The dynamic engineering of band structures for ultracold atoms in optical lattices represents an innovative approach to understanding and exploring the fundamental principles of topological matter. In particular, the folded Floquet spectrum determines the associated band topology via band inversion. We experimentally and theoretically study two-frequency phase modulation to asymmetrically hybridize the lowest two bands of a one-dimensional lattice. Using quasidegenerate perturbation theory in the extended Floquet space we derive an effective two-band model that quantitatively describes our setting. The energy gaps are experimentally probed via Landau-Zener transitions between Floquet-Bloch bands using an accelerated Bose-Einstein condensate. Separate and simultaneous control over the closing and reopening of these band gaps is demonstrated. We find good agreement between experiment and theory, establishing an analytic description for resonant Floquet-Bloch engineering that includes single- and multiphoton couplings, as well as interference effects between several commensurate drives.

We study black and white hole analogs in Weyl semimetals with inhomogeneous nodal tilts. We study how the presence of a microscopic lattice, giving rise to low-energy fermion doubler states at large momenta that are not present for elementary particles, affects the analogy between Weyl Hamiltonians and general relativity. Using a microscopic tight-binding lattice model, we find the doubler states to give rise to Hawking fragmentation and Hawking attenuation of wave packets by the analog event horizon. These phenomena depend on an analog Hawking temperature and can be measured in metamaterials and solids, as we confirm by numerical simulations.

Classical shadow tomography provides an efficient method for predicting functions of an unknown quantum state from a few measurements of the state. It relies on a unitary channel that efficiently scrambles the quantum information of the state to the measurement basis. Facing the challenge of realizing deep unitary circuits on near-term quantum devices, we explore the scenario in which the unitary channel can be shallow and is generated by a quantum chaotic Hamiltonian via time evolution. We provide an unbiased estimator of the density matrix for all ranges of the evolution time. We analyze the sample complexity of the Hamiltonian-driven shadow tomography. For Pauli observables, we find that it can be more efficient than the unitary-2-design-based shadow tomography in a sequence of intermediate time windows that range from an order-1 scrambling time to a timescale of $D^{1/6}$, given the Hilbert space dimension $D$. In particular, the efficiency of predicting diagonal Pauli observables is improved by a factor of $D$ without sacrificing the efficiency of predicting off-diagonal Pauli observables.

Fracton systems exhibit restricted mobility of their excitations due to the presence of higher-order conservation laws. Here we study the time evolution of a one-dimensional fracton system with charge and dipole moment conservation using a random unitary circuit description. Previous work has shown that when the random unitary operators act on four or more sites, an arbitrary initial state eventually thermalizes via a universal subdiffusive dynamics. In contrast, a system evolving under three-site gates fails to thermalize due to strong “fragmentation” of the Hilbert space. Here we show that three-site gate dynamics causes a given initial state to evolve toward a highly nonthermal state on a timescale consistent with Brownian diffusion. Strikingly, the dynamics produces an effective attraction between isolated fractons or between a single fracton and the boundaries of the system, as in the Casimir effect of quantum electrodynamics. We show how this attraction can be understood by exact mapping to a simple classical statistical mechanics problem, which we solve exactly for the case of an initial state with either one or two fractons.

We present a quantum-classical hybrid algorithm that simulates electronic structures of periodic systems such as ground states and quasiparticle band structures. By extending the unitary coupled cluster (UCC) theory to describe crystals in arbitrary dimensions, for a hydrogen chain, we numerically demonstrate that the UCC ansatz implemented on a quantum circuit can be successfully optimized with a small deviation from the exact diagonalization over the entire range of the potential energy curves. Furthermore, by using the quantum subspace expansion method, in which we truncate the Hilbert space within the linear response regime from the ground state, the quasiparticle band structure is computed as charged excited states. Our work establishes a powerful interface between the rapidly developing quantum technology and modern material science.

Parity-time-symmetric ($\mathcal{PT}$-symmetric) Hamiltonians are generally non-Hermitian and give rise to exotic behavior in quantum systems at exceptional points, where eigenvectors coalesce. The recent realization of $\mathcal{PT}$-symmetric Hamiltonians in quantum systems has ignited efforts to simulate and investigate many-particle quantum systems across exceptional points. Here, we use a programmable integrated photonic chip to simulate a model composed of twin pairs of $\mathcal{PT}$-symmetric Hamiltonians, with each the time reverse of its twin. We simulate quantum dynamics across exceptional points including two- and three-particle interference, and a particle-trembling behavior that arises due to interference between subsystems undergoing time-reversed evolutions. These results show how programmable quantum simulators can be used to investigate foundational questions in quantum mechanics.

