Topological interface states (TISs) in multivalley systems are studied to unravel their valley sensitivity. For this purpose, multivalley IV–VI topological crystalline insulator (TCI) heterostructures are explored using magnetooptical Landau level spectroscopy up to 34 teslas. We characterize the TISs emerging from the distinct $L$ valleys in ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Se}$ multiquantum wells grown along the [111] direction. It is shown that the shape of the two-dimensional (2D) Fermi surfaces of TISs residing at the TCI-trivial insulator interfaces are strongly affected by the valley anisotropy of topologically trivial ${\mathrm{Pb}}_{1-y}{\mathrm{Eu}}_y\mathrm{Se}$ barriers. This phenomenon is shown to be due to the deep penetration of the TISs into the barriers. For the valleys tilted with respect to the confinement direction, a significant interaction between topological states and the conventional massive quantum well states is observed, evidenced by the resulting large anticrossings between Landau levels. These are theoretically well described by a $\mathbf{k}·\mathbf{p}$ model that considers tilt and anisotropy of the valleys in 2D. Therefore, in this paper, we provide a precise characterization of the TIS valley splitting as well as an accurate determination of the anisotropy of their Dirac cone dispersion.

The opportunity to build quantum technologies operating with elementary quantum systems with more than two levels is now increasingly being examined, not least because of the availability of such systems in the laboratory. It is therefore essential to understand how these single systems initially in highly nonclassical states decohere on different timescales due to their coupling with the environment. In this work, we consider the depolarization, both isotropic and anisotropic, of a quantum spin of arbitrary spin quantum number $j$ and focus on the study of the most superdecoherent states. We approach this problem from the perspective of the collective dynamics of a system of $N=2j$ constituent spin $\frac{1}{2}$, initially in a symmetric state, undergoing collective depolarization. This allows us to use the powerful language of quantum information theory to analyze the fading of quantum properties of spin states caused by depolarization. In this framework, we establish a pre- cise link between superdecoherence and entanglement. We present extensive numerical results on the scaling of the entanglement survival time with the Hilbert space dimension for collective depolarization. We also highlight the specific role played by anticoherent spin states and show how their Markovian isotropic depolarization alone can lead to the generation of bound entangled states.

Junctions composed of excitonic condensate (EC) bilayers exhibit intriguing physics due to the presence of interlayer tunneling. Together with the excitonic superflow, this tunneling induces a special kind of topological object which is analogous to the Josephson vortices in extended superconducting Josephson junction. We name such objects the “tunneling vortices.” In this work we propose to characterize the topological properties of these vortices in the current-injecting scheme under which multiple vortices can easily be introduced through current injection from one end of the EC junctions. We find theoretically that current responses demonstrate the quantized nature of vortices: the ejected currents show periodicity as a function of injected currents(or vortex numbers). Moreover, the injected-ejected current difference shows plateaus corresponding to different vortex numbers when varying tunneling strength ${\mathrm{\Delta }}_t$ or superfluid density ${\rho }_S$. Since all parameters are experimentally accessible, we expect real-world demonstrations of these enthralling quantization behaviors to come in the near future.

Parametric amplifiers are known to squeeze the vacuum state of the electromagnetic field, thus the statistics of the photocounts at their output should be that of squeezed vacuum. However, several theoretical works predict a very different statistical distribution. We have measured the photocount statistics of a Josephson parametric amplifier and recover the expected squeezed vacuum statistics. We explain this discrepancy by showing theoretically how the photocount statistics is dictated by the detection process. Namely, most experiments will naturally measure a single mode of the electromagnetic field, while the aforementioned theoretical works implicitly describe massively multimode detection.

We have carried out neutron spectroscopic measurements on single crystals of ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ from $0.12\le x\le 0.26$ using time-of-flight techniques. These measurements allow us to follow the evolution of parallel spin stripe fluctuations with energies less than $\sim 33$ meV, from $x=0.12$ to 0.26. Samples at these hole-doping levels are known to display static (on the neutron-scattering time scale) parallel spin stripes at low temperature, with onset temperatures and intensities which decrease rapidly with increasing $x$. Nonetheless, we report remarkably similar dynamic spectral weight for the corresponding dynamic parallel spin stripes, between 5 and 33 meV, from the 1/8 anomaly near $x=0.12$, to optimal doping near $x=0.19$ to the quantum critical point for the pseudogap phase near $x=0.24$, and finally to the approximate end of superconductivity near $x=0.26$. This observed dynamic magnetic spectral weight is structured in energy with a peak near 17 meV at all dopings studied. Earlier neutron and resonant x-ray scattering measurements on related cuprate superconductors have reported both a disappearance with increasing doping of magnetic fluctuations at ($\pi$, $\pi$) wave vectors characterizing parallel spin stripe structures and persistant paramagnon scattering away from this wave vector, respectively. Our results for ${\mathrm{La}}_{1.6-x}{\mathrm{Nd}}_{0.4}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$ from 0.12 $\le x\le 0.26$ clearly show persistent parallel spin stripe fluctuations at and around at ($\pi$, $\pi$), and across the full range of doping studied. These results are also compared to recent theory. Together with a rapidly declining $x$ dependence to the static parallel spin stripe order, the persistent parallel spin stripe fluctuations show a remarkable similarity to the expectations of a quantum spin glass, random t-J model, recently introduced to describe strong local correlations in cuprates.

