As a quantum analog of Cherenkov radiation, an inertial photon detector moving through a medium with constant refractive index $n$ may perceive the electromagnetic quantum fluctuations as real photons if its velocity $v$ exceeds the medium speed of light $c/n$. For dispersive Hopfield-type media, we find this Ginzburg effect to extend to much lower $v$ because the phase velocity of light is very small near the medium resonance. In this regime, however, dissipation effects become important. Via an extended Hopfield model, we present a consistent treatment of quantum fluctuations in dispersive and dissipative media and derive the Ginzburg effect in such systems. Finally, we propose an experimental test.

It is not widely appreciated that an acoustic wave can “jump” or “tunnel” across a vacuum gap between two piezoelectric solids, nor has the general case been formulated or studied in detail. Here, we remedy that situation, by presenting a general formalism and approach to study such an acoustic tunneling effect between two arbitrarily oriented anisotropic piezoelectric semi-infinite crystals. The approach allows one to solve for the reflection and transmission coefficients of all the partial-wave modes, and is amenable to practical numerical or even analytical implementation, as we demonstrate by a few chosen examples. The formalism can be used in the future for quantitative studies of the tunneling effect in connection not only with the manipulation of acoustic waves, but with many other areas of physics of vibrations such as heat transport, for example.

Materials with a kagome lattice structure display a wealth of intriguing magnetic properties due to their geometric frustration and intrinsically flat band structure. Recently, topological and superconducting states have also been observed in kagome systems. The kagome lattice may also host a “breathing” mode that leads to charge density wave (CDW) states, if there is strong electron-phonon coupling, electron-electron interaction, or external excitation of the material. This “breathing” mode can give rise to candidate distortions such as the star of David (SoD) or its inverse structure [trihexagonal (TrH)]. To date, in most materials, only a single type of distortion has been observed. Here, we present angle-resolved photoemission spectroscopy measurements on the kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ at multiple temperatures and photon energies to reveal the nature of the CDW in this material. It is shown that ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ displays two intertwined CDW orders corresponding to the SoD and TrH distortions. These two distinct types of distortions are stacked along the $c$ direction to form a three-dimensional CDW order where the two 2-fold CDWs are phase shifted along the $c$ axis. The presented results provide not only key insights into the nature of the unconventional CDW order in ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, but also an important reference for further studies on the relationship between the CDW and superconducting order.

We present a comprehensive theory for interrogation of the quantum state of a two-level system (TLS) based on a free-electron–bound-electron resonant interaction scheme. The scheme is based on free electrons, whose quantum electron wavefunction is preshaped or optically modulated by lasers in an electron microscope setup and then inelastically scattered by a quantum TLS target (e.g., atom, quantum dot, and crystal defect center) upon traversing in proximity to the target. Measurement of the postinteraction energy spectrum of the electrons probes and quantifies the full Bloch sphere parameters of a pre-excited TLS and enables coherent control of the qubit states. The exceptional advantage of this scheme over laser-based ones is atomic-scale spatial resolution of addressing individual TLS targets. Thus, this scheme opens horizons for electron microscopy in material interrogation and quantum information technology.

Local memory—the ability to extract information from a subsystem about its initial state—is a central feature of many-body localization. We introduce, investigate, and compare several information-theoretic quantifications of memory and discover a hierarchical relationship among them. We also find that while the Holevo quantity is the most complete quantifier of memory, vastly outperforming the imbalance, its decohered counterpart is significantly better at capturing the critical properties of the many-body localization transition at small system sizes. This motivates our suggestion that one can emulate the thermodynamic limit by artificially decohering otherwise quantum quantities. Applying this method to the von Neumann entropy results in critical exponents consistent with analytic predictions, a feature missing from similar small finite-size system treatments. In addition, the decohering process makes experiments significantly simpler by avoiding quantum state tomography.

