We develop the hydrodynamic theory for number conserving asymmetric exclusion processes with short-range random quenched disordered hopping rates, which is a one-dimensional Kardar-Parisi-Zhang (KPZ) equation with quenched columnar disorder. We show that when the system is away from half-filling, the universal spatiotemporal scaling of the density fluctuations is indistinguishable from its pure counterpart, with the model belonging to the one-dimensional Kardar-Parisi-Zhang universality class. In contrast, close to half-filling, the quenched disorder is relevant, leading to a new universality class. We physically argue that the irrelevance of the quenched disorder when away from half-filling is a consequence of the averaging of the disorder by the propagating density fluctuations in the system. In contrast, close to half-filling the density fluctuations are overdamped, and as a result, are strongly influenced by the quenched disorder.

The resilience of quantum entanglement to a classicality-inducing environment is tied to fundamental aspects of quantum many-body systems. The dynamics of entanglement has recently been studied in the context of measurement-induced entanglement transitions, where the steady-state entanglement collapses from a volume law to an area law at a critical measurement probability $p_c$. Interestingly, there is a distinction in the value of $p_c$ depending on how well the underlying unitary dynamics scramble quantum information. For strongly chaotic systems, $p_c>0$, whereas for weakly chaotic systems, such as integrable models, $p_c=0$. In this work, we investigate these measurement-induced entanglement transitions in a system where the underlying unitary dynamics are many-body localized (MBL). We demonstrate that the emergent integrability in an MBL system implies a qualitative difference in the nature of the measurement-induced transition depending on the measurement basis, with $p_c>0$ when the measurement basis is scrambled and $p_c=0$ when it is not. This feature is not found in Haar-random circuit models, where all local operators are scrambled in time. When the transition occurs at $p_c>0$, we use finite-size scaling to obtain the critical exponent $\nu =1.3\left(2\right)$, close to the value for (2+0)-dimensional percolation. We also find a dynamical critical exponent of $z=0.98\left(4\right)$ and logarithmic scaling of the Rényi entropies at criticality, suggesting an underlying conformal symmetry at the critical point. This work further demonstrates how the nature of the measurement-induced entanglement transition depends on the scrambling nature of the underlying unitary dynamics. This leads to further questions on the control and simulation of entangled quantum states by measurements in open quantum systems.

We investigate spinon band topology and engineering from the interplay between long-ranged magnetic order and fractionalized spinons, as well as Zeeman coupling under external magnetic fields, in honeycomb lattice magnets. The synergism of Néel order and magnetic fields could reconstruct the spinon bands and drive a topological phase transition from the coexisting phase of long-ranged order and chiral spin liquid with semion topological order to the conventional magnetic order. Our prediction can be immediately tested through thermal Hall transport measurements among the honeycomb lattice magnets that are tuned to be proximate to the quantum critical point. Our theory should also shed light on the critical behavior of honeycomb Kitaev materials with emergent Majorana fermion bands. We suggest a possible relevance to the spin-1/2 honeycomb spin liquid candidate material ${\mathrm{In}}_3{\mathrm{Cu}}_2{\mathrm{VO}}_9$.

Both linguistic and genetic evolution involve copying and mutation of variants. The simplest copying process assumes that variants are reproduced at a rate equal to their current frequency, exemplified by Kimura's stepping stone model of neutral evolution, and the voter model. In this case, spatial patterns are driven by noise. In the linguistic context, an alternative possibility is that speakers preferentially select variants which are already popular, yielding patterns driven by surface tension, exemplified by the Ising model. In this paper, we model language change using a spatial network of speakers, inspired by the Hopfield neural network. The model's universality class—Voter or Ising—is determined by speakers' learning function. We view maps generated by the Survey of English Dialects as samples from our network. Maximum likelihood analysis, and comparison of spatial auto-correlations between real and simulated maps, indicates that the underlying copying processes is more likely to belong to the conformity-driven Ising class.

