We investigate the impact on basic quantum transport properties of disorder-induced exceptional points (EPs) that emerge in a disorder-averaged Green's function description of two-dimensional Dirac semimetals with spin- or orbital-dependent potential scattering. Remarkably, we find that EPs may promote the nearly vanishing conductance of a finite sample at the Dirac point to a sizable value that increases with disorder strength. This striking behavior exhibits a strong directional anisotropy that is closely related to the Fermi arcs connecting the EPs. We corroborate our results by numerically exact simulations, thus revealing the fingerprints of characteristic non-Hermitian spectral features also on the localization properties of the considered systems. Finally, several candidates for the experimental verification of our theoretical analysis are discussed, including disordered electronic square-net materials and cold atoms in spin-dependent optical lattices.

When a crystal becomes thin to the atomic level, peculiar phenomena discretely depending on its layer numbers ($n$) start to appear. Here, we investigate the electronic band dispersions of multilayer ${\mathrm{WTe}}_2$ (2–5 layers), by performing laser-based microfocused angle-resolved photoelectron spectroscopy on exfoliated flakes sorted by $n$. We observe that the holelike valence bands start to cross the Fermi level when the number of layers is increased from 2- to 3 layers, which should be related to the insulator-semimetal transition, as well as the 30–70-meV spin splitting of valence bands manifesting in even $n$ as a signature of stronger structural asymmetry. Our result fully demonstrates the possibility of the large energy-scale band and spin manipulation through the finite-$n$ stacking procedure.

We consider the effects of thermal noise on the Batchelor-Kraichnan theory of high Schmidt-number mixing in the viscous dissipation range of turbulent flows at sub-Kolmogorov scales. Starting with the nonlinear Landau-Lifshitz fluctuating hydrodynamic equations for a binary fluid mixture at low Mach numbers, we justify linearization around the deterministic Navier-Stokes solution in the dissipation range. For the latter solution we adopt the standard Kraichnan model, a Gaussian random velocity with spatially constant strain but white noise in time. Then, following prior work of Donev, Fai, and Vanden-Eijnden [J. Stat. Mech.: Theory Exp. (2014) P04004], we derive asymptotic high Schmidt limiting equations for the concentration field, in which the thermal velocity fluctuations are exactly represented by a Gaussian random velocity that is likewise white in time. We obtain the exact solution for concentration spectrum in this high Schmidt limiting model, showing that the Batchelor prediction in the viscous-convective range is unaltered. Thermal noise dramatically renormalizes the bare diffusivity in this range, but the effect is the same as in laminar flow and thus hidden phenomenologically. However, in the viscous-diffusive range at scales below the Batchelor length (typically micron scales) the predictions based on deterministic Navier-Stokes equations are drastically altered by thermal noise. Whereas the classical theories predict rapidly decaying spectra in the viscous-diffusive range, either Gaussian or exponential, we obtain a $k^{-2}$ power-law spectrum over a couple of decades starting just below the Batchelor length. This spectrum corresponds to nonequilibrium giant concentration fluctuations, which are due to the imposed concentration variations being advected by thermal velocity fluctuations and which are experimentally well observed in quiescent fluids. At higher wave numbers, the concentration spectrum instead goes to a $k^2$ equipartition spectrum due to equilibrium molecular fluctuations. We work out detailed predictions for water-glycerol and water-fluorescein mixtures. Finally, we discuss broad implications for turbulent flows and applications of our methods to experimentally accessible laminar flows.

Single strontium atoms held in optical tweezers have so far only been imaged using the broad ${}^1{S}_0{\text{−}}^1{P}_1$transition. For Yb, use of the narrow (183-kHz wide) ${}^1{S}_0{\text{−}}^3{P}_1$transition for simultaneous imaging and cooling has been demonstrated in tweezers with a magic wavelength for the imaging transition. We demonstrate high-fidelity imaging of single Sr atoms using its even narrower (7.4-kHz wide) ${}^1{S}_0{\text{−}}^3{P}_1$transition. The atoms are trapped in nonmagic-wavelength tweezers. We detect the photons scattered during Sisyphus cooling, thus keeping the atoms near the motional ground state of the tweezer throughout imaging. The fidelity of detection is 0.9991(4) with a survival probability of 0.97(2). An atom in a tweezer can be held under imaging conditions for 79(3) seconds allowing for hundreds of images to be taken, limited mainly by background gas collisions. The use of a fully closed (cycling) transition for imaging will provide a useful tool for state specific detection. We detect atoms in an array of 36 tweezers with 813.4-nm light and trap depths of 135(20) $\mu$K. This trap depth is three times shallower than typically used for imaging on the broad ${}^1{S}_0{\text{−}}^1{P}_1$transition. Narrow-line imaging opens the possibility to even further reduce this trap depth, as long as all trap frequencies are kept larger than the imaging transition linewidth. Imaging using a narrow-linewidth transition in a nonmagic-wavelength tweezer also allows for selective imaging of a given tweezer. As a demonstration, we selectively image (hide) a single tweezer from the array. This provides a useful tool for quantum error correction protocols.

