Average atom (AA) models allow one to efficiently compute electronic and optical properties of materials over a wide range of conditions and are often employed to interpret experimental data. However, at high pressure, predictions from AA models have been shown to disagree with results from ab initio computer simulations. Here we reconcile these deviations by developing an innovative type of AA model, AvIon, that computes the electronic eigenstates with novel boundary conditions within the ion sphere. Bound and free states are derived consistently. We drop the common AA image that the free-particle spectrum starts at the potential threshold, which we found to be incompatible with ab initio calculations. We perform ab initio simulations of crystalline and liquid carbon and aluminum over a wide range of densities and show that the computed band structure is in very good agreement with predictions from AvIon.

The phenomenon of dielectric friction predicts slowing down of rotational and translational diffusion of a dipolar tracer in a polar medium due to retarded response of the medium polarization. This problem is studied here by numerical simulations in which the dipole moment of the tracer (solute) is continuously increased. The rotational time of the solute increases linearly with its squared dipole moment. A more pronounced effect of the solute dipole is found on the relaxation time of the electric field of the medium acting on the dipole: We report two orders of magnitude retardation for the electric field dynamics. With this strong retardation, classical theories of dielectric friction fail to describe slowing down of rotational diffusion. This failure is traced back to the breakdown of additivity between friction produced by van der Waals and electrostatic forces and torques. In contrast to the neglect of their correlations by traditional models, electrostatic and van der Waals forces and torques are strongly correlated. Electrostatic interactions bring to linear transport coefficients a number of features typically associated with the dynamics of low-temperature and supercooled liquids. The translational diffusion coefficient becomes strongly anisotropic in the solute's body frame, which translates to non-Gaussian translational dynamics. The Stokes-Einstein-Debye relation connecting the translational and rotational single-particle dynamics deviates from the hydrodynamic limit in linear proportion to the squared solute dipole. It is also found to increase with lowering temperature in qualitative agreement with phenomenology of supercooled liquids.

Applying quantum field theory on a classical, curved space-time has led to the enthralling theoretical predictions of, e.g., black hole evaporation and particle creation in the early, expanding universe. However, the prospects of observing such in the near to mid future seem far from practical. This is discouraging because extrapolation of general relativity “down to the Hilbert space level,” though expected, carries no empirical support as of yet. On the other hand, fueled by the continuing developments in quantum technology, there has been growing interest in the prospect of observing more modest effects of general relativity at the level of quantum interference phenomena. Motivated by these recent developments, in this work, we investigate the Hong-Ou-Mandel (HOM) effect—a two-photon quantum-interference effect—in the space-time of a rotating spherical mass. In particular, we propose and analyze a common-path setup restricted to the surface of the Earth and show that, in principle, gravitational frame dragging induces observable shifts in the two-photon HOM dip. For completeness and correspondence with current literature, we also analyze the emergence of gravitational time dilation, for a dual-arm interferometer. The formalism thus presented establishes a basis for encoding general-relativistic effects into local, multiphoton, quantum-interference experiments. We also consider signatures of noninertial reference-frame effects on two-photon interference such as, e.g., the Sagnac effect induced by Earth's rotation, which seems detectable with current quantum-optical technology. This can be viewed as a stepping-stone towards the more ambitious goal of observing gravitational frame dragging at the level of individual photons.

We describe a mechanism by which the longitudinal thermal conductivity ${\kappa }_{xx}$, measured in an in-plane magnetic field, oscillates as a function of field angle in layered nodal superconductors. These oscillations occur when the spin-orbit splitting at the nodes is larger than the nodal scattering rate, and are complementary to vortex-induced oscillations identified previously. In sufficiently anisotropic materials, the spin-orbit mechanism may be dominant. As a particular application, we focus on the cuprate high-temperature superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+x}$. This material belongs to the class of Rashba bilayers, in which individual ${\mathrm{CuO}}_2$ layers lack inversion symmetry although the crystal itself is globally centrosymmetric. We show that spin-orbit coupling endows ${\kappa }_{xx}/T$ with a characteristic dependence on magnetic field angle that should be easily detected experimentally, and argue that for underdoped samples the spin-orbit contribution is larger than the vortex contribution. A key advantage of the magnetothermal conductivity is that it is a bulk probe of spin-orbit physics, and therefore not sensitive to inversion breaking at surfaces.

