Three-dimensional, commensurate long-range magnetic order in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ quickly evolves to quasi-two-dimensional, incommensurate correlations upon doping with mobile holes, and superconductivity follows for $x$ as small as 0.05 in the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x/{\mathrm{Ba}}_x{\mathrm{CuO}}_4$ family of superconductors. The onset of superconductivity in these systems is known to be coincident with a remarkable rotation of the incommensurate spin order from “diagonal stripes” below $x$ = 0.05 to “parallel stripes” above. However, little is known about the spin correlations at optimal and high doping levels, around and beyond the proposed quantum critical point for the pseudogap phase, $p^{*}$. Here, we present elastic and inelastic neutron scattering measurements on single crystals of ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ with $x$ = 0.125, 0.19, 0.24, and 0.26 and show that two-dimensional, quasistatic, parallel spin stripes have an onset at temperatures such that the parallel spin stripe phase extends beyond $p^{*}$ and envelops the entirety of superconducting ground states in this system. We also show that the elastic order parameter for parallel spin stripes at optimum doping, $x$ = 0.19, displays an inflection point at superconducting $T_c$, while the low-energy dynamic spectral weight of parallel stripe fluctuations grows with decreasing temperature and saturates below $T_c$.

The spontaneously oscillating hair bundle of sensory cells in the inner ear is an example of a stochastic, nonlinear oscillator driven by internal active processes. Moreover, this internal activity is state dependent in nature—it measures the current state of the system and changes its power input accordingly. We study the breakdown of two fluctuation-dissipation relations in these nonequilibrium limit cycle oscillators with and without state-dependent drives. First, in the simple model of the hair cell oscillator where the internal drive of the system is state independent, we observe the expected violation of the well-known, equilibrium fluctuation-dissipation theorem (FDT), and verify the existence of a generalized fluctuation-dissipation theorem (GFDT). This generalized theorem is analogous to one proposed earlier by Seifert and Speck. It requires the system to be analyzed in the co-moving frame associated with the mean limit cycle of the stochastic oscillator. We then demonstrate, via numerical simulations and analytic calculations, that in the presence of a state-dependent drive, the dynamical hair cell model violates both the FDT and this GFDT. We go on to show, using stochastic, finite-state, dynamical models, that such a drive in stochastic limit cycle oscillators generically violates both the FDT and GFDT. We propose that one may in fact use the breakdown of this particular GFDT as a tool to more broadly look for and quantify the effect of state-dependent drives within (nonequilibrium) biological dynamics.

Dynamical mean-field theory (DMFT) has been employed in conjunction with density functional theory (DFT $+$ DMFT) to investigate the metal-insulator transition (MIT) of strongly correlated $3d$ electrons due to quantum confinement. We shed light on the microscopic mechanism of the MIT and previously reported anomalous subband mass enhancement, both of which arise as a direct consequence of the quantization of V $xz\left(yz\right)$ states in the ${\mathrm{SrVO}}_3$ layers. We therefore show that quantum confinement can sensitively tune the strength of electron correlations, leading the way to applying such approaches in other correlated materials.

An efficient representation of a quantum circuit is of great importance in achieving a quantum advantage on current noisy intermediate scale quantum (NISQ) devices and the classical simulation of quantum many-body systems. The quantum circuits are playing the key ingredient in the performance of variational quantum algorithms and quantum dynamics in problems of physics and chemistry. In this paper, we study the role of the network structure of a quantum circuit in its performance. We discuss the variational optimization of quantum circuit (a unitary tensor-network circuit) with different network structures. The ansatz is performed based on a generalization of well-developed multiscale entanglement renormalization algorithm and also the conjugate-gradient method with an effective line search. We present the benchmarking calculations for different network structures by studying the Heisenberg model in a strongly disordered magnetic field and a tensor-network $QR$ decomposition. Our work can contribute to achieve the most out of NISQ hardware and to classically develop isometric tensor network states.