The recently discovered vanadium-based kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A$ = K, Rb, Cs) exhibit superconductivity at low temperatures and charge density wave (CDW) order at high temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below $T_{\mathrm{TRSB}}=T_{\mathrm{CDW}}\simeq 95$ K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the $ab$ plane to the $c$ axis, was detected below $T^{*}\simeq 30$ K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8 T is applied along the crystallographic $c\mathrm{axis}$, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ by charge order with an onset of $\simeq 95$ K, followed by an enhanced electronic response below $\simeq 30$ K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy.

Unitary coupled cluster (UCC) approaches are an appealing route to utilizing quantum hardware to perform quantum chemistry calculations, as quantum computers can in principle perform UCC calculations in a polynomially scaling fashion, as compared with the exponential scaling required on classical computers. Current noisy intermediate scale quantum computers are limited by both hardware capacity in number of logical qubits and the noise introduced by the deep circuits required for UCC calculations using the variational quantum eigensolver (VQE) approach. We present a combined classical-quantum approach where a stochastic classical UCC preprocessing step is used to determine the important excitations in the UCC Ansatz. The reduced number of selected excitations are then used in a UCC-based VQE calculation. This approach gives a systematically improvable approximation, and we show that significant reductions in quantum resources can be achieved, with simulations on the ${\mathrm{CH}}_2, {\mathrm{N}}_2$, and ${\mathrm{N}}_2{\mathrm{H}}_2$ molecules giving submillihartree errors.

A complex fractional derivative can be derived by formally extending the integer $k$ in the $k\mathrm{th}$ derivative of a function, computed via Cauchy's integral, to complex $\alpha$. This straightforward approach reveals fundamental problems due to inherent nonanalyticity. A consequence is that the complex fractional derivative is not uniquely defined. We explain in detail the anomalies (not closed paths, branch cut jumps) and try to interpret their meaning physically in terms of entropy, friction and deviations from ideal vector fields. Next, we present a class of non-Markovian continuous-time processes by replacing the standard derivative by a Caputo fractional derivative in the classical Chapman-Kolmogorov governing equation of a continuous-time Markov process. The fractional derivative leads to a replacement of the set of exponential base functions by a set of Mittag-Leffler functions, but also creates a complicated dependence structure between states. This fractional non-Markovian process may be applied to generalize the Markovian SIS epidemic process on a contact graph to a more realistic setting.

The electronic band structure of ${\mathrm{Au}}_2\mathrm{Pb}$ has a Dirac cone which gaps when undergoing a structural transition into a low temperature superconducting phase. This suggests that the superconducting phase ($T_c=1.1$ K) might hold topological properties at the surface. Here we make Scanning Tunneling Microscopy experiments on the surface of superconducting ${\mathrm{Au}}_2\mathrm{Pb}$. We measure the superconducting gap and find a sizable superconducting density of states at the Fermi level. We discuss possible origins for this finding in terms of superconductivity induced into surface states.

Statistical inference is central to many scientific endeavors, yet how it works remains unresolved. Answering this requires a quantitative understanding of the intrinsic interplay between statistical models, inference methods, and the structure in the data. To this end, we characterize the efficacy of direct coupling analysis (DCA)—a highly successful method for analyzing amino acid sequence data—in inferring pairwise interactions from samples of ferromagnetic Ising models on random graphs. Our approach allows for physically motivated exploration of qualitatively distinct data regimes separated by phase transitions. We show that inference quality depends strongly on the nature of data-generating distributions: optimal accuracy occurs at an intermediate temperature where the detrimental effects from macroscopic order and thermal noise are minimal. Importantly our results indicate that DCA does not always outperform its local-statistics-based predecessors; while DCA excels at low temperatures, it becomes inferior to simple correlation thresholding at virtually all temperatures when data are limited. Our findings offer insights into the regime in which DCA operates so successfully, and more broadly, how inference interacts with the structure in the data.

Multiple intriguing low-temperature phenomena have recently been discovered in the family of chiral cubic Co-Zn-Mn compounds with a $\beta$-Mn-type structure. In particular, ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ displays a reduction of the helical spiral pitch on cooling along with lattice shape transformations of metastable skyrmions and the manifestation of peculiar magnetic textures due to strong magnetocrystalline anisotropy. Here we report on temperature-dependent measurements of helimagnon excitations in the field polarized regime ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ using the spin-wave (SW) small-angle neutron-scattering technique. By applying a new analytical expression to interpret the data, quantitative estimates for both SW stiffness and damping are extracted across a wide temperature range between 70 and 250 K. We speculate that their nontrivial temperature dependencies arise due to the effects of magnetic frustration arising from Mn magnetic moments, which is further reflected in continuous variations of both exchange and Dzyaloshinskii-Moriya interactions.

