We theoretically study the dynamical phase diagram of the Dicke model in both classical and quantum limits using large, experimentally relevant system sizes. Our analysis elucidates that the model features dynamical critical points that are strongly influenced by features of chaos and emergent integrability in the model. Moreover, our numerical calculations demonstrate that mean-field features of the dynamics remain valid in the exact quantum dynamics, but we also find that in regimes where quantum effects dominate signatures of the dynamical phases and chaos can persist in purely quantum metrics such as entanglement and correlations. Our predictions can be verified in current quantum simulators of the Dicke model including arrays of trapped ions.

A high gain parametric amplifier with a single-pass pulsed pump is known to generate broadband twin photon fields that are entangled in amplitude and phase but have complicated spectral correlation. Fortunately, they can be decomposed into independent temporal modes. However, the early treatment of the parametric interaction Hamiltonian at strong pumping does not consider the issue of the time-ordering problem of the interaction Hamiltonian and thus leads to the inaccurate conclusion that the mode structure and the temporal mode functions do not change as the gain increases. Recent studies have revealed the change of the spectrum with the gain of the process. In this paper we use an approach that is usually employed for treating nonlinear interferometers and avoids the time-ordering issue. This allows us to derive an evolution equation in differential-integral form. Numerical solutions for the high gain situation indicate a gain-dependent mode structure that has its mode distributions changed and mode functions broadened as the gain increases. This study will enable us to have a complete picture of the mode structure of parametric processes and produce high quality quantum sources for a variety of applications of quantum technology.

Non-Hermitian optical metasurfaces have recently attracted interest as a platform for controlling light in amplitude, phase, and polarization. Here we predict that breaking parity-time symmetry in honeycomblike active plasmonic metasurfaces can lead to nonunitary circular dichroism at oblique incidence. This extraordinary chiroptical response is achieved through band folding that enables coupling of incident light to spin-polarized flat bands surrounded by exceptional rings, formed at Dirac points of plasmonic lattice resonances. The reported spin polarization differs from previously reported chiral and pseudochiral phenomena in that it solely emerges from spatial distribution of gain and loss and at the same time it does not involve the inherent chirality of isolated exceptional points, but instead it can be considered as a non-Hermitian counterpart of spin polarization of $K$-point valleys. Our findings may become useful in designing and realizing novel polarization-controlling optical elements and spin-polarized exceptional ring lasers.

The nonlinear response associated with the current dependence of the superconducting kinetic inductance was studied in capacitively shunted NbTiN microstrip transmission lines. It was found that the inductance per unit length of one microstrip line could be changed by up to 20% by applying a dc current, corresponding to a single-pass time delay of 0.7 ns. To investigate nonlinear dissipation, Bragg reflectors were placed on either end of a section of this type of transmission line, creating resonances over a range of frequencies. From the change in the resonance linewidth and amplitude with dc current, the ratio of the reactive to the dissipative response of the line was found to be 788. The low dissipation makes these transmission lines suitable for a number of applications that are microwave- and millimeter-wave band analogs of nonlinear optical processes. As an example, by applying a millimeter-wave pump tone, very-wide-band parametric amplification was observed between about 3 and 34 GHz. Use as a current variable delay line for an on-chip millimeter-wave Fourier transform spectrometer is also considered.

We consider one-dimensional quantum walks in optical linear networks with synthetically introduced disorder and tunable system parameters allowing for the engineered realization of distinct topological phases. The option to directly monitor the walker's probability distribution makes this optical platform ideally suited for the experimental observation of the unique signatures of the one-dimensional topological Anderson transition. We analytically calculate the probability distribution describing the quantum critical walk in terms of a (time-staggered) spin polarization signal and propose a concrete experimental protocol for its measurement. Numerical simulations back the realizability of our blueprint with current date experimental hardware.

The fundamental band gaps of liquid water and hexagonal ice are calculated through advanced electronic-structure methods. We compare specifically the performance of state-of-the-art $GW$ calculations with nonempirical hybrid functionals. For the latter, we fix the free parameters either through the dielectric response of the material or through enforcing Koopmans' condition to localized states. The various approaches yield consistent band gaps, in good agreement with available experimental references. Furthermore, we discuss the critical aspects of each approach that underlie the band-gap predictions.

