The electrodynamic response of ultrapure materials at low temperatures becomes spatially nonlocal. This nonlocality gives rise to phenomena such as hydrodynamic flow in transport and the anomalous skin effect in optics. In systems characterized by an anisotropic electronic dispersion, the nonlocal dynamics becomes dependent on the relative orientation of the sample with respect to the applied field, in ways that go beyond the usual, homogeneous response. Such orientational dependence should manifest itself not only in transport experiments, as recently observed, but also in optical spectroscopy. In this paper, we develop a kinetic theory for the distribution function and the transverse conductivity of two- and three-dimensional Fermi systems with anisotropic electronic dispersion. By expanding the collision integral into the eigenbasis of a collision operator, we include momentum-relaxing scattering as well as momentum-conserving collisions. We examine the isotropic 2D case as a reference, as well as anisotropic hexagonal and square Fermi-surface shapes. We apply our theory to the quantitative calculation of the skin depth and the surface impedance, in all regimes of skin effect. We find qualitative differences between the frequency dependence of the impedance in isotropic and anisotropic systems. Such differences are shown to persist even for more complex 2D Fermi surfaces, including the “supercircle” geometry and an experimental parametrization for ${\mathrm{PdCoO}}_2$, which deviate from an ideal polygonal shape. We study the orientational dependence of skin effect due to Fermi-surface anisotropy, thus providing guidance for the experimental study of nonlocal optical effects.

While quantum machine learning (ML) has been proposed to be one of the most promising applications of quantum computing, how to build quantum ML models that outperform classical ML remains a major open question. Here, we demonstrate a Bayesian algorithm for constructing quantum kernels for support vector machines that adapts quantum gate sequences to data. The algorithm increases the complexity of quantum circuits incrementally by appending quantum gates selected with Bayesian information criterion as circuit selection metric and Bayesian optimization of the parameters of the locally optimal quantum circuits identified. The performance of the resulting quantum models for the classification problems considered here significantly exceeds that of optimized classical models with conventional kernels.

An intriguing chiral soliton phase has recently been identified in the $S=\frac{1}{2}$ Kitaev spin chain. Here we show that for $S$ = 1, 2, 3, 4, 5 an analogous phase can be identified, but contrary to the $S=\frac{1}{2}$ case the chiral soliton phases appear as islands within the sea of the polarized phase. In fact, a small field applied in a general direction will adiabatically connect the integer spin Kitaev chain to the polarized phase. Only at sizable intermediate fields along symmetry directions does the soliton phase appear centered around the special point $h_x^{★}=h_y^{★}=S$ where two exact product ground states can be identified. The large-$S$ limit can be understood from a semiclassical analysis, and variational calculations provide a detailed picture of the $S=1$ soliton phase. Under open boundary conditions, the chain has a single soliton in the ground state, which can be excited, leading to a proliferation of in-gap states. In contrast, even length periodic chains exhibit a gap above a twice-degenerate ground state. The presence of solitons leaves a distinct imprint on the low-temperature specific heat.

We present an exact first-order perturbation theory for eigenmodes in systems with interfaces causing material discontinuities. We show that when interfaces deform, higher-order terms of the perturbation series can contribute to the eigenmode frequencies in first order in the deformation depth. In such cases, the first-order approximation is different from the usual diagonal approximation and its single-mode result. Extracting additional first-order corrections from all higher-order terms enables us to recover the diagonal formalism in a modified form. A general formula for the single-mode first-order correction to electromagnetic eigenmodes in systems with interfaces is derived, capable of treating dispersive, magnetic, and chiral materials of arbitrary shape.

Pattern formation induced by symmetry breaking is a fundamental concept underlying biological phenomena across different scales, from single cells to tissues. However, the mechanics behind the pattern formation of the actomyosin system remains elusive due to complex biochemical regulations in living cells. In this study, we report the transition between distinct patterns of cytoplasmic actomyosin networks: steady actin flow and periodic actin waves, which are confined to a quasi-two-dimensional cell-like compartment. By combining molecular perturbations and numerical simulations of the active fluid model, we show that contractility and actin polymerization rate are the critical factors for the state transition from the steady actin flow to periodic actin waves. These patterns emerge either when active stress outweighs the diffusive relaxation of actin filaments or when the actin polymerization rate is sufficiently slow to accumulate actin filaments close to the surface of the circular confinement. Furthermore, our active fluid model predicts that the spatial heterogeneity at the onset of contraction leads to a rotational actin wave, which is stable only at the phase boundary between the steady actin flow and periodic actin waves. This study provides an integrative understanding of the distinct pattern formation of active gels confined in cell-sized spaces.