We study the quench dynamics of non-Hermitian topological models with non-Hermitian skin effects. Adopting the non-Bloch band theory and projecting quench dynamics onto the generalized Brillouin zone, we find that emergent topological structures, in the form of dynamic skyrmions, exist in the generalized momentum-time domain, and are correlated with the non-Bloch topological invariants of the static Hamiltonians. The skyrmion structures anchor on the fixed points of dynamics whose existence is conditional on the coincidence of generalized Brillouin zones of the pre- and post-quench Hamiltonians. Global features of dynamic skyrmions, however, persist well beyond such a condition, constituting signatures for a general dynamic detection scheme of non-Bloch topology in the presence of non-Hermitian skin effects. Applying our theory to an experimentally relevant, nonunitary quantum walk, we explicitly demonstrate how the non-Bloch topological invariants can be revealed through the non-Bloch quench dynamics.

We show how one can deterministically generate photonic matrix product states with high bond and physical dimensions with an atomic array if one has access to a Rydberg-blockade mechanism. We develop both a quantum gate and an optimal control approach to universally control the system and analyze the photon retrieval efficiency of atomic arrays. Comprehensive modeling of the system shows that our scheme is capable of generating a large number of entangled photons. We further develop a multi-port photon emission approach that can efficiently distribute entangled photons into free space in several directions, which can become a useful tool in future quantum networks.

Given a class of $q$-local Hamiltonians, is it possible to find a simple variational state whose energy is a finite fraction of the ground-state energy in the thermodynamic limit? Whereas product states often provide an affirmative answer in the case of bosonic (or qubit) models, we show that Gaussian states fail dramatically in the fermionic case, such as for the Sachdev-Ye-Kitaev (SYK) models. This prompts us to propose a new class of wave functions for SYK models inspired by the variational coupled cluster algorithm. We introduce a static “(0+0)-dimensional” large-$N$ field theory to study the energy, two-point correlators, and entanglement properties of these states. Most importantly, we demonstrate a finite disorder-averaged approximation ratio of $r\approx 0.62$ between the variational and ground-state energy of the SYK model for $q=4$. Moreover, the variational states provide an exact description of spontaneous symmetry breaking in a related two-flavor SYK model.

Due to the capability of tolerating high error rate and generating more key bits per trial, high-dimensional quantum key distribution attracts wide interest. Despite great progresses in high-dimensional quantum key distribution, there are still some gaps between theory and experiment. One of these is that the security of the secret key heavily depends on the number of the emitted signals. So far, the existing security proofs are only suitable in the case with an infinite or unpractically large number of emitted signals. Here, by introducing the idea of “key classification” and developing relevant techniques based on the uncertainty relation for smooth entropies, we propose a tight finite-key analysis suitable for generalized high-dimensional quantum key distribution protocols. Benefitting from our theory, high-dimensional quantum key distribution protocols with finite resources become experimentally feasible.

The cryogenic buffer gas beam (CBGB) is an important tool in the study of cold and ultracold molecules. While there are known techniques to enhance desired beam properties of a CBGB, such as high flux, low velocity, or reduced divergence, they have generally not undergone detailed numerical optimization. Numerical simulation of buffer gas beams is challenging, because the relevant dynamics occur in regions where the density varies by orders of magnitude, rendering typical numerical methods unreliable or intractable. Here we simulate CBGBs with a hybrid approach that combines gas dynamics methods with particle tracing. The simulations capture important properties, such as velocitiy and divergence, across an assortment of designs, including two-stage slowing cells and de Laval nozzles. This approach should therefore be a useful tool for optimizing CBGB designs across a wide range of applications.

Quantum incompatibility, as an important quantum resource, is believed to play a key role in quantum information processing tasks. In order to detect this property, an entanglement-free incompatibility witness is proposed via constructing a combined quantum state discrimination task with prior-measurement and post-measurement information strategies. Here we experimentally implement the witness of the incompatibility of mutually unbiased measurements under different noise levels in a qutrit system. Moreover, we estimate the robustness of incompatibility using collected data. Our work paves the path for understanding quantum incompatibility and exploring the features of the quantum resource.