Magnetic nano- and microswimmers provide a powerful platform to study driven colloidal systems in fluidic media and are relevant to futuristic medical technologies requiring precise yet minimally invasive motion control at small scales. Upon the action of a rotating magnetic field, the helical microswimmers rotate and translate, generating flow in the surrounding fluid. In this paper, we study the fluid flow induced by the rotating helices using a combination of experiments, numerical simulations, and theory. The microhelices are actuated either in a fluid bulk or in proximity to the bottom wall using typical microfluidic device setup. We conclude that the mean hydrodynamic flow due to the helix actuation can be closely approximated by a system of rotlets line distributed along the helical axis (i.e., representing the flow due to rotating cylinder) which gets modified close to a wall through appropriate contributions from image multipoles. As the mean flow can be obtained in closed form, this study can be further applied towards modeling of the dynamics in a swarm of driven microswimmers interacting hydrodynamically near a bounding surface.

We propose a set of Bell-type nonlocal games that can be used to prove an unconditional quantum advantage in an objective and hardware-agnostic manner. In these games, the circuit depth needed to prepare a cyclic cluster state and measure a subset of its Pauli stabilizers on a quantum computer is compared to that of classical Boolean circuits with the same, nearest-neighboring gate connectivity. Using a circuit-based trapped-ion quantum computer, we prepare and measure a six-qubit cyclic cluster state with an overall fidelity of 60.6% and 66.4%, before and after correcting for measurement-readout errors, respectively. Our experimental results indicate that while this fidelity readily passes conventional (or depth-0) Bell bounds for local hidden-variable models, it is on the cusp of demonstrating a higher probability of success than what is possible by depth-1 classical circuits. Our games offer a practical and scalable set of quantitative benchmarks for quantum computers in the pre-fault-tolerant regime as the number of qubits available increases.

Based on the first-principles calculations and model analysis, we propose that the isostructural compounds ${\mathrm{Cu}}_2{\mathrm{SnS}}_3$ and ${\mathrm{Cu}}_2{\mathrm{SnSe}}_3$ are both the simplest nodal-line semimetals with only one nodal line in their crystal momentum space when spin-orbit coupling (SOC) is ignored. The inclusion of SOC drives ${\mathrm{Cu}}_2{\mathrm{SnS}}_3$ into a Weyl semimetal (WSM) state with only two pairs of Weyl nodes, the minimum number required for a WSM with time-reversal symmetry. In contrast, SOC leads ${\mathrm{Cu}}_2{\mathrm{SnSe}}_3$ to a strong topological insulator (STI) state. This difference can be well understood as there is a topological phase transition (TPT). In it, the Weyl nodes are driven by tunable SOC and annihilate in a mirror plane, resulting in a STI. This TPT, together with the evolution of Weyl nodes, the changing of mirror Chern numbers of the mirror plane, and the $Z_2$ indices protected by time-reversal symmetry, has been demonstrated by the calculation of ${\mathrm{Cu}}_2\mathrm{Sn}{\left({\mathrm{S}}_{1-x}{\mathrm{Se}}_x\right)}_3$ within virtual crystal approximation and an effective $k·p$ model analysis. Though our first-principles calculations have overestimated the topological states in both compounds, we believe that the theoretical demonstration of controlling the TPT and the evolution of Weyl nodes will stimulate further efforts to explore them.

The theory of chemical kinetics forms the basis to describe the dynamics of chemical reaction networks. Owing to physical and thermodynamic constraints, the networks possess various structures, which can be utilized to characterize important properties of the networks. In this work, we reveal the Hessian geometry which underlies chemical reaction networks and demonstrate how it originates from the interplay of stoichiometric and thermodynamic constraints. Our derivation is based on kinetics, we assume the law of mass action and characterize the equilibrium states by the detailed balance condition. The obtained geometric structure is then related to thermodynamics via the Hessian geometry appearing in a pure thermodynamic derivation. We demonstrate, based on the fact that both equilibrium and complex balanced states form toric varieties, how the Hessian geometric framework can be extended to nonequilibrium complex balanced steady states. We conclude that Hessian geometry provides a natural framework to capture the thermodynamic aspects of chemical reaction networks.