Measurement-induced phase transitions arise due to a competition between the scrambling of quantum information in a many-body system and local measurements. In this work we investigate these transitions in different classes of fast scramblers, systems that scramble quantum information as quickly as is conjectured to be possible—on a timescale proportional to the logarithm of the system size. In particular, we consider sets of deterministic sparse couplings that naturally interpolate between local circuits that slowly scramble information and highly nonlocal circuits that achieve the fast-scrambling limit. We find that circuits featuring sparse nonlocal interactions are able to withstand substantially higher rates of local measurement than circuits with only local interactions, even at comparable gate depths. We also study the quantum error-correcting codes that support the volume-law entangled phase and find that our maximally nonlocal circuits yield codes with nearly extensive contiguous code distance. Use of these sparse, deterministic circuits opens pathways towards the design of noise-resilient quantum circuits and error correcting codes in current and future quantum devices with minimum gate numbers.

Variational quantum eigensolver (VQE) is an appealing candidate for the application of near-term quantum computers. A technique introduced in [Higgot et al., Quantum 3, 156 (2019)], which is named variational quantum deflation (VQD), has extended the ability of the VQE framework for finding excited states of a Hamiltonian. However, no method to evaluate transition amplitudes between the eigenstates found by the VQD without using any costly Hadamard-test-like circuit has been proposed despite its importance for computing properties of the system such as electronic oscillator strengths of molecules. Here we propose a method to evaluate transition amplitudes between the eigenstates obtained by the VQD avoiding any Hadamard-test-like circuit. Our method relies only on the ability to estimate overlap between two states, so it does not restrict to the VQD eigenstates and applies for general situations. To support the significance of our method, we provide a comparison of three previously proposed methods to find excited states with numerical simulation of three molecules (lithium hydride, diazene, and azobenzene) and find that the VQD method exhibits the best performance among the three methods within the scope of noiseless simulation. Finally, we demonstrate the validity of our method by calculating the oscillator strength of lithium hydride, comparing results from numerical simulations and real-hardware experiments on the cloud enabled quantum computer IBMQ Rome. Our results illustrate the superiority of the VQD to find excited states and widen its applicability to various quantum systems.

The Su-Schrieffer-Heeger (SSH) model has recently received considerable attention in condensed matter. Here, we investigate SSH-based charging protocols of quantum batteries (QBs) with $N$ spins. The hopping interaction induces ground state splitting, leading to different effects on the dimerization parameter to the QB in different quantum phase regimes. In the degenerate ground state regime, the dimerization parameter has little influence on the QB. In contrast, the dimerization parameter has a significantly quantum advantage in the fully nondegenerate ground state regime on energy and ergotropy. It also results in the dimerization spin pairs having larger occupations than other spins. Although the dimerization parameter can enhance the energy and ergotropy, the QB's capacity will decrease.

Local quantum master equations provide a simple description of interacting subsystems coupled to different reservoirs. They have been widely used to study nonequilibrium critical phenomena in open quantum systems. We here investigate the validity of such a local approach by analyzing a paradigmatic system made of two harmonic oscillators each in contact with a heat bath. We evaluate the steady-state mean occupation number for varying temperature differences and find that local master equations generally fail to reproduce the results of an exact quantum-Langevin-equation description. We relate this property to the inability of the local scheme to properly characterize intersystem correlations, which we quantify with the quantum mutual information.

In this article, we develop a description of topological pumps as slow classical dynamical variables coupled by a quantum system. We discuss the cases of quantum Hall pumps, Thouless pumps, and the more recent Floquet pumps based frequency converters. This last case corresponds to a quantum topological coupling between classical modes described by action-angle variables on which we focus. We propose a realization of such a topological coupler based on a superconducting qutrit suitably driven by three modulated drives. A detailed experimental protocol allowing to measure the quantized topological power transfer between the different modes of a superconducting circuit is discussed.