The correlation between electronic and crystal structures of $1T\text{−}\mathrm{Ti}{\mathrm{Se}}_2$ in the charge-density wave (CDW) state is studied by x-ray diffraction in order to clarify basic properties in the CDW state, transport properties, and chirality. Three families of reflections are used to probe atomic displacements and the orbital asymmetry in Se. Two distinct onset temperatures are found: $T_{\mathrm{CDW}}$ and a lower $T^{*}$ indicative for an onset of Se out-of-plane atomic displacements. $T^{*}$ coincides with a DC resistivity maximum and the onset of the proposed gyrotropic (chiral) electronic structure. However, no indication for chirality is found. The relation between the atomic displacements and the transport properties is discussed in terms of Ti $3d$ and Se $4p$ states that only weakly couple to the CDW order.

Kitaev materials are promising materials for hosting quantum spin liquids and investigating the interplay of topological and symmetry-breaking phases. We use an unsupervised and interpretable machine-learning method, the tensorial-kernel support vector machine, to study the honeycomb Kitaev-$\mathrm{\Gamma }$ model in a magnetic field. Our machine learns the global classical phase diagram and the associated analytical order parameters, including several distinct spin liquids, two exotic $S_3$ magnets, and two modulated $S_3\times Z_3$ magnets. We find that the extension of Kitaev spin liquids and a field-induced suppression of magnetic order already occur in the large-$S$ limit, implying that critical parts of the physics of Kitaev materials can be understood at the classical level. Moreover, the two $S_3\times Z_3$ orders are induced by competition between Kitaev and $\mathrm{\Gamma }$ spin liquids and feature a different type of spin-lattice entangled modulation, which requires a matrix description instead of scalar phase factors. Our work provides a direct instance of a machine detecting new phases and paves the way towards the development of automated tools to explore unsolved problems in many-body physics.

Universality is one of the key concepts in understanding critical phenomena. However, for interacting inhomogeneous systems described by complex networks, a clear understanding of the relevant parameters for universality is still missing. Here we discuss the role of a fundamental network parameter for universality, the spectral dimension. For this purpose, we construct a complex network model where the probability of a bond between two nodes is proportional to a power law of the nodes' distances. By explicit computation we prove that the spectral dimension for this model can be tuned continuously from 1 to infinity, and we discuss related network connectivity measures. We propose our model as a tool to probe universal behavior on inhomogeneous structures and comment on the possibility that the universal behavior of correlated models on such networks mimics the one of continuous field theories in fractional Euclidean dimensions.

A collective flashing ratchet transports Brownian particles using a spatially periodic, asymmetric, and time-dependent on-off switchable potential. The net current of the particles in this system can be substantially increased by feedback control based on the particle positions. Several feedback policies for maximizing the current have been proposed, but optimal policies have not been found for a moderate number of particles. Here, we use deep reinforcement learning (RL) to find optimal policies, with results showing that policies built with a suitable neural network architecture outperform the previous policies. Moreover, even in a time-delayed feedback situation where the on-off switching of the potential is delayed, we demonstrate that the policies provided by deep RL provide higher currents than the previous strategies.

Interacting single-level quantum dots connected to BCS superconducting leads represent a well-controllable system to study the interplay between the correlation effects and the electron pairing that can result in a $0-\pi$ (singlet-doublet) quantum phase transition (QPT). The physics of this system can be well described by the impurity Anderson model. We present an analysis of the statistics of the perturbation expansion order of the continuous-time hybridization expansion quantum Monte Carlo algorithm in the vicinity of such a first-order impurity QPT. By calculating the moments of the histograms of the expansion order which deviate from the ideal Gaussian shape, we provide an insight into the thermodynamic mixing of the two phases at low but finite temperatures, which is reflected in the bimodal nature of the histograms.

An indispensable step to understand collective magnetic phenomena in rare-earth compounds is the determination of spatially anisotropic single-ion properties resulting from spin-orbit coupling and crystal field (CF). The CF Hamiltonian has a discrete energy spectrum—accessible to spectroscopic probes such as neutron scattering—controlled by a number of independent parameters reflecting the point symmetry of the magnetic sites. Determining these parameters in low-symmetry systems is often challenging. Here, we describe a general method to analyze CF excitation spectra using adjustable effective point-charges. We benchmark our method to existing neutron-scattering measurements on pyrochlore rare-earth oxides and obtain a universal point-charge model that describes a large family of related materials. We adapt this model to the newly discovered tripod kagome magnets ($R_3{\mathrm{Mg}}_2{\mathrm{Sb}}_3{\mathrm{O}}_{14}, R$ = Tb, Ho, Er, Yb) for which we report broadband inelastic neutron-scattering spectra. Analysis of these data using adjustable point-charges yields the CF wave functions for each compound. From this, we calculate thermomagnetic properties that accurately reflect our measurements on powder samples and predict the effective gyromagnetic tensor for pseudospin degrees of freedom—a crucial step to understand the exotic collective properties of these kagome magnets at low temperature. We present further applications of our method to other tripod kagome materials and triangular rare-earth compounds $R{\mathrm{MgGaO}}_4$ ($R$ =Yb, Tm). Overall, this study establishes a widely applicable methodology to predict CF and single-ion properties of rare-earth compounds based on interpretable and adjustable models of effective point charges.

