Real materials always contain, to some extent, randomness in the form of defects or irregularities. It is known since the seminal work of Anderson that randomness can drive a metallic phase to an insulating one, and the mechanism responsible for this transition is intrinsically different from the one of the interaction-induced transitions discovered by Mott. Lattice Hamiltonians, with their conceptual and computational advantages, permitted to investigate broadly the interplay of both mechanisms. However, a clear understanding of the differences (or not) with their real-space counterparts is lacking, especially in the presence of long-range Coulomb interactions. This work aims at shedding light on this challenging question by investigating a real-space one-dimensional model of interacting electrons in the presence of a disordered potential. The transition between delocalized and localized phases is characterized using two different indicators, namely, the single-particle occupation entropy and the position-space information entropy. In addition, the performance of density functional approximations to reproduce the exact ground-state densities of this many-body localization model are gauged.

We generalize the hidden-fermion family of neural network quantum states to encompass both continuous and discrete degrees of freedom and solve the nuclear many-body Schrödinger equation in a systematically improvable fashion. We demonstrate that adding hidden nucleons to the original Hilbert space considerably augments the expressivity of the neural-network architecture compared to the Slater-Jastrow ansatz. The benefits of explicitly encoding in the wave function point symmetries such as parity and timereversal are also discussed. Leveraging on improved optimization methods and sampling techniques, the hidden-nucleon ansatz achieves an accuracy comparable to the numericallyexact hyperspherical harmonic method in light nuclei and to the auxiliary field diffusion Monte Carlo in ${}^{16}\mathrm{O}$. Thanks to its polynomial scaling with the number of nucleons, this method opens the way to highly-accurate quantum Monte Carlo studies of medium-mass nuclei.

Vorticity in closed quantum fluid circuits is known to arise in the form of persistent currents. In this work, we develop a method to engineer transport of the quantized vorticity between density-coupled ring-shaped atomic Bose-Einstein condensates in experimentally accessible regimes. Introducing a tunable weak link between the rings, we observe and characterize the controllable periodic transfer of the current and investigate the role of temperature on suppressing these oscillations via a range of complementary state-of-the-art numerical methods. Our setup paves the way for precision measurements of local acceleration and rotation.

Topological gauge theories provide powerful effective descriptions of certain strongly correlated systems, a prime example being the Chern-Simons gauge theory of fractional quantum Hall states. Engineering topological gauge theories in controlled quantum systems is of both conceptual and practical importance, as it would provide access to systems with exotic excitations such as anyons without the need for strong correlations. Here, we discuss a scheme to engineer the chiral BF theory, a minimal model of a topological gauge theory corresponding to a one-dimensional reduction of the Chern-Simons theory, with ultracold atoms. Using the local conservation laws of the theory, we encode its quantum Hamiltonian into an ultracold quantum gas with chiral interactions. Building on a seminal proposal by Edmonds et al. [Phys. Rev. Lett. 110, 085301 (2013)], we show how to implement it in a Raman-coupled Bose-Einstein condensate with imbalanced scattering lengths, as we have recently realized experimentally [Frölian et al., Nature (London) 608, 293 (2022)]. We discuss the properties of the chiral condensate from a gauge theory perspective and assess the validity of the effective quantum description for accessible experimental parameters via numerical simulations. Our approach lays the foundation for realizing topological gauge theories in higher dimensions with Bose-Einstein condensates.

Particle transport across junctions between two superconductors is commonly described using a simplifying approximation (often called the Andreev approximation), which assumes that excitations are fixed at the Fermi momentum and only Andreev reflections, with no normal reflections, occur at interfaces. While this approximation is appropriate for superconductors with high carrier density (for which the chemical potential vastly exceeds the pairing gap), it breaks down for superconductors with low carrier density, such as topological superconductors, doped semiconductors, or superfluid quantum gases. Here, we present a general analytical framework for transport in superconducting junctions that does not rely on this limiting Andreev approximation. We apply our framework to describe transport in junctions between $s$-wave superconductors along the BCS-BEC crossover, which interpolates between the conventional high-carrier-density (BCS-)regime and moderate- as well as low-carrier-density regimes (unitary and BEC regimes), for which the high-carrier-density (Andreev) approximation is not valid. As the system is tuned from the BCS to the BEC regime, we find that the overall magnitude of a subgap current, which is attributed to multiple Andreev reflections, decreases. However, nonlinearities in the current-voltage characteristic become more pronounced near the intermediate unitary limit, giving rise to sharp peaks and dips in the differential conductance with even negative differential conductance at specific voltages. Microscopically, the negative differential conductance is related to the van Hove points in the band structures, at which enhanced normal reflection occurs and that become accessible only when the chemical potential is comparable to or smaller than the pairing gap. The subgap current due to multiple Andreev reflections vanishes at a critical interaction strength on the BEC side, which we identify as the splitting point where the particle dispersion changes curvature. Our work shows that a description of transport in low-density superconducting junctions necessarily requires a treatment beyond the standard Andreev approximation.