We establish a Hessian geometric structure in chemical thermodynamics, which describes chemical reaction networks (CRNs) with equilibrium states. In our setup, the ideal gas assumption and mass action kinetics are not required. The existence and uniqueness condition of the equilibrium state is derived by using the Legendre duality inherent to the Hessian structure. The entropy production during a relaxation to the equilibrium state can be evaluated by the Bregman divergence. Furthermore, the equilibrium state is characterized by four distinct minimization problems of the divergence, which are obtained from the generalized Pythagorean theorem originating in the dual flatness. For the ideal gas case, we confirm that our existence and uniqueness condition implies Birch's theorem, and that the entropy production represented by the divergence coincides with the generalized Kullback-Leibler divergence. In addition, under mass action kinetics, our general framework reproduces the local detailed balance condition.

The transition of the viscosity $\eta$ from a collisional gas through a minimum value to a correlated liquid is investigated using computer simulations with the Green-Kubo relation. It is discovered that, as the temperature varies, the transition of $\eta$ is well described by the unity ratio of the instantaneous transverse sound speed $C_T$ to the average particle speed ${\overline{v}}_p$. While $C_T/{\overline{v}}_p<1, \eta$ increases with the temperature, since in this regime the viscosity is dominated by the gaslike individual dynamics. However, when $C_T/{\overline{v}}_p>1$ where the cooperative dynamics dominates, the fundamental origin of viscosity of liquids is found to be just losing or gaining neighbors for individual particles, so that the viscosity of a typical liquid reasonably decreases with the temperature. Our results reveal that the viscosity transition point of $C_T/{\overline{v}}_p=1$ is just $\approx 20$ times the corresponding melting point for both two-dimensional and three-dimensional Yukawa liquids with various screening parameters, which probably can be used as a new criterion to distinguish the strong and weak couplings in plasma physics.

Shear thickening denotes the reversible increase in viscosity of a suspension of rigid particles under external shear. This ubiquitous phenomenon has been documented in a broad variety of multiphase particulate systems, while its microscopic origin has been successively attributed to hydrodynamic interactions and frictional contact between particles. The relative contribution of these two phenomena to the magnitude of shear thickening is still highly debated, and we report here a discriminating experimental study using a model shear-thickening suspension that allows us to independently tune both the surface chemistry and the surface roughness of the particles. We show here that both properties matter when it comes to continuous shear thickening (CST) and that the presence of hydrogen bonds between the particles is essential to achieve discontinuous shear thickening (DST) by enhancing solid friction between closely contacting particles. Moreover, a simple argument allows us to predict the onset of CST, which for these very rough particles occurs at a critical volume fraction much lower than that previously reported in the literature. Finally, we demonstrate how mixtures of particles with opposing surface chemistry make it possible to finely tune the shear-thickening response of the suspension at a fixed volume fraction, paving the way for a fine control of the shear-thickening transition in engineering applications.

Recent measurements in several different laboratories report the observation of an approximately linear-in-temperature resistivity with a large twist-angle-dependent slope (or temperature coefficient) in moiré twisted bilayer graphene down to a few K and sometimes to much lower temperatures. In this paper, we theoretically discuss this “strange metal” linear-in-temperature transport behavior from the perspective of resistive scattering by acoustic phonons, emphasizing the aspects of the transport data, which are and which are not consistent with the phonon scattering mechanism. An extensive theoretical comparison with a recent experiment [A. Jaoui et al., Nat. Phys. 18, 633 (2022)] is the central new aspect of this work.