We report ferromagnetism in $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.22}\mathrm{Ta}{\mathrm{S}}_2$ single crystals where Co atoms are intercalated in the van der Waals gap, and a systematic study of its magnetic critical behavior in the vicinity of $T_c\sim 28$ K. The obtained critical exponents $\beta =0.43\left(2\right)$, $\gamma =1.15\left(1\right)$, and $\delta =3.54\left(1\right)$ fulfill the Widom scaling relation $\delta =1+\gamma /\beta$ and follow the scaling equation. This indicates that the spin coupling in $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.22}\mathrm{Ta}{\mathrm{S}}_2$ is of three-dimensional Heisenberg type coupled with long-range magnetic interaction, and that the exchange interaction decays with distance as $J\left(r\right)\approx r^{-4.69}$. $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.22}\mathrm{Ta}{\mathrm{S}}_2$ exhibits a weak temperature-dependent metallic behavior in resistivity and negative values of thermopower with dominant electron-type carriers, in which obvious anomalies were observed below $T_c$ as well as the anomalous Hall effect (AHE). The linear scaling behavior between the modified anomalous Hall resistivity ${\rho }_{xy}/{\mu }_{0}H$ and longitudinal resistivity ${\rho }_{xx}^{2}M/{\mu }_{0}H$ implies that the origin of the AHE in $2\mathrm{H}\text{−}{\mathrm{Co}}_{0.22}\mathrm{Ta}{\mathrm{S}}_2$ should be dominated by the extrinsic side-jump mechanism.

Quasicrystals lack translational symmetry, but can still exhibit long-range order, promoting them to candidates for unconventional physics beyond the paradigm of crystals. Here, we apply a real-space functional renormalization group approach to the prototypical quasicrystalline Penrose tiling Hubbard model treating competing electronic instabilities in an unbiased, beyond-mean-field fashion. Our work reveals a delicate interplay between charge and spin degrees of freedom in quasicrystals. Depending on the range of interactions and hopping amplitudes, we unveil a rich phase diagram including antiferromagnetic orderings, charge density waves, and subleading, superconducting pairing tendencies. For certain parameter regimes, we find a competition of phases, which is also common in crystals, but additionally encounter phases coexisting in a spatially separated fashion and ordering tendencies which mutually collaborate to enhance their strength. We therefore establish that quasicrystalline structures open up a route towards this rich ordering behavior uncommon to crystals and that an unbiased, beyond-mean-field approach is essential to describe this physics of quasicrystals correctly.

Superconductivity in a crystalline lattice without inversion is subject to complex spin-orbit-coupling effects, which can lead to mixed-parity pairing and an unusual magnetic response. In this study, the properties of a layered superconductor with alternating Rashba spin-orbit coupling in the stacking of layers, hence (globally) possessing a center of inversion, is analyzed in an applied magnetic field, using a generalized Ginzburg-Landau model. The superconducting order parameter consists of an even- and an odd-parity pairing component which exchange their roles as dominant pairing channel upon increasing the magnetic field. This leads to an unusual kink feature in the upper critical field and a first-order phase transition within the mixed phase. We investigate various signatures of this internal phase transition. The physics we discuss here could explain the recently found $H\text{−}T$ phase diagram of the heavy-fermion superconductor $\mathrm{Ce}{\mathrm{Rh}}_2{\mathrm{As}}_2$.

Matter-wave interferometers with large momentum transfers, irrespective of specific implementations, will face a universal dephasing due to relative accelerations between the interferometric mass and the associated apparatus. Here we propose a solution that works even without actively tracking the relative accelerations: putting both the interfering mass and its associated apparatus in a freely falling capsule, so that the strongest inertial noise components vanish due to the equivalence principle. In this setting, we investigate two of the most important remaining noise sources: (a) the noninertial jitter of the experimental setup and (b) the gravity-gradient noise. We show that the former can be reduced below desired values by appropriate pressures and temperatures, while the latter can be fully mitigated in a controlled environment. We finally apply the analysis to a recent proposal for testing the quantum nature of gravity [S. Bose et al., Phys. Rev. Lett. 119, 240401 (2017)] through the entanglement of two masses undergoing interferometry. We show that the relevant entanglement witnessing is feasible with achievable levels of relative acceleration noise.

Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An excellent example of a controlled QPT is the field-induced ordering of a gapped quantum magnet. Although numerous “quasi-one-dimensional” coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3D), quasi-two-dimensional (2D) systems are special for multiple reasons. Motivated by the ancient pigment Han purple (${\mathrm{BaCuSi}}_2{\mathrm{O}}_6$), a quasi-2D material displaying anomalous critical properties, we present a complete analysis of ${\mathrm{Ba}}_{0.9}{\mathrm{Sr}}_{0.1}{\mathrm{CuSi}}_2{\mathrm{O}}_6$. We measure the zero-field magnetic excitations by neutron spectroscopy and deduce the spin Hamiltonian. We probe the field-induced transition by combining magnetization, specific-heat, torque, and magnetocalorimetric measurements with nuclear magnetic resonance studies near the QPT. With a Bayesian statistical analysis and large-scale Quantum Monte Carlo simulations, we demonstrate unambiguously that observable 3D quantum critical scaling is restored by the structural simplification arising from light Sr substitution in Han purple.

We study the spectral statistics of spatially extended many-body quantum systems with on-site Abelian symmetries or local constraints, focusing primarily on those with conserved dipole and higher moments. In the limit of large local Hilbert space dimension, we find that the spectral form factor $K\left(t\right)$ of Floquet random circuits can be mapped exactly to a classical Markov circuit, and, at late times, is related to the partition function of a frustration-free Rokhsar-Kivelson (RK) type Hamiltonian. Through this mapping, we show that the inverse of the spectral gap of the RK Hamiltonian lower bounds the Thouless time $t_{\mathrm{Th}}$ of the underlying circuit. For systems with conserved higher moments, we derive a field theory for the corresponding RK Hamiltonian by proposing a generalized height field representation for the Hilbert space of the effective spin chain. Using the field theory formulation, we obtain the dispersion of the low-lying excitations of the RK Hamiltonian in the continuum limit, which allows us to extract $t_{\mathrm{Th}}$. In particular, we analytically argue that in a system of length $L$ that conserves the $m\mathrm{th}$ multipole moment, $t_{\mathrm{Th}}$ scales subdiffusively as $L^{2(m+1)}$. We also show that our formalism directly generalizes to higher dimensional circuits, and that in systems that conserve any component of the $m\mathrm{th}$ multipole moment, $t_{\mathrm{Th}}$ has the same scaling with the linear size of the system. Our work therefore provides a general approach for studying spectral statistics in constrained many-body chaotic systems.

Information-theoretic uncertainty relations formulate the joint immeasurability of two noncommuting observables in terms of information entropies. The tradeoff of the accuracy in the outcome of two successive measurements manifests in entropic noise-disturbance uncertainty relations. Recent theoretical analysis predicts that projective measurements are not optimal, with respect to the noise-disturbance tradeoffs. Therefore, the results in our previous paper [Phys. Rev. Lett. 115, 030401 (2015)] are outperformed by general quantum measurements. Here, we experimentally test a tight information-theoretic measurement uncertainty relation for three-outcome positive-operator-valued measures, using neutron spin-$\frac{1}{2}$ qubits. The obtained results violate the lower bound for projective measurements as theoretically predicted.