We study coherent dynamics of two interacting Bose-Bose droplets by means of the extended Gross-Pitaevskii equation. The relative motion of the droplets couples to the phases of their components. The dynamics can be understood in terms of the evolution of zero-energy modes recovering symmetries spontaneously broken by the mean-field solution. These are translational symmetry and two U(1) symmetries, associated with the phases of the droplets' two components. A phase-dependent interaction potential and double Josephson-junction equations are introduced to explain the observed variety of different scenarios of collision. We show that the evolution of the droplets is a macroscopic manifestation of the hidden dynamics of their phases. The occurrence of nondissipative drag between the two supercurrents (Andreev-Bashkin effect) is mentioned.

We show that foams and emulsions can display a fundamentally different normal response to a simple shear deformation. While foams dilate or push outwards on the shearing surfaces, known as a positive Poynting effect, in emulsions the Poynting effect can have either sign and can be tuned by changing the emulsion properties. We relate the sign of the Poynting effect to the presence of a compressible contact network supported by adhesive contacts. When the concentration of surfactant in the continuous phase is low, the emulsions are nonadhesive and push outward on their shearing surfaces, as do the foams. When the surfactant concentration is increased, the emulsions become adhesive due to depletion interactions, and the Poynting effect changes sign. We argue that the adhesive contact network develops a shear modulus that stiffens in response to dilation, which leads to the negative Poynting effect.

We develop a workflow to use current quantum computing hardware for solving quantum many-body problems, using the example of the fermionic Hubbard model. Concretely, we study a four-site Hubbard ring that exhibits a transition from a product state to an intrinsically interacting ground state as hopping amplitudes are changed. We locate this transition and solve for the ground-state energy with high quantitative accuracy using a variational quantum algorithm executed on an IBM quantum computer. Our results are enabled by a variational ansatz that takes full advantage of the maximal set of commuting ${\mathbb{Z}}_2$ symmetries of the problem and a Lanczos-inspired error mitigation algorithm. They are a benchmark on the way to exploiting near term quantum simulators for quantum many-body problems.

Moiré materials, and in particular twisted bilayer graphene (TBG), exhibit a range of fascinating phenomena that emerge from the interplay of band topology and interactions. We show that the nonlinear second-order photoresponse is an appealing probe of this rich interplay. A dominant part of the photoresponse is the shift current, which is determined by the geometry of the electronic wave functions and carrier properties and thus becomes strongly modified by electron-electron interactions. We analyze its dependence on the twist angle and doping and investigate the role of interactions. In the absence of interactions, the response of the system is dictated by two energy scales: (i) the mean energy of direct transitions between the hole and electron flat bands and (ii) the gap between flat and dispersive bands. Including electron-electron interactions both enhances the response at the noninteracting characteristic frequencies and produces new resonances. We attribute these changes to the filling-dependent band renormalization in TBG. Our results highlight the connection between nontrivial geometric properties of TBG and its optical response, as well as demonstrate how optical probes can access the role of interactions in moiré materials.

Magnetic topological materials have attracted significant attention due to their potential realization of a variety of novel quantum phenomena. ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ has recently been theoretically recognized as a long-awaited intrinsic antiferromagnetic bulk axion insulator. However, experimental studies of the transport properties arising from the topological states in this material are scarce. In this paper, we perform detailed magnetoresistance (MR) and Hall measurements to study the magnetotransport properties of this material. We find that the transport is strongly influenced by the spin configuration of the Eu moments from the concomitant change in the field dependence of the MR and that of the magnetization below the Néel temperature. Most importantly, an anomalous Hall effect (AHE) and a large topological Hall effect (THE) are observed. We suggest that the AHE is originated from a nonvanishing net Berry curvature due to the helical spin structure and that the THE is attributed to the formation of a noncoplanar spin texture with a finite scalar spin chirality induced by the external magnetic field in ${\mathrm{EuIn}}_2{\mathrm{As}}_2$. Our studies provide a platform to understand the influence of the interplay between the topology of electronic bands and the field-induced magnetic structure on magnetoelectric transport properties. In addition, our observations give a hint to realize axion insulator states and higher-order topological insulator states through manipulating the magnetic state of ${\mathrm{EuIn}}_2{\mathrm{As}}_2$.