We develop a nonperturbative approach to the bulk polarization of crystalline electric insulators in $d\ge 1$ dimensions. Formally, we define polarization via the response to background fluxes of both charge and lattice translation symmetries. In this approach, the bulk polarization is related to properties of magnetic monopoles under translation symmetries. Specifically, in 2D, the monopole is a source of $2\pi$ flux, and the polarization is determined by the crystal momentum of the $2\pi$ flux. In 3D, the polarization is determined by the projective representation of translation symmetries on Dirac monopoles. Our approach also leads to a concrete scheme to calculate polarization in 2D, which in principle can be applied even to strongly interacting systems. For open boundary conditions, the bulk polarization leads to an altered boundary Luttinger theorem (constraining the Fermi surface of surface states) and also to modified Lieb-Schultz-Mattis theorems on the boundary, which we derive.

Noisy, intermediate-scale quantum (NISQ) computing devices have become an industrial reality in the last few years, and cloud-based interfaces to these devices are enabling the exploration of near-term quantum computing on a range of problems. As NISQ devices are too noisy for many of the algorithms with a known quantum advantage, discovering impactful applications for near-term devices is the subject of intense research interest. We explore a quantum-assisted machine learning (QAML) workflow using NISQ devices through the perspective of tensor networks (TNs), which offer a robust platform for designing resource-efficient and expressive machine learning models to be dispatched on quantum devices. In particular, we lay out a framework for designing and optimizing TN-based QAML models using classical techniques, and then compiling these models to be run on quantum hardware, with demonstrations for generative matrix product state (MPS) models. We put forth a generalized canonical form for MPS models that aids in compilation to quantum devices, and demonstrate greedy heuristics for compiling with a given topology and gate set that outperforms known generic methods in terms of the number of entangling gates, e.g., CNOTs, in some cases by an order of magnitude. We present an exactly solvable benchmark problem for assessing the performance of MPS QAML models and also present an application for the canonical MNIST handwritten digit dataset. The impacts of hardware topology and day-to-day experimental noise fluctuations on model performance are explored by analyzing both raw experimental counts and statistical divergences of inferred distributions. We also present parametric studies of depolarization and readout noise impacts on model performance using hardware simulators.

A strongly interacting many-body system of bosons exhibiting the quantum carpet pattern is investigated exactly by using Gaudin solutions. We show that this highly coherent design usually present in noninteracting, single-body scenarios gets destroyed by weak to moderate interatomic interactions in an ultracold bosonic gas trapped in a box potential. However, it becomes revived in a very strongly interacting regime, when the system undergoes fermionization. We track the whole single- to many-body crossover, providing an analysis of de- and rephasing present in the system.

Sorting atoms stochastically loaded in optical tweezer arrays via an auxiliary mobile tweezer is an efficient approach to preparing intermediate-scale defect-free atom arrays in arbitrary geometries. However, the high filling fraction of atom-by-atom assemblers is impeded by redundant sorting moves with imperfect atom transport, especially for scaling the system size to larger atom numbers. Here we propose a sorting algorithm [the heuristic cluster algorithm (HCA)] which provides the near-fewest number of moves $N_m\approx N$ ($N$ is the number of defect sites to be filled) and experimentally demonstrate a $5\times 6$ defect-free atom array with 98.4(7)$\%$ filling fraction for one rearrangement cycle. Furthermore, we compare the HCA with two other algorithms by simulating results and find that the HCA is well suited for large-scale atom assemblers. Our method is essential to scale hundreds of assembled atoms for bottom-up quantum computation, quantum simulation, and precision measurement.