Finding the closest separable state to a given target state is a notoriously difficult task, even more difficult than deciding whether a state is entangled or separable. To tackle this task, we parametrize separable states with a neural network and train it to minimize the distance to a given target state with respect to a differentiable distance, such as the trace distance or Hilbert–Schmidt distance. By examining the output of the algorithm, we obtain an upper bound on the entanglement of the target state, and construct an approximation for its closest separable state. We benchmark the method on a variety of well-known classes of bipartite states and find excellent agreement, even up to local dimension of $d=10$, while providing conjectures and analytic insight for isotropic and Werner states. Moreover, we show our method to be efficient in the multipartite case, considering different notions of separability. Examining three- and four-party GHZ and W states, we recover known bounds and obtain additional ones, for instance, for triseparability.

The system of linear equations problem (SLEP) is specified by a complex invertible matrix $A$, the condition number $\kappa$ of $A$, a vector $\mathit{b}$, a Hermitian matrix $M$, and an accuracy $\epsilon$, and the task is to estimate ${\mathit{x}}^{†}M\mathit{x}$, where $\mathit{x}$ is the solution vector to the equation $A\mathit{x}=\mathit{b}$. We aim to establish a lower bound on the complexity of the end-to-end quantum algorithms for SLEP with respect to $\epsilon$, and devise a quantum algorithm that saturates this bound. To make lower bounds attainable, we consider query complexity in the setting in which a block encoding of $M$ is given, i.e., a unitary black box $U_M$ that contains $M/\alpha$ as a block for some $\alpha \in {\mathbb{R}}^{+}$. We show, by constructing a quantum algorithm and deriving a lower bound, that the quantum query complexity for SLEP in this setting is $\mathrm{\Theta }(\alpha /\epsilon )$. Our lower bound is established by reducing the problem of estimating the mean of a black box function to SLEP. Our $\mathrm{\Theta }(\alpha /\epsilon )$ result tightens and proves the common assertion of polynomial accuracy dependence $\left[\mathrm{poly}\right(1/\epsilon \left)\right]$ for SLEP without making any complexity-theoretic assumptions, and shows that improvement beyond linear dependence on accuracy is not possible if $M$ is provided via block encoding.

We propose to use the Hall response of topological defects, such as merons and antimerons, to spin currents in two-dimensional magnetic insulator with in-plane anisotropy for identification of the Berezinskii-Kosterlitz-Thouless (BKT) transition in a transistorlike geometry. Our numerical results relying on a combination of Monte Carlo and spin dynamics simulations show transition from spin superfluidity to conventional spin transport, accompanied by the universal jump of the spin stiffness and exponential growth of the transverse vorticity current. We propose a superfluid spin transistor in which the spin and vorticity currents are modulated by changes in density of free topological defects, e.g., by injection of vorticity or by tuning the in-plane magnet across the BKT transition by changing the exchange interaction, magnetic anisotropy, or temperature.

Disordered dielectric materials with short-range spatial correlations on length scales comparable to the wavelength of light display a rich variety of optical phenomena: photonic band gaps, structural coloring, strong scattering, and whiteness. However, a lack of reliable and straightforward analytical models complicates the rationale design of optical materials for specific applications. Here, we demonstrate how to accurately introduce collective scattering and define the effective medium in single scattering from heterogeneous dielectrics with a substantial refractive index contrast, starting from fundamental principles. Our model captures the effective medium's role in the momentum transfer definition for the particle form and structure factor in the forward scattering regime. We support our claims through transfer matrix calculations scattering from particle clusters.

Nonequilibrium transportation of particles through a restricted space (such as porous media or narrow channels) significantly differs from free space. With a simple model of two types of particles competing to transport via a passive single-lane channel connecting two chemical baths, we find two dynamical modes of transportation—the dud mode and the ratchet mode. At the ratchet mode, the gradient flow of one type of particle forces the other into a transient antiequilibrium flow against its gradient. At the dud mode, the two types of particles both flow according to their gradients. This counterintuitive nonequilibrium effect can be explained by a sloppy gear mechanism. In addition to the antigradient flow observed in the ratchet mode, we find that the nonequilibrium sloppy gear mechanism suppresses the flow of one particle while allowing the flow for the other. This mechanism provides a general theoretical framework to explain and design the selective transportation of particles via passive channels.

We study the topological photonic states in one-dimensional lattices analog to the Su-Schrieffer-Heeger model beyond the dipole approximation. The electromagnetic resonances of the lattices supported by near-field interactions between the plasmonic nanoparticles are studied analytically with coupled dipole-quadrupole method. The topological phase transition in the bipartite lattices is determined by the change of Zak phase. Our results reveal the contribution of quadrupole moments to the near-field interactions and the band topology. It is found that the topological edge states in nontrivial lattices have both dipolar and quadrupolar nature. The quadrupolar edge states are not only orthogonal to the dipolar edge states, but also spatially localized at different sublattices. Furthermore, the quadrupolar topological edge states, which coexist at the same energy with the quadrupolar flat band have shorter localization length and hence smaller mode volume than the conventional dipolar edge states. The findings deepen our understanding in topological systems that involve higher-order multipoles, or in analogy to the wave functions in quantum systems with higher-orbital angular momentum, and may be useful in designing topological systems for confining light robustly and enhancing light-matter interactions.