Disorder is intrinsic to weakly bound ionic systems and gives rise to specific electronic processes. In recently developed perovskite ionic crystals, this dynamic lattice disorder is inferred to give rise to properties of interest, such as defect tolerance. Here, the elementary excitation of interest is the polaron, a localized lattice distortion. We employ state-resolved pump/probe spectroscopy to monitor electron and lattice dynamics in bulk $\mathrm{CsPb}{\mathrm{Br}}_3$ perovskite crystals. The data report surprising line-shape dynamics. Rather than causing redshifting of the band edge exciton, polaron formation gives rise to confinement induced dynamical processes leading to a blueshift in the band edge bleach feature. In these ionic nanocrystals, the formation of quantum confined excitons arises from the polaronic potential, as opposed to physical confinement in conventional covalent quantum dots, resulting in an excitation of a quantum confined exciton polaron. This state may represent an alternative quasiparticle.

One of the major approaches to spike-based neuromorphic computing is using memristors as analog synapses. We propose unitary quantum gates that exhibit memristive behaviors, including Ohm's law, pinched hysteresis loop and synaptic plasticity. Hysteresis depending on the quantum phase and long-term plasticity that encodes the quantum state are observed. We also propose a three-layer neural network with the capability of universal quantum computing. Quantum state classification on the memristive neural network is demonstrated. These results pave the way towards quantum spiking neural network built on unitary processes. We obtain these results in numerical simulations and experiments on the superconducting quantum computer ibmq_vigo.

A molecular Mott insulator $\kappa \text{−}{\left(\mathrm{ET}\right)}_2\mathrm{B}{\left(\mathrm{CN}\right)}_4$ [ET = bis(ethylenedithio)tetrathiafulvalene] with a distorted triangular lattice exhibits a quantum disordered state with gapped spin excitation in the ground state. ${}^{13}\mathrm{C}$ nuclear magnetic resonance, magnetization, and magnetic torque measurements reveal that magnetic field suppresses valence bond order and induces long-range magnetic order above a critical field $\sim 8$ T. The nuclear spin-lattice relaxation rate $1/T_1$ shows persistent evolution of antiferromagnetic correlation above the transition temperature, highlighting a quantum spin liquid state with fractional excitations. The field-induced transition as observed in the spin-Peierls phase suggests that the valence bond order transition is driven through renormalized one-dimensionality and spin-lattice coupling.

We propose a way to construct a thermal pure quantum matrix product state (TPQ-MPS) that can simulate finite-temperature quantum many-body systems with a minimal numerical cost comparable to the matrix product algorithm for the ground state. The MPS was originally designed for the wave function with area law entanglement. However, by attaching the auxiliary sites to the edges of the random matrix product state, we find that the degree of entanglement is automatically tuned so as to recover the volume law of the entanglement entropy that characterizes the TPQ state. The finite-temperature physical quantities of the transverse Ising and the spin-1/2 Heisenberg chains evaluated by a TPQ-MPS show excellent agreement even for bond dimension $\sim 10–20$ with those of the exact results.

We report the observation of a nontrivial emergent state in a chain of nonidentical, heterogeneously coupled oscillators where a set of weakly coupled oscillators becomes phase synchronized while the strongly coupled ones remain drifting. This intriguing “weak-winner” synchronization phenomenon can be explained by the interplay between nonisochronicity and the natural frequency of the oscillator, as coupling strength is varied. Furthermore, we present sufficient conditions under which the weak-winner phase synchronization can occur for limit cycles as well as chaotic oscillators. Employing a model system from ecology as well as a paradigmatic model from physics, we demonstrate that this phenomenon is a generic feature for a large class of coupled oscillator systems. The realization of this peculiar, yet quite generic weak-winner dynamics can have far-reaching consequences in a wide range of scientific disciplines that deal with the phenomenon of phase synchronization, including synchronization of networks. Our results also highlight the role of nonisochronicity (shear) as a fundamental feature of an oscillator in shaping emergent dynamical patterns in complex networks.

Incompatible measurements, i.e., measurements that cannot be simultaneously performed, are necessary to observe nonlocal correlations. It is natural to ask, e.g., how incompatible the measurements have to be to achieve a certain violation of a Bell inequality. In this paper, we provide the direct link between Bell nonlocality and the quantification of measurement incompatibility. This includes quantifiers for both incompatible and genuine-multipartite incompatible measurements. Our method straightforwardly generalizes to include constraints on the system's dimension (semi-device-independent approach) and on projective measurements, providing improved bounds on incompatibility quantifiers, and to include the prepare-and-measure scenario.