Interevent times in temporal contact data from humans and animals typically obey heavy-tailed distributions, which impacts contagion and other dynamical processes on networks. We theoretically show that distributions of interevent times heavier-tailed than exponential distributions are a consequence of the most basic metapopulation model used in epidemiology and ecology, in which individuals move from one patch to another according to the simple random walk. Our results hold true irrespective of the network structure and also for more realistic mobility rules such as high-order random walks and the recurrent mobility patterns used for modeling human dynamics.

Device-independent certification, also known as self-testing, aims at guaranteeing the proper functioning of untrusted and uncharacterized devices. For example, the quality of an unknown source expected to produce two-qubit maximally entangled states can be evaluated in a bipartite scenario, each party using two binary measurements. The most robust approach consists in deducing the fidelity of produced states with respect to a two-qubit maximally entangled state from the violation of the CHSH inequality. In this paper, we show how the self-testing of two-qubit maximally entangled states is improved by a refined analysis of measurement statistics. The use of suitably chosen Bell tests, depending on the observed correlations, allows one to conclude higher fidelities than ones previously known. In particular, nontrivial self-testing statements can be obtained from correlations that cannot be exploited by a CHSH-based self-testing strategy. Our results not only provide insight into the set of quantum correlations suited for self-testing, but also facilitate the experimental implementations of device-independent certifications.

We report a detailed study of electrical and thermal transport properties in $2\mathrm{H}\text{−}{\mathrm{M}}_x{\mathrm{TaS}}_2 (\mathrm{M}=\mathrm{Mn}, \mathrm{Co})$ magnets where M atoms are intercalated in the van der Waals gap. The intercalation induces ferromagnetism (FM) with an easy-plane anisotropy in $2\mathrm{H}\text{−}{\mathrm{Mn}}_x{\mathrm{TaS}}_2$, but FM with a strong uniaxial anisotropy in $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.22}{\mathrm{TaS}}_2$, which finally evolves into a three-dimensional antiferromagnetism (AFM) in $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.34}{\mathrm{TaS}}_2$. Temperature-dependent electrical resistivity shows metallic behavior for all samples. Thermopower is negative in the whole temperature range for $2\mathrm{H}\text{−}{\mathrm{Co}}_x{\mathrm{TaS}}_2$, whereas the sign changes from negative to positive with increasing Mn for $2\mathrm{H}\text{−}{\mathrm{Mn}}_x{\mathrm{TaS}}_2$. The diffusive thermoelectric response dominates in both high- and low-temperature ranges for all samples. A clear kink in electrical resistivity, a weak anomaly in thermal conductivity, as well as a slope change in thermopower were observed at the magnetic transitions for $2\mathrm{H}\text{−}{\mathrm{Mn}}_{0.28}{\mathrm{TaS}}_2$ ($T_{\text{c}}\approx$ 82 K) and $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.34}{\mathrm{TaS}}_2$ ($T_{\text{N}}\approx$ 36 K), respectively, albeit weaker for lower $x$ crystals. Co-intercalation promoted FM to AFM transition is further confirmed by Hall resistivity; the sign change of the ordinary Hall coefficient indicates a multiband behavior in $2\mathrm{H}\text{−}{\mathrm{Co}}_x{\mathrm{TaS}}_2$.

We investigate the effects of quantum fluctuations in a parity-time $\left(\mathcal{PT}\right)$ symmetric two-species Bose-Einstein condensate (BEC). It is found that the $\mathcal{PT}$ symmetry, though preserved by the macroscopic condensate, can be spontaneously broken by its Bogoliubov quasiparticles under quantum fluctuations. The associated $\mathcal{PT}$-breaking transitions in the Bogoliubov spectrum can be conveniently tuned by the interaction anisotropy in spin channels and the strength of $\mathcal{PT}$ potential. In the $\mathcal{PT}$-unbroken regime, the real Bogoliubov modes are generally gapped, in contrast to the gapless phonon mode in Hermitian case. Moreover, the presence of $\mathcal{PT}$ potential is found to enhance the mean-field collapse and thereby intrigue the droplet formation after incorporating the repulsive force from quantum fluctuations. These remarkable interplay effects of $\mathcal{PT}$ symmetry and interaction can be directly probed in cold atoms experiments, which shed light on related quantum phenomena in general $\mathcal{PT}$-symmetric systems.

