality for purely dephasing evolutions.

    • Hong-Bin Chen
    • , Ping-Yuan Lo
    •  & Franco Nori

  • Article
    | Open Access

    Time has a fundamentally different character in quantum mechanics and in general relativity. Here, the authors consider a thought experiment where a massive body in a spatial superposition leads to entanglement of temporal orders between time-like events, resulting in a violation of a Bell-type inequality.

    • Magdalena Zych
    • , Fabio Costa
    •  & Časlav Brukner

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