We study properties of the lowest energy states at nonzero total momentum (yrast states) of the Hubbard model for spin-$\frac{1}{2}$ fermions in the quantum ring configuration with attractive on-site interaction at low density. In the one-dimensional (1D) case we solve the Hubbard model using the Bethe ansatz, while for the crossover into the 2D regime we use the Full-Configuration-Interaction Quantum Monte Carlo method to obtain the yrast states for the spin-balanced Fermi system. We show how the yrast excitation spectrum changes from the 1D to the 2D regime and how pairing affects the yrast spectra. We also find signatures of fragmented condensation for certain yrast states usually associated with dark solitons.

We report spin-polarized tunneling density of states measurements of the proximity modulated superconductor-insulator transition in ultrathin Be-Al bilayers. The bilayer samples consisted of a Be film of varying thickness, $d_{\mathrm{Be}}=0.8–4.5\mathrm{nm}$, on which a 1 nm thick capping layer of Al was deposited. Detailed measurements of the Zeeman splitting of the BCS coherence peaks in samples with sheet resistances $R\sim h/4e^2$ revealed a superlinear Zeeman shift near the critical field. Our data suggests that critically disordered samples have a broad distribution of gap energies and that only the higher portion of the distribution survives as the Zeeman critical field is approached. This produces a counterintuitive field dependence in which the gap apparently increases with increasing parallel field.

Given the parallelism between the physical properties of Ce- and Yb-based magnets and heavy fermions due to the electron-hole symmetry, it has been rather odd that the transition temperature of the Yb-based compounds is normally very small, as low as $\sim 1\mathrm{K}$ or even lower, whereas Ce counterparts may often have the transition temperature well exceeding 10 K. Here, we report our experimental discovery of the transition temperature reaching 20 K in a Yb-based compound at ambient pressure. The Mn substitution at the Al site in an intermediate valence state of $\alpha \text{−}{\mathrm{YbAlB}}_4$ not only induces antiferromagnetic transition at a record high temperature of 20 K but also transforms the heavy-fermion liquid state in $\alpha \text{−}{\mathrm{YbAlB}}_4$ into a highly resistive metallic state proximate to a Kondo insulator.

In practical quantum networks, a variety of multiqubit stabilized states emitted from independent sources are distributed among the agents, and the correlations across the entire network can be derived from each agent's local measurements on the shared composite quantum systems. To reveal the Bell nonlocality in such cases as a quantum feature, minimal knowledge of the emitted stabilizer state is required. Here, we demonstrate that knowing the stabilizing and logical operators indeed provides a way of exploring Bell nonlocality in quantum networks. For the qubit distribution in quantum networks, the associated nonlinear Bell inequalities are derived. On the other hand, to violate these inequalities, one can design local incompatible observables using minimal knowledge of the emitted states. The tilted nonlinear Bell inequalities tailored for specific nonmaximal entangled stabilizer states and a way of achieving the maximal violation are also explored.

$\mathrm{SU}\left(N\right)$ Mott insulators have been proposed and/or realized in solid-state materials and with ultracold atoms on optical lattices. We study the two-dimensional $\mathrm{SU}\left(N\right)$ antiferromagnets on the triangular lattice. Starting from an $\mathrm{SU}\left(N\right)$ Heisenberg model with the fundamental representation on each site in the large-$N$ limit, we perform a self-consistent calculation and find a variety of ground states including the valence cluster states, stripe ordered states with a doubled unit cell, and topological chiral spin liquids. The system favors a cluster or ordered ground state when the number of flavors $N$ is less than 6. It is shown that increasing the number of flavors enhances quantum fluctuations and eventually transfers the clusterized ground states into topological chiral spin liquids. This chiral spin liquid ground state has an equivalent for the square lattice $\mathrm{SU}\left(N\right)$ magnets. We further identify the corresponding lowest competing states that represent another distinct type of chiral spin liquid state. We conclude with a discussion of the relevant systems and the experimental probes.