Detailed radiative transfer simulations of kilonovae are difficult to apply directly to observations; they only sparsely cover simulation parameters, such as the mass, velocity, morphology, and composition of the ejecta. On the other hand, semianalytic models for kilonovae can be evaluated continuously over model parameters, but neglect important physical details which are not incorporated in the simulations, thus introducing systematic bias. Starting with a grid of two-dimensional anisotropic simulations of kilonova light curves covering a wide range of ejecta properties, we apply adaptive learning techniques to iteratively choose new simulations and produce high-fidelity surrogate models for those simulations. These surrogate models allow for continuous evaluation across model parameters while retaining the microphysical details about the ejecta. Using a code formultimessenger inference developed by our group, we demonstrate how to use our interpolated models to infer kilonova parameters. Comparing to inferences using simplified analytic models, we recover different ejecta properties. We discuss the implications of this analysis which is qualitatively consistent with similar previous work using detailed ejecta opacity calculations and which illustrates systematic challenges for kilonova modeling. An associated data and code release provides our interpolated light-curve models, interpolation implementation which can be applied to reproduce our work or extend to new models, and our multimessenger parameter inference engine.

We study the emergence of dissipative localized states in phase mismatched singly resonant optical parametric oscillators. These states arise in two different bistable configurations due to the locking of front waves connecting the two coexisting states. In one of these configurations, the bistability is mediated by the coexistence of two uniform states. Here the localized states are organized in a collapsed snaking bifurcation structure. Moreover, these states undergo oscillatory instabilities which lead to a breathing behavior. When the bistability is related to the coexistence of a uniform state and a spatially periodic pattern, localized states are organized in a bifurcation structure similar to the standard homoclinic snaking. Performing an exhaustive bifurcation analysis, we characterize in detail the previous structures, their linear stability, and the modification of their dynamics as a function of the control parameters of the system.

The Bloch theorem is a general theorem restricting the persistent current associated with a conserved U(1) charge in a ground state or in a thermal equilibrium. It gives an upper bound of the magnitude of the current density, which is inversely proportional to the system size. In a recent paper, Else and Senthil applied the argument for the Bloch theorem to a generalized Gibbs ensemble, assuming the presence of an additional conserved charge, and predicted a nonzero current density in the nonthermal steady state [Else and Senthil, Phys. Rev. B 104, 205132 (2021)]. In this paper, we provide a complementary derivation based on the canonical ensemble, given that the additional charge is strictly conserved within the system by itself. Furthermore, using the example where the additional conserved charge is the momentum operator, we discuss that the persistent current tends to vanish when the system is in contact with an external momentum reservoir in the comoving frame of the reservoir.

We develop a geometric approach to operator growth and Krylov complexity in many-body quantum systems governed by symmetries. We start by showing a direct link between a unitary evolution with the Liouvillian and the displacement operator of appropriate generalized coherent states. This connection maps operator growth to a purely classical motion in phase space. The phase spaces are endowed with a natural information metric. We show that, in this geometry, operator growth is represented by geodesics, and Krylov complexity is proportional to a volume. This geometric perspective also provides two novel avenues toward computation of Lanczos coefficients, and it sheds new light on the origin of their maximal growth. We describe the general idea and analyze it in explicit examples, among which we reproduce known results from the Sachdev-Ye-Kitaev model, derive operator growth based on SU(2) and Heisenberg-Weyl symmetries, and generalize the discussion to conformal field theories. Finally, we use techniques from quantum optics to study operator evolution with quantum information tools such as entanglement and Renyi entropies, negativity, fidelity, relative entropy, and capacity of entanglement.

The dynamics of cascading activation, such as rapid changes in public opinion and the outbreak of disease epidemics, have a crucial dependence on the connectivity patterns among the agents. We study cascading dynamics in modular, degree-heterogeneous networks, and consider the impact of intramodule seeding strategy on intermodule spread. Specifically, we establish that although activating the highest-degree nodes is more effective than random selection at growing a cascade locally, there is a critical level of intermodule connectivity required for a cascade to cross from one module to another, irrespective of the seeding strategy. We present an analytical proof of this statement for the case that each module has the same degree distribution and all module pairs have the same intermodule connectivity, while our simulation results suggest its validity for more general situations, including a ring of modules. Interestingly, we find that on a network comprised of two modules, this critical level is primarily determined by the degree distribution of the alter module, as opposed to the seed module. Our analytical approach extends a method developed by Gleeson, but is able to capture different seeding strategies using only one dynamical variable per module, namely, the conditional exposure probability. Our work shows that the possibility of a global cascade depends sensitively on intermodule connectivity, and less on the intramodule seeding strategy. This suggests, for example, that slight changes to intermodule connectivity can be a feasible intervention strategy to promote or inhibit global cascades.