A class of photocurrents is predicted to occur in both type-I and type-II Weyl semimetals. Unlike the previously studied photocurrents in chiral materials, the proposed current requires neither circularly polarized light nor an absence of symmetry with respect to a plane of reflection. We show that if a Weyl semimetal has a broken inversion symmetry then linearly polarized light can induce a photocurrent transverse to the direction of an applied magnetic field, in spite of the symmetry with respect to a reflection plane and the time reversal symmetry. The class of materials in which we expect this to occur is sufficiently broad and includes the transition metal monopnictides such as TaAs. The effect stems from the dynamics of Weyl chiral quasiparticles in a magnetic field, restricted by the symmetries described above; because the resulting current is transverse to the direction of magnetic field, we call it the transverse chiral magnetic photocurrent. The magnitude of the resulting photocurrent is predicted to be significant in the THz frequency range, about $0.75\mu \mathrm{A}$ for type-I and $2.5\mu \mathrm{A}$ for type-II Weyl semimetals. This opens the possibility to utilize the predicted transverse chiral magnetic photocurrent for sensing unpolarized THz radiation.

In recent theoretical and experimental investigations, researchers have linked the low-energy field theory of a Weyl semimetal gapped with a charge-density wave (CDW) to high-energy theories with axion electrodynamics. However, it remains an open question whether a lattice regularization of the dynamical Weyl-CDW is in fact a single-particle axion insulator (AXI). In this Rapid Communication, we use analytic and numerical methods to study both lattice-commensurate and incommensurate minimal (magnetic) Weyl-CDW phases in the mean-field state. We observe that, as previously predicted from field theory, the two inversion ($\mathcal{I}$)-symmetric Weyl-CDWs with $\varphi =0,\pi$ differ by a topological axion angle $\delta {\theta }_{\varphi }=\pi$. However, we crucially discover that neither of the minimal Weyl-CDW phases at $\varphi =0,\pi$ is individually an AXI; they are instead quantum anomalous Hall (QAH) and “obstructed” QAH insulators that differ by a fractional translation in the modulated cell, analogous to the two phases of the Su-Schrieffer-Heeger model of polyacetylene. Using symmetry indicators of band topology and non-Abelian Berry phase, we demonstrate that our results generalize to multiband systems with only two Weyl fermions, establishing that minimal Weyl-CDWs unavoidably carry nontrivial Chern numbers that prevent the observation of a static magnetoelectric response. We discuss the experimental implications of our findings and provide models and analysis generalizing our results to nonmagnetic Weyl- and Dirac-CDWs.

We study a circularly moving impurity in an atomic condensate for the realization of superradiance phenomena in tabletop experiments. The impurity is coupled to the density fluctuations of the condensate and, in a quantum field theory language, it serves as an analog of a detector for the quantum phonon field. For sufficiently large rotation speeds, the zero-point fluctuations of the phonon field induce a sizable excitation rate of the detector even when the condensate is initially at rest in its ground state. For spatially confined condensates and harmonic detectors, such a superradiant emission of sound waves provides a dynamical instability mechanism, leading to a phonon lasing concept. Following an analogy with the theory of rotating black holes, our results suggest a promising avenue to quantum simulate basic interaction processes involving fast-moving detectors in curved space-times.

The effective description of the weak interaction between an emitter and a bosonic field as a sequence of two-body collisions provides a simple intuitive picture compared to traditional quantum optics methods as well as an effective calculation tool of the joint emitter-field dynamics. Here this collisional approach is extended to many emitters (atoms or resonators), each generally interacting with the field at many coupling points (giant emitter). In the regime of negligible delays, the unitary describing each collision in particular features a contribution of a chiral origin resulting in an effective Hamiltonian. The picture is applied to derive a Lindblad master equation (ME) of a set of giant atoms coupled to a (generally chiral) waveguide field in an arbitrary white-noise Gaussian state, which condenses into a single equation and extends a variety of quantum optics and waveguide QED MEs. The effective Hamiltonian and jump operators corresponding to a selected photodetection scheme are also worked out.