The intensity and the features of sensory stimuli are encoded in the activity of neurons in the cortex. In the visual and piriform cortices, the stimulus intensity rescales the activity of the population without changing its selectivity for the stimulus features. The cortical representation of the stimulus is therefore intensity invariant. This emergence of network-invariant representations appears robust to local changes in synaptic strength induced by synaptic plasticity, even though (i) synaptic plasticity can potentiate or depress connections between neurons in a feature-dependent manner, and (ii) in networks with balanced excitation and inhibition, synaptic plasticity determines the nonlinear network behavior. In this study we investigate the consistency of invariant representations with a variety of synaptic states in balanced networks. By using mean-field models and spiking network simulations, we show how the synaptic state controls the emergence of intensity-invariant or intensity-dependent selectivity. In particular, we demonstrate that an effective power-law synaptic transformation at the population level is necessary for invariance. In a range of firing rates, purely depressing short-term synapses fulfills this condition, and in this case, the network is contrast-invariant. Instead, facilitating short-term plasticity generally narrows the network selectivity. We found that facilitating and depressing short-term plasticity can be combined to approximate a power-law that leads to contrast invariance. These results explain how the physiology of individual synapses is linked to the emergence of invariant representations of sensory stimuli at the network level.

How do different reset protocols affect ergodicity of a diffusion process in single-particle-tracking experiments? We here address the problem of resetting of an arbitrary stochastic anomalous-diffusion process (ADP) from the general mathematical points of view and assess ergodicity of such reset ADPs for an arbitrary resetting protocol. The process of stochastic resetting describes the events of the instantaneous restart of a particle's motion via randomly distributed returns to a preset initial position (or a set of those). The waiting times of such resetting events obey the Poissonian, Gamma, or more generic distributions with specified conditions regarding the existence of moments. Within these general approaches, we derive general analytical results and support them by computer simulations for the behavior of the reset mean-squared displacement (MSD), the new reset increment-MSD (iMSD), and the mean reset time-averaged MSD (TAMSD). For parental nonreset ADPs with the MSD($t$)$\propto t^{\mu }$ we find a generic behavior and a switch of the short-time growth of the reset iMSD and mean reset TAMSDs from $\propto {\mathrm{\Delta }}^{\mu }$ for subdiffusive to $\propto {\mathrm{\Delta }}^1$ for superdiffusive reset ADPs. The critical condition for a reset ADP that recovers its ergodicity is found to be more general than that for the nonequilibrium stationary state, where obviously the iMSD and the mean TAMSD are equal. The consideration of the new statistical quantifier, the iMSD—as compared to the standard MSD—restores the ergodicity of an arbitrary reset ADP in all situations when the $\mu \mathrm{th}$ moment of the waiting-time distribution of resetting events is finite. Potential applications of these new resetting results are, inter alia, in the area of biophysical and soft-matter systems.

We study an impurity with a resonance level whose position coincides with the Fermi energy of the surrounding Fermi gas. An impurity causes a rapid variation of the scattering phase shift for fermions at the Fermi surface, introducing a new characteristic length scale into the problem. We investigate manifestations of this length scale in the self-energy of the impurity and in the density of the bath. Our calculations reveal a model-independent deformation of the density of the Fermi gas, which is determined by the width of the resonance. To provide a broader picture, we investigate time evolution of the density in quench dynamics, and study the behavior of the system at finite temperatures. Finally, we briefly discuss implications of our findings for the Fermi-polaron problem.

The violation of a Bell inequality is the paradigmatic example of device-independent quantum information: The nonclassicality of the data is certified without the knowledge of the functioning of devices. In practice, however, all Bell experiments rely on the precise understanding of the underlying physical mechanisms. Given that, it is natural to ask: Can one witness nonclassical behavior in a truly black-box scenario? Here, we propose and implement, computationally and experimentally, a solution to this ab initio task. It exploits a robust automated optimization approach based on the stochastic Nelder-Mead algorithm. Treating preparation and measurement devices as black boxes, and relying on the observed statistics only, our adaptive protocol approaches the optimal Bell inequality violation after a limited number of iterations for a variety photonic states, measurement responses, and Bell scenarios. In particular, we exploit it for randomness certification from unknown states and measurements. Our results demonstrate the power of automated algorithms, opening a venue for the experimental implementation of device-independent quantum technologies.

An infodemic—an outpouring of information, including misleading and also fake news—is accompanying the current pandemic caused by SARS-CoV-2. In the absence of valid therapeutic approaches, behavioral responses may seriously affect the social dynamics of contagion, so the infodemic may cause confusion and disorientation in the public, leading to possible individually and socially harmful choices. This new phenomenon requires specific modeling efforts to better understand the complex intertwining of the epidemic and infodemic components of a pandemic crisis, with a view to building an integrative public health approach. We propose three models, from epidemiology to game theory, as potential candidates for the onset of the infodemics and statistically assess their accuracy in reproducing real infodemic waves observed in a data set of 390 million tweets collected worldwide. Our results show that evolutionary game-theory models are the most suitable ones to reproduce the observed infodemic modulations around the onset of the local epidemic wave. Furthermore, we find that the number of confirmed COVID-19 reported cases in each country and worldwide are driving the modeling dynamics with opposite effects.