We report on transport experiments through high-mobility gate-tunable undoped InSb quantum wells (QWs). Due to the elimination of any Si modulation doping, the gate-defined two-dimensional electron gases in the QWs display a significantly increased mobility of $260000\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$ at a rather low density of $2.4\times {10}^{11}\mathrm{c}{\mathrm{m}}^{\text{–}2}$. Using magnetotransport experiments, we characterize spin-orbit interactions by measuring weak antilocalization. Furthermore, by measuring Shubnikov–de Haas oscillations in tilted magnetic fields, we find that the $g$ factor agrees with $\mathbf{k}·\mathbf{p}$ theory calculations at low magnetic fields but grows with spin polarization and carrier density at high magnetic fields. Additionally, signatures of Ising quantum Hall ferromagnetism are found at filling factor $\nu =2$ for tilt angles where the Landau level energy equals the Zeeman energy. Despite the high mobility, the undoped InSb QWs exhibit no fractional quantum Hall effect up to magnetic fields of 25 T.

The outlook of protected quantum computing spurred enormous progress in the search for topological materials, sustaining a continued race to find the most experimentally feasible platform. Here we show that one of the simplest quantum circuits, the Cooper-pair transistor, exhibits a nontrivial Chern number which has not yet been discussed, in spite of the exhaustive existing literature. Surprisingly, the resulting quantized current response is robust with respect to a large number of external perturbations, most notably low-frequency charge noise and quasiparticle poisoning. Moreover, the fact that the higher bands experience crossings with higher topological charges leads to all the bands having the same Chern number, such that there is no restriction to stay close to the ground state. Remaining small perturbations are investigated based on a generic master equation approach. Finally, we discuss a feasible protocol to measure the quantized current.

The indistinguishability of successively generated photons from a single quantum emitter is most commonly measured using two-photon interference at a beam splitter. Whilst for sources excited in the pulsed regime the measured bunching of photons reflects the full wave-packet indistinguishability of the emitted photons, for continuous wave (cw) excitation, the inevitable dependence on detector timing resolution and driving strength obscures the underlying photon interference process. Here we derive a method to extract full photon wave-packet indistinguishability from cw measurements by considering the relevant correlation functions. The equivalence of both methods is experimentally verified through a comparison of cw and pulsed excitation measurements on an archetypal source of photons, a single molecule.

Measuring time lags between time series or light curves at different wavelengths from a variable or transient source in astronomy is an essential probe of physical mechanisms causing multiwavelength variability. Time lags are typically quantified using discrete correlation functions (DCFs), which are appropriate for linear relationships. However, in variable sources such as x-ray binaries, active galactic nuclei (AGNs), and other accreting systems, the radiative processes and the resulting multiwavelength light curves often have nonlinear relationships. For such systems it is more appropriate to use nonlinear information-theoretic measures of causation such as mutual information, routinely used in other disciplines. We demonstrate with toy models the limitations of using the standard DCF and show improvements when using a discrete mutual information function (DMIF). For nonlinear correlations, the latter accurately and sharply identifies the lag components as opposed to the DCF, which can be erroneous. Following that, we apply the DMIF to the multiwavelength light curves of AGN NGC 4593. We find that x-ray fluxes are leading UVW2 fluxes by $\sim 0.2$ days, closer to model predictions from reprocessing by the accretion disk than the DCF estimate. The uncertainties with the current light curves are too large, though, to rule out negative lags. Additionally, we find another delay component at approximately $-1$ day, i.e., UVW2 leading x rays, consistent with inward propagating fluctuations in the accretion disk scenario. This is not detected by the DCF. Keeping in mind the nonlinear relation between x ray and UVW2, this is worthy of further theoretical investigation. From both the toy models and real observations, it is clear that the mutual-information-based estimator is highly sensitive to complex nonlinear relations. With sufficiently high temporal resolution and signal-to-noise ratio, we will precisely detect each of the lag features corresponding to these relations.