Mathematical approaches from dynamical systems theory are used in a range of fields. This includes biology where they are used to describe processes such as protein-protein interaction and gene regulatory networks. As such networks increase in size and complexity, detailed dynamical models become cumbersome, making them difficult to explore and decipher. This necessitates the application of simplifying and coarse graining techniques to derive explanatory insight. Here we demonstrate that Zwanzig-Mori projection methods can be used to arbitrarily reduce the dimensionality of dynamical networks while retaining their dynamical properties. We show that a systematic expansion around the quasi-steady-state approximation allows an explicit solution for memory functions without prior knowledge of the dynamics. The approach not only preserves the same steady states but also replicates the transients of the original system. The method correctly predicts the dynamics of multistable systems as well as networks producing sustained and damped oscillations. Applying the approach to a gene regulatory network from the vertebrate neural tube, a well-characterized developmental transcriptional network, identifies features of the regulatory network responsible for its characteristic transient behavior. Taken together, our analysis shows that this method is broadly applicable to multistable dynamical systems and offers a powerful and efficient approach for understanding their behavior.

We experimentally realize the internal and external entanglement tradeoff, which is a kind of entanglement monogamy relation different from that usually discussed. Using a source of twin photons, we find that the external entanglement in the twin photons and the path-polarization internal entanglement of one photon limit each other. In the extreme case, when the internal state is maximally entangled, the external entanglement must be vanishing, which illustrates entanglement monogamy. Our results of the experiment coincide with the theoretical predictions and therefore provide a direct experimental observation of the internal and external entanglement monogamy relations.

We consider the phase locking of two delay-coupled quasi-cycles. A coupled envelope-phase system obtained via stochastic averaging enables a stability analysis. While for deterministic limit-cycle oscillators the coupling can produce in-phase, antiphase, and the intermediate “out-of-phase” locking (OPL) behavior via spontaneous symmetry breaking, such outcomes for the quasi-cycle case are shown to require instead both noise and coupling delay. The theory, which applies the stochastic averaging method to delayed dynamics, generates stochastic stability functions that predict the numerically observed OPL behavior as a function of all the system parameters. OPL for coupled quasi-cycles occurs for additive or multiplicative noise, and for coupled networks of excitatory and inhibitory neurons as well as networks of inhibitory neurons coupled to one another. Our theory also predicts that the bifurcation at which the in-phase state becomes unstable lies at smaller delays for stronger noise. The noise produces the realistic quasi-cycle rhythms and out-of-phase behavior, all the while causing random reversals of the phase leader. Asymmetry in the coupling between networks, as well as heterogeneity within each network, also allows for quasi-cycle OPL, although it produces asymmetric bifurcations that bias the leadership towards one of the networks. These results are relevant to communication between brain areas and other networks that rely on noise-induced rather than noise-perturbed rhythms.

In solid-state physics, the Grüneisen parameter (GP) was first introduced to study the effect of volume change of a crystal lattice on its vibrational frequencies and has since been widely used to investigate the characteristic energy scales of systems associated with the changes of external potentials. However, the GP is less investigated in gas systems and especially strongly interacting quantum gases. Here we report on some general results on the origin of the GP, an identity, and caloric effects in ultracold quantum gases. We prove that there exists a simple identity among three different types of GPs, quantifying the caloric effect induced by variations of volume, magnetic field, and interaction, respectively. Using exact Bethe ansatz solutions, we present a rigorous study of these different GPs and the quantum refrigeration in one-dimensional Bose and Fermi gases. Based on the exact equations of states of these systems, we further obtain analytic results for singular behavior of the GPs and the caloric effects at quantum criticality. We also predict the existence of the lowest temperature for cooling near a quantum phase transition. It turns out that the interaction ramp up and down in quantum gases provide a promising protocol of quantum refrigeration in addition to the usual adiabatic demagnetization cooling in solid-state materials.