The cycle current is a crucial quantity in stochastic thermodynamics. The absolute and net cycle currents of a Markovian system can be defined in the loop-erased (LE) or spanning tree (ST) manner. Here we make a comparative study between the large deviations and fluctuation theorems for LE and ST currents, i.e., cycle currents defined in the LE and ST manners. First, we derive the exact joint distribution and large deviation rate function for the LE currents of a system with a cyclic topology and also obtain the exact rate function for the ST currents of a general system. The relationship between the rate functions for LE and ST currents is clarified and the analytical results are applied to examine the fluctuations in the product rate of a three-step reversible enzyme reaction. Furthermore, we examine various types of fluctuation theorems satisfied by LE and ST currents and clarify their ranges of applicability. We show that both the absolute and net LE currents satisfy the strong form of all types of fluctuation theorems. In contrast, the absolute ST currents do not satisfy fluctuation theorems, while the net ST currents only satisfy the weak form of fluctuation theorems under the periodic boundary condition. Finally, the fluctuation theorems for cycle currents are applied to study the fluctuations in entropy production along a single stochastic trajectory.

A single-molecular motor kinesin harnesses thermal and nonthermal fluctuations to transport various cargoes along microtubules, converting chemical energy to directed movements. To describe the nonthermal fluctuations generated by the complex environment in living cells, we propose a bottom-up model which combines a Markov model of the kinesin and a mean-field model for the active bath, to mimic the kinesin movements in the intracellular environment. Simulations of the model system show that the kinesin and the probe attached to it are accelerated by such active bath. Further, we provide a theoretical insight into the simulation result by deriving a generalized Langevin equation (GLE) for the probe with a mean-field method, wherein an effective friction kernel and fluctuating noise terms are obtained explicitly. Numerical solutions of the GLE show very good agreement with simulation results. We sample such noises, calculate their variances and non-Gaussian parameters, and reveal that the dominant contribution to probe acceleration is attributed to noise variance.

We consider a multiqubit system of atoms or ions with two computational ground states and an interacting excited state in the so-called blockade regime, such that only one qubit can be excited at any one time. Examples of such systems are rare-earth-ion-doped crystals and neutral atoms trapped in tweezer arrays. We present a simple laser excitation protocol that yields a complex phase factor on any desired multiqubit product state, and which can be used to implement multiqubit gates such as the $n$-bit Toffoli gates. The operation is performed using only two pulses, where each pulse simultaneously address all qubits involved in the operation. By relying on the well-established complex hyperbolic secant pulse shape, our scheme is also robust and permits complete transfers to and from the excited states despite variations in Rabi frequencies and transition frequencies. A detailed analysis of the multiqubit gate performance is provided.

By using intense coherent electromagnetic radiation, it may be possible to manipulate the properties of quantum materials very quickly, or even induce new and potentially useful phases that are absent in equilibrium. For instance, ultrafast control of magnetic dynamics is crucial for a number of proposed spintronic devices, and it can also shed light on the possible dynamics of correlated phases out of equilibrium. Inspired by recent experiments on spin-orbital ferromagnet ${\mathrm{YTiO}}_3$, we consider the nonequilibrium dynamics of a Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model the dynamics of the magnon excitations in this system following an optical pulse that resonantly excites infrared-active phonon modes. As the phonons ring down, they can dynamically couple the orbitals with the low-lying magnons, leading to a dramatically modified effective bath for the magnons. We show that this transient coupling can lead to a dynamical acceleration of the magnetization dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the parameter space more, we find that the magnon dynamics can also even completely reverse, leading to a negative relaxation rate when the pump is blue-detuned with respect to the orbital bath resonance. We therefore show that by using specially targeted optical pulses, one can exert a much greater degree of control over the magnetization dynamics, allowing one to optically steer magnetic order in this system. We conclude by discussing interesting parallels between the magnetization dynamics we find here and recent experiments on photoinduced superconductivity, where it is similarly observed that depending on the initial pump frequency, an apparent metastable superconducting phase emerges.

Hexagonal ${\mathrm{YMnO}}_3$ is a prototype antiferromagnet which exhibits multiferroic behavior with the ferroelectric and magnetic transitions occurring at different temperatures. We observe an out-of-plane canting of the ${\mathrm{Mn}}^{3+}$ magnetic moments using resonant x-ray diffraction (RXD) in a single crystal of this material. These canted moments result in the symmetry-forbidden (0,0,1) magnetic Bragg reflection, which is observed at the Mn $L_{2,3}$ absorption edges. We also observe an unexpected difference in the RXD spectral shapes at different temperatures. Using ab initio calculations, we explore the possibility that this behavior arises due to the interference between scattering from the canted magnetic moments and parity-odd atomic multipoles on the ${\mathrm{Mn}}^{3+}$ ions.