The minimum number of photons necessary for activating the sense of vision has been a topic of research for over a century. The ability of rod cells to sense a few photons has implications for understanding the fundamental capabilities of the human visual and nervous system and creating new vision technologies based on photonics. We investigate the fundamental metrological capabilities of different quantum states of light to probe the retina, which is modeled using a simple neural network. Stimulating the rod cells by Fock, coherent, and thermal states of light, and calculating the Cramer-Rao lower bound and Fisher information matrix for the signal produced by the ganglion cells in various conditions, we determine the volume of minimum error ellipsoid. Comparing the resulting ellipsoid volumes, we determine the metrological performance of different states of light for probing the retinal network. The results indicate that the thermal state yields the largest error ellipsoid volume and hence the worst metrological performance, and the Fock state yields the best performance for all parameters. This advantage persists even if another layer is added to the network or optical losses are considered in the calculations.

We provide a pure state formulation for hydrodynamics of isolated quantum many-body systems. A pure state describing quantum systems in local thermal equilibrium is constructed, which we call a local thermal pure quantum ($\ell \mathrm{TPQ}$) state. We show that the thermodynamic functional and the expectation values of local operators (including a real-time correlation function) calculated from the $\ell \mathrm{TPQ}$ state converge to those from a local Gibbs ensemble in the large fluid-cell limit. As a numerical demonstration, we investigate a one-dimensional spin chain and observe the hydrodynamic relaxation obeying Fourier's law. We further prove the second law of thermodynamics and the quantum fluctuation theorem, which are also validated numerically. The $\ell \mathrm{TPQ}$ formulation gives a useful theoretical basis to describe the emergent hydrodynamic behavior of quantum many-body systems furnished with a numerical efficiency applicable to both the nonrelativistic and relativistic regimes.

Rapid proliferation of hyperspectral imaging in scanning probe microscopy creates unique opportunities to systematically capture and categorize higher dimensional datasets, toward insights into electronic, mechanical, and chemical properties of materials with nano- and atomic-scale resolution. Effective hyperspectral imaging requires a consistent framework for data analysis that would be broadly applicable, reproducible, and transferrable, conceptually resembling the success of integral transforms in image analysis. Here, we demonstrate application of similarity learning for resolving the structure of tunneling spectroscopy data, characterizing a superconducting material with sparse density of defects. Popular methods for unsupervised learning and discrete representation of the data in terms of clusters of characteristic behaviors were found to produce inconsistencies with respect to capturing the location and tunneling characteristics of defect sites. The underlying reason for their ambiguity was traced to continuous variation of the electronic properties across the surface and therefore the absence of clear structural boundaries in the low-dimensional latent spaces of the data. We supported this hypothesis by direct analysis of the distributions of Euclidean distances within the dataset. We further proposed distance rescaling with probabilistic description as a possible approach to mitigate the detrimental effect of the long tails of the distributions on the performance of clustering methods. Subsequently, we applied a more general, nonlinear similarity learning, where dimension reduction was explicitly trained to amplify similarities and dissimilarities among individual spectra. This approach was found to outperform several widely used methods for dimensionality reduction and produce a clear categorization of tunneling spectra. Significant spectral weight transfer associated with the electronic reconstruction by the vacancy sites was systematically captured, as was the spatial extent of the vacancy region. Given that a great variety of electronic materials will exhibit similarly smooth variation of spectral response due to random or engineered inhomogeneities, we believe our approach will be useful for systematic analysis of hyperspectral imaging with minimal prior knowledge as well as prospective comparison of experimental measurements with theoretical calculations with explicit consideration of disorder.

We develop an analytical description of static ferrodark solitons, the ${\mathbb{Z}}_2$ topological defects in the magnetic order, in the easy-plane phase of ferromagnetic spin-1 Bose-Einstein condensates. We find that the type-I ferrodark soliton has a single width while the type-II ferrodark soliton exhibits two characteristic length scales. The proposed Ansätze show excellent agreement with numerical results. Spin-singlet amplitudes, nematic tensor densities, and nematic currents of ferrodark solitons are also discussed. The ${\mathbb{Z}}_2$ topological defects in the mass superfluid order, dark-dark-dark vector solitons, are obtained exactly in the parameter regime where exact ferrodark solitons exist. The dark-dark-dark vector soliton has higher excitation energy than ferrodark solitons.