We report on the emergence of stable self-propelled bound defects in monolayers of active nematics, which form virtual full-integer topological defects in the form of vortices and asters. Through numerical simulations and analytical arguments, we identify the phase space of the bound defect formation in active nematic monolayers. It is shown that an intricate synergy between the nature of active stresses and the flow-aligning behavior of active particles can stabilize the motion of self-propelled positive half-integer defects into specific bound structures. Our findings therefore, uncover conditions for the formation of full integer topological defects in active nematics with the potential for triggering further experiments and theories.

The honeycomb compound $\alpha \text{−}{\mathrm{RuCl}}_3$ is widely discussed as a proximate Kitaev spin-liquid material. This scenario builds on spin-orbit entangled $j$ = 1/2 moments arising for a $t_{2g}^5$ electron configuration with strong spin-orbit coupling $\lambda$ and a large cubic crystal field. The actual low-energy electronic structure of $\alpha \text{−}{\mathrm{RuCl}}_3$, however, is still puzzling. In particular, infrared absorption features at 0.30, 0.53, and 0.75 eV seem to be at odds with a $j$ = 1/2 scenario. Also the energy of the spin-orbit exciton, the excitation from $j$ = 1/2 to 3/2, and thus the value of $\lambda$, are controversial. Combining infrared and Raman data, we show that the infrared features can be attributed to single, double, and triple spin-orbit excitons. We find $\lambda$ = 0.16 eV and $\mathrm{\Delta }$ = 42(4) meV for the observed noncubic crystal-field splitting, supporting the validity of the $j$ = $1/2$ picture for $\alpha \text{−}{\mathrm{RuCl}}_3$. The unusual strength of the double excitation is related to the underlying hopping interactions, which form the basis for dominant Kitaev exchange.

We propose emulation of Hawking radiation (HR) by means of acoustic excitations propagating on top of persistent current in an atomic Bose-Einstein condensate (BEC) loaded in an annular confining potential. The setting is initially created as a spatially uniform one, and then switches into a nonuniform configuration, while maintaining uniform BEC density. The eventual setting admits the realization of sonic black and white event horizons with different slopes of the local sound speed. A smooth slope near the white-hole horizon suppresses instabilities in the supersonic region. It is found that tongue-shaped patterns of the density-density correlation function, which represent the acoustic analog of HR, are strongly affected by the radius of the ring-shaped configuration and number of discrete acoustic modes admitted by it. There is a minimum radius that enables the emulation of HR. We also briefly discuss a possible similarity of properties of the matter-wave sonic black holes to the known puzzle of the stability of Planck-scale primordial black holes in quantum gravity.

Imaging the structure of molecules during a photoinduced reaction is essential for elucidating reaction mechanisms. This requires high spatiotemporal resolution to capture nuclear motions on the femtosecond and subangstrom scale, and a sufficiently high signal level to sample their continuous evolution with high fidelity. Here we show that, using high-repetition-rate ultrafast electron diffraction, we can accurately reconstruct a movie of the coherent rotational motion of laser-aligned nitrogen molecules. We have used a tabletop 90-keV photoelectron gun to simultaneously achieve high temporal resolution of 240 fs full width at half maximum and an electron beam current that is more than an order of magnitude above the previous state of the art in gas-phase ultrafast electron diffraction. With this, we have made an essentially continuous real-space experimental movie of the rotational motion of the molecular wave packet as it evolves from initial alignment and past multiple revivals.

Adhesion interactions mediated by multiple bond types are relevant for many biological and soft matter systems, including the adhesion of biological cells and functionalized colloidal particles to various substrates. To elucidate advantages and disadvantages of multiple bond populations for the stability of heterogeneous adhesion clusters of receptor-ligand pairs, a theoretical model for a homogeneous parallel adhesion bond cluster under constant loading is extended to several bond types. The stability of the entire cluster can be tuned by changing densities of different bond populations as well as their extensional rigidity and binding properties. In particular, bond extensional rigidities determine the distribution of total load to be shared between different subpopulations. Under a gradual increase of the total load, the rupture of a heterogeneous adhesion cluster can be thought of as a multistep discrete process, in which one of the bond subpopulations ruptures first, followed by similar rupture steps of other subpopulations or by immediate detachment of the remaining cluster. This rupture behavior is qualitatively independent of involved bond types, such as slip and catch bonds. Interestingly, an optimal stability is generally achieved when the total cluster load is shared such that loads on distinct bond populations are equal to their individual critical rupture forces. We also show that cluster heterogeneity can drastically affect cluster lifetime.