Competing ground states may lead to topologically constrained excitations such as domain walls or quasiparticles, which govern metastable states and their dynamics. Domain walls and more exotic topological excitations are well studied in magnetic systems such as artificial spin ice, in which nanoscale magnetic dipoles are placed on geometrically frustrated lattices, giving rise to highly degenerate ground states. We propose a mechanical spin-ice constructed from a lattice of floppy, bistable square unit cells. We compare the domain wall excitations that arise in this metamaterial to their magnetic counterparts, finding that new behaviors emerge in this overdamped mechanical system. By tuning the ratios of the internal elements of the unit cell, we control the curvature and propagation speed of internal domain walls. We change the domain wall morphology from a binary, strictly spinlike regime, to a more continuous, elastic regime. In the elastic regime, we inject, manipulate, and expel domain walls via textured forcing at the boundaries. The system exhibits dynamical hysteresis, and we find a first-order dynamical transition as a function of the driving frequency. We demonstrate a forcing protocol that produces multiple, topologically distinct steady cycles, which are protected by the differences in their internal domain wall arrangements. These distinct steady cycles rapidly proliferate as the complexity of the applied forcing texture is increased, thus suggesting that such mechanical systems could serve as useful model systems to study multistability, glassiness, and memory in materials.

The fermion sign problem, when severe, prevents the computation of physical quantities of a system of interacting fermions via stochastic evaluation of its path integral. This is due to the oscillatory nature of the integrand $\exp (-S),$ where $S$ is the imaginary-time action. This issue is a major obstacle to first-principles lattice quantum Monte Carlo studies of excited states of electrons in matter. However, in the Kohn-Sham orbital basis, which is the output of a density-functional theory simulation, the path integral for electrons in a semiconductor nanoparticle has only a mild fermion sign problem and is amenable to evaluation by standard stochastic methods. This is evidenced by our simulations of silicon hydrogen-passivated nanocrystals such as ${\mathrm{Si}}_{35}{\mathrm{H}}_{36},{\mathrm{Si}}_{87}{\mathrm{H}}_{76},{\mathrm{Si}}_{147}{\mathrm{H}}_{100}$, and ${\mathrm{Si}}_{293}{\mathrm{H}}_{172},$ which range in size $1.0-2.4$ nm and contain 176 to 1344 valence electrons. We find that approximating the fermion action by its leading order polarization term results in a positive-definite integrand in the functional integral, and is a very good approximation of the full action. We compute imaginary-time electron propagators and extract the energies of low-lying electron and hole levels. Our quasiparticle gap predictions agree with the results of previous high-precision $G_0{W}_0$ calculations. This formalism allows calculations of more complex excited states such as excitons and trions.

Multimaterial optical coatings are a promising viable option to meet the challenging requirements (in terms of transmittance, absorbance, and thermal noise) of next-generation gravitational wave detector mirrors. In this paper we focus on ternary coatings consisting of quarter-wavelength-thick layers, where a third material ($H^{\prime }$) is added to the two presently in use, namely, silica ($L$) and titania-doped tantala ($H$), featuring higher dielectric contrast (against silica) and lower thermal noise (compared with titania-doped tantala), but higher optical losses. We seek the optimal material sequences, featuring minimal thermal (Brownian) noise under prescribed transmittance and absorbance constraints, by exhaustive simulation over all possible configurations, for different values of the optical density and extinction coefficient of the third material, including the case of amorphous silicon and silicon nitride operating at ambient and cryogenic temperatures. In all cases studied, the optimal designs consist of a stack of $\left(H^{\prime }\right|L)$ doublets topped by a stack of $\left(H\right|L)$ doublets, confirming previous heuristic assumptions, and the achievable coating noise power spectral density reduction factor ranges from $\sim 0.5$ at 290 $\text{K}$ down to $\sim 0.1$ at 20 $\text{K}$. The robustness of the found optimal designs against layer thickness deposition errors and uncertainties and/or fluctuations in the optical losses of the third material is also investigated. Possible margins for further thermal noise reduction by layer thickness optimization, and strategies to implement it, are discussed.