The problem of quantum first detection time has been extensively investigated in recent years. Employing the stroboscopic measurement protocol, we consider such a monitored quantum walk on sequentially or periodically perturbed rings and focus on the statistics of first detected return time, namely, the time it takes a particle to return to the initial state for the first time. Using time-independent perturbation theory, we obtain the general form of the eigenvalues and eigenvectors of the Hamiltonian. For the case of a sequentially perturbed ring system, we find steplike behaviors of ${\sum }_{n=1}^{\mathcal{N}}n{F}_n$ ($\to \langle n\rangle$ as $\mathcal{N}\to \infty$) when $\mathcal{N}$ increases, with two plateaus corresponding to integers, where $F_n$ is the first detected return probability at the $n\mathrm{th}$ detection attempt. Meanwhile, if the initial condition preserves the reflection symmetry, the mean return time is the same as the unperturbed system. For the periodically perturbed system, similar results can also appear in the case where the symmetry is preserved; however, the size of the ring, the interval between adjacent perturbations, and the initial position may change the mean return time in most cases. In addition, we find that the decay rate of the first detection probability $F_n$ decreases with the increase in perturbation amplitude. More profoundly, the symmetry-preserving setup of the initial conditions leads to the coincidence of $F_n$. The symmetry of the physical systems under investigation is deeply reflected in the quantum detection time statistics.

Single contagion processes are known to display a continuous transition from an epidemic-free state to an epidemic one, for contagion rates above a critical threshold. This transition can become discontinuous when two simple contagion processes are coupled in a bidirectional symmetric way. However, in many cases, the coupling is not symmetric and the nature of the processes can differ. For example, risky social behaviors—such as not wearing masks or engaging in large gatherings—can affect the spread of a disease, and their adoption dynamics via social reinforcement mechanisms are better described by complex contagion models rather than by simple contagions, more appropriate for disease spreading. Here, we consider a simplicial contagion (describing the adoption of a behavior) that unidirectionally drives a simple contagion (describing a disease propagation). We show, both analytically and numerically, that, above a critical driving strength, such a driven simple contagion can exhibit both discontinuous transitions and bistability, absent otherwise. Our results provide a route for a simple contagion process to display the phenomenology of a higher-order contagion, through a driving mechanism that may be hidden or unobservable in practical instances.

In this work, we present a quantum algorithm for ground-state energy calculations of periodic solids on error-corrected quantum computers. The algorithm is based on the sparse qubitization approach in second quantization and developed for Bloch and Wannier basis sets. We show that Wannier functions require less computational resources with respect to Bloch functions because (i) the ${\mathrm{L}}_1$ norm of the Hamiltonian is considerably lower and (ii) the translational symmetry of Wannier functions can be exploited in order to reduce the amount of classical data that must be loaded into the quantum computer. The resource requirements of the quantum algorithm are estimated for periodic solids such as NiO and PdO. These transition metal oxides are industrially relevant for their catalytic properties. We find that ground-state energy estimation of Hamiltonians approximated using 200–900 spin orbitals requires ca. ${10}^{10}$–${10}^{12}$ T gates and up to $3\times {10}^8$ physical qubits for a physical error rate of $0.1\%$.

Based on density functional theory calculations, we model the x-ray absorption spectra of the $\mathrm{O} K$ edge and the $\mathrm{Ti} L_{2,3}$ edge in bulk ${\mathrm{SrTiO}}_3$. Taking into account excitonic effects by solving the Bethe-Salpeter equation is found to be pivotal for obtaining concurrence with the experimental data with respect to the energetic positions and relative intensity of the peaks. Moreover, analysis of the underlying interband transitions in reciprocal space reveals the origin of the prominent peaks and features in the spectra, and provides a deeper understanding of the electronic structure. For example, the characteristic four-peak structure of the $\mathrm{Ti} L_{2,3}$ edge results from transitions from $\mathrm{Ti} 2p_{3/2}$ states to the unoccupied $\mathrm{Ti} 3d{t}_{2g}$ (456.1 eV) and $\mathrm{Ti} 3d{e}_g$ states (458.2 eV), followed by transitions from $\mathrm{Ti} 2p_{1/2}$ states to $\mathrm{Ti} 3d{t}_{2g}$ (461.8 eV) and $\mathrm{Ti} 3d{e}_g$ states (463.7 eV). The first bound exciton is strongly localized in real space, and is confined to essentially one unit cell with $3d_{xz}$ character near the Ti sites. On the other hand, the first bound exciton of the O K edge is identified as a charge-transfer type with a dominant contribution from the $\mathrm{Ti} 3d_{xy}$ states hybridized with O $p$ states. Moreover, the spatial distribution of the exciton wave function shows an intriguing two-dimensional spread in the $x\text{−}y$ plane, despite the three-dimensional nature of the material.