We propose a qubit-environment entanglement measure which is tailored for evolutions that lead to pure dephasing of the qubit, such as are abundant in solid-state scenarios. The measure can be calculated directly form the density matrix without minimization of any kind. In fact, it does not require knowledge of the full density matrix and it is enough to know the initial qubit state and the states of the environment conditional on qubit pointer states. This yields a computational advantage over standard entanglement measures, which becomes large when there are no correlations between environmental components in the conditional environmental states. In contrast to all other measures of mixed-state entanglement, the measure has a straightforward physical interpretation directly linking the amount of information about the qubit state which is contained in the environment to the amount of qubit-environment entanglement. This allows for a direct extension of the pure-state interpretation of entanglement generated during pure dephasing to mixed states, even though pure-state conclusions about qubit decoherence are not transferable.

The connection between spherical harmonics and symmetric tensors is explored. For each spherical harmonic, a corresponding traceless symmetric tensor is constructed. These tensors are then extended to include nonzero traces, providing an orthonormal angular-momentum eigenbasis for symmetric tensors of any rank. The relationship between the spherical-harmonic tensors and spin-weighted spherical harmonics is derived. The results facilitate the spherical-harmonic expansion of a large class of tensor-valued functions. Several simple illustrative examples are discussed, and the formalism is used to derive the leading-order effects of violations of Lorentz invariance in Newtonian gravity.

We develop a comprehensive quantitative theory for magnetoelectricity in magnetically ordered quasi-two-dimensional (quasi-2D) systems whereby in thermal equilibrium an electric field can induce a magnetization and a magnetic field can induce an electric polarization. This effect requires that both space-inversion and time-reversal symmetry are broken. Antiferromagnetic order plays a central role in this theory. We define a Néel operator $\mathit{\tau }$ such that a nonzero expectation value $\langle \mathit{\tau }\rangle$ signals collinear antiferromagnetic order in the same way a magnetization signals ferromagnetic order. While a magnetization is even under space inversion and odd under time reversal, the operator $\mathit{\tau }$ describes a toroidal moment that is odd both under space inversion and under time reversal. Thus the magnetization and the toroidal moment $\langle \mathit{\tau }\rangle$ quantify complementary aspects of collinear magnetic order in solids. Focusing on quasi-2D systems, itinerant-electron ferromagnetic order can be attributed to dipolar equilibrium currents that give rise to a magnetization. In the same way, antiferromagnetic order arises from quadrupolar equilibrium currents that generate the toroidal moment $\langle \mathit{\tau }\rangle$. In the magnetoelectric effect, the electric-field-induced magnetization can then be attributed to the electric manipulation of the quadrupolar equilibrium currents. We develop a $\mathrm{k}·\mathrm{p}$ envelope-function theory for the antiferromagnetic diamond structure that allows us to derive explicit expressions for the Néel operator $\mathit{\tau }$. Considering ferromagnetic zincblende structures and antiferromagnetic diamond structures, we derive quantitative expressions for the magnetoelectric responses due to electric and magnetic fields that reveal explicitly the inherent duality of these responses required by thermodynamics. Magnetoelectricity is found to be small in realistic calculations for quasi-2D electron systems. The magnetoelectric response of quasi-2D hole systems turns out to be sizable, however, with moderate electric fields being able to induce a magnetic moment of one Bohr magneton per charge carrier. Our theory provides a broad framework for the manipulation of magnetic order by means of external fields.