We investigate topological signatures in the short-time nonequilibrium dynamics of symmetry protected topological (SPT) systems starting from initial states which break a protecting symmetry. Naively one might expect that topology loses meaning when a protecting symmetry is broken. Defying this intuition, we illustrate, in an interacting Su-Schrieffer-Heeger (SSH) model, how this combination of symmetry breaking and quench dynamics can give rise to both single-particle and many-body signatures of topology. From the dynamics of the symmetry broken state, we find that we are able to dynamically probe the equilibrium topological phase diagram of a symmetry respecting projection of the post-quench Hamiltonian. In the ensemble dynamics we demonstrate how spontaneous symmetry breaking (SSB) of a protecting symmetry can result in a quantized many-body topological “invariant” which is not pinned under unitary time evolution. We dub this “dynamical many-body topology” (DMBT). We show numerically that both the pure state and ensemble signatures are remarkably robust, and argue that these nonequilibrium signatures should be quite generic in SPT systems, regardless of protecting symmetries or spatial dimension.

Helium is ideal for studies of quantum liquids and solids, with its significant advantage of being extremely pure at low temperatures—all except isotopic impurities are frozen out. However, even tiny concentrations of isotopic impurities have significant effects on both dynamic and thermodynamic properties. Difficulties in predicting and measuring impurities' distributions and motions make it hard to systematically study such effects. Here we report a sensitive technique combining dielectric and pressure measurements to resolve concentration changes of isotopic impurities in liquid helium with a resolution of 40 ppm. Using this technique, we observed that ${}^3\mathrm{He}$ impurities concentrate in liquid phase of solid-liquid coexisting ${}^4\mathrm{He}$ at low temperatures, and their migration between the liquid and solid phases when temperature is changed. This migration process is much slower than ${}^3\mathrm{He}$ diffusion in the liquid phase, suggesting the bottleneck is in the solid phase and possibly transmission across the liquid-solid interface.

Motivated by recent experiments reporting the suppression of the critical current in superconducting Dayem bridges by the application of strong electrostatic fields, in this paper we study the impact on the superconducting gap of charge redistribution in response to an applied electric field in thin crystalline metals. By numerically solving the BCS gap equation and the Poisson equation in a fully self-consistent way, we find that the gap becomes sensitive to the applied electric field when the size of the gap becomes an order of magnitude larger than the average level spacing of the spectrum in the normal state. In this case, the gap shows sudden rises and falls that are compatible with surface modifications of the local density of states. The effect is washed out by increasing the pairing strength toward the weak-to-moderate coupling limit or by introduction of a weak smearing in the density of states that effectively mimics a thicker sample and a weakly disordered system.

Ehrenfest urns with interaction that are connected in a ring is considered as a paradigm model for nonequilibrium thermodynamics and is shown to exhibit two distinct nonequilibrium steady states (NESS) of uniform and nonuniform particle distributions. As the interparticle attraction varies, a first-order nonequilibrium phase transition occurs between these two NESSs characterized by a coexistence regime. The phase boundaries, the NESS particle distributions near saddle points and the associated particle fluxes, average urn population fractions, and the relaxational dynamics to the NESSs are obtained analytically and verified numerically. A generalized nonequilibrium thermodynamics law is also obtained, which explicitly identifies the heat, work, energy, and entropy of the system.

The physics governing the formation of extreme coherent events, i.e., the systemwide emergence of an observable taking extraordinary values in a short time window, is a relevant yet elusive problem to a variety of disciplines ranging from climate science to neuroscience. Despite their inherent differences, systems exhibiting episodes of extreme coherence can be abstracted as a set of coupled nonlinear elements in a noisy and networked environment. Here, we propose a model describing the generation of extreme coherence by exploring theoretically and numerically the capacity of noise and network correlations to amplify a critical core of the system and trigger an extreme event. Although we principally center our study in modeling bursting phenomena in neuronal circuits, we extend our analysis to other systems such as algae blooms and infectious diseases. We show that extreme events originate in a relatively small core of the system and that different cores may coexist. We also show that the amplification mechanisms within a system are highly robust, so that the deletion of central nodes leads to other nodes taking leadership.