Networks of strongly coupled neurons with random connectivity exhibit chaotic, asynchronous fluctuations. In a previous study, we showed that when endowed with an additional component of low-rank connectivity consisting of the outer product of orthogonal vectors, these networks generate macroscopic coherent fluctuations. Although a striking phenomenon, that result depended on a fine-tuned choice of low-rank structure. Here we extend that result by generalizing the theory of excitation-inhibition balance to networks with arbitrary low-rank structure and show that macroscopic fluctuations emerge intrinsically through what we call “incomplete recurrent alignment.” We say that a low-rank connectivity structure exhibits incomplete alignment if its row space is not contained in its column space. In the generic setting of incomplete alignment, recurrent connectivity can be decomposed into a “subspace-recurrent” component and an “effective-feedforward” component. We show that the balance equations of excitation-inhibition networks generalize naturally to the setting of arbitrary strongly coupled low-rank structure and are determined by the subspace-recurrent component of connectivity. The effective-feedforward component, meanwhile, projects high-dimensional, microscopic fluctuations to a low-dimensional subspace where they are dynamically balanced by emergent macroscopic fluctuations. We present biologically plausible examples from excitation-inhibition networks and networks with heterogeneous degree distributions. Finally, we define the “alignment matrix” as the matrix of overlaps between left and right singular vectors of the structured connectivity, and show that the singular values of the alignment matrix determine the amplitude of macroscopic, low-dimensional variability, while its singular vectors determine the structure. Our work shows how low-dimensional fluctuations can emerge generically in strongly coupled networks with low-rank structure. Furthermore, by generalizing excitation-inhibition balance to arbitrary low-rank structure, our work may find relevance in any setting with strongly interacting units, whether in biological, social, or technological networks.

Quantum game theory has stimulated some interest in recent years with the advancement of quantum information theory. This interest has led to a resurgence of quantum Parrondo's games. With two losing games combining to give a winning game, this paradoxical idea is known as Parrondo's paradox. By using chaotic switching between the two losing quantum games, we show that it is possible to achieve Parrondo's paradox involving a quantum walker playing two-sided quantum coin tossing games. Furthermore, we show that the framework of chaotic switching in quantum coin tosses can be applied to encryption. This is a proposal to deploy a quantum coin toss with chaotic switching for semiclassical encryption.

Topological Dirac and Weyl semimetals are currently attracting intense interest due to their exotic physical properties. Transition metal diarsenides such as ${\mathrm{MoAs}}_2$ and ${\mathrm{WAs}}_2$ have been reported to harbor very high magnetoresistance suggesting the possible existence of a topological quantum state, although this conclusion remains dubious. Here, using systematic angle-resolved photoemission spectroscopy (ARPES) measurements and parallel first-principles calculations, we discuss the electronic structures of ${\mathrm{TAs}}_2$ (T = Mo, W). Two different cleavage planes of ${\mathrm{MoAs}}_2$ are found to harbor distinctly different surface states. Our experiments show the presence of Dirac-like dispersions on the (001) plane, which our first-principles calculations ascribe to trivial surface states. Our in-depth study also finds ${\mathrm{WAs}}_2$ to possess a trivial electronic structure. Our study emphasizes the importance of identifying the cleavage plane in low-symmetry systems and indicates that topological semimetallic states are not the key for generating high magnetoresistance in ${\mathrm{MoAs}}_2$ and ${\mathrm{WAs}}_2$.

Efficiency of search for randomly distributed targets is a prominent problem in many branches of the sciences. For the stochastic process of Lévy walks, a specific range of optimal efficiencies was suggested under variation of search intrinsic and extrinsic environmental parameters. In this paper, we study fractional Brownian motion as a search process, which under parameter variation generates all three basic types of diffusion, from sub- to normal to superdiffusion. In contrast to Lévy walks, fractional Brownian motion defines a Gaussian stochastic process with power-law memory yielding antipersistent, respectively persistent motion. Computer simulations of search by time-discrete fractional Brownian motion in a uniformly random distribution of targets show that maximising search efficiencies sensitively depends on the definition of efficiency, the variation of both intrinsic and extrinsic parameters, the perception of targets, the type of targets, whether to detect only one or many of them, and the choice of boundary conditions. In our simulations, we find that different search scenarios favor different modes of motion for optimising search success, defying a universality across all search situations. Some of our numerical results are explained by a simple analytical model. Having demonstrated that search by fractional Brownian motion is a truly complex process, we propose an overarching conceptual framework based on classifying different search scenarios. This approach incorporates search optimization by Lévy walks as a special case.