Quantum annealing is a promising approach for obtaining good approximate solutions to difficult optimization problems. Folding a protein sequence into its minimum-energy structure represents such a problem. For testing new algorithms and technologies for this task, the minimal lattice-based [hydrophobic (H) or polar (P) beads] HP model is well suited, as it represents a considerable challenge despite its simplicity. The HP model has favorable interactions between adjacent, not directly bound hydrophobic residues. Here, we develop a novel spin representation for lattice protein folding tailored for quantum annealing. With a distributed encoding onto the lattice, it differs from earlier attempts to fold lattice proteins on quantum annealers, which were based upon chain growth techniques. With our encoding, the Hamiltonian by design has the quadratic structure required for calculations on an Ising-type annealer, without having to introduce any auxiliary spin variables. This property greatly facilitates the study of long chains. The approach is robust to changes in the parameters required to constrain the spin system to chainlike configurations, and performs very well in terms of solution quality. The results are evaluated against existing exact results for HP chains with up to $N=30$ beads with 100% hit rate, thereby also outperforming classical simulated annealing. In addition, the method allows us to recover the lowest known energies for $N=48$ and $N=64$ HP chains, with similar hit rates. These results are obtained by the commonly used hybrid quantum-classical approach. For pure quantum annealing, our method successfully folds an $N=14$ HP chain. The calculations were performed on a D-Wave Advantage quantum annealer.

We study the electric field around a continuous one-dimensional loop of static charge, under the assumption that the charge is distributed uniformly along the loop. For rectangular or stadium-shaped loops in the plane, we find that the electric field can undergo a symmetry-breaking pitchfork bifurcation as the loop is elongated; the field can have either one or three zeros, depending on the loop's aspect ratio. For knotted charge distributions in three-dimensional space, we compute the electric field numerically and compare our results to previously published theoretical bounds on the number of equilibrium points around charged knots. Our computations reveal that the previous bounds are far from sharp. The numerics also suggest conjectures for the actual minimum number of equilibrium points for all charged knots with five or fewer crossings. In addition, we provide the first images of the equipotential surfaces around charged knots and visualize their topological transitions as the level of the potential is varied.

The concept of exceptional points-based optical amplifiers (EPOAs) has been recently proposed as a new paradigm for miniaturizing optical amplifiers while simultaneously enhancing their gain-bandwidth product. While the operation of this new family of amplifiers in the classical domain provides a clear advantage, their performance in the quantum domain has not yet been evaluated. Particularly, it is not clear how the quantum noise introduced by vacuum fluctuations will affect their operation. Here, we investigate this problem by considering three archetypal EPOA structures that rely either on unidirectional coupling, parity-time symmetry, or particle-hole symmetry for implementing the exceptional point. By using the Heisenberg-Langevin formalism, we calculate the added quantum noise in each of these devices and compare it with that of a quantum-limited amplifier scheme that does not involve any exceptional points. Our analysis reveals several interesting results: most notably that while the quantum noise of certain EPOAs can be comparable to those associated with conventional amplifier systems, in general the noise does not follow a universal scaling as a function of the exceptional point but rather varies from one implementation to another.

We demonstrate a generic mechanism to realize topological flat minibands by confining massive Dirac fermions in a periodic moiré potential, which can be achieved in a heterobilayer of transition metal dichalcogenides. We show that the topological phase can be protected by the symmetry of moiré potential and survive to arbitrarily large Dirac band gap. We take the ${\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$ heterobilayer as an example and find that the topological phase can be driven by a vertical electric field. By projecting the Coulomb interaction onto the topological fat minibands, we identify a correlated Chern insulator at half filling and a quantum valley-spin Hall insulator at full filling which explains the topological states observed in the ${\mathrm{MoTe}}_2/{\mathrm{WSe}}_2$ in the experiment. Our work clarifies the importance of Dirac structure for the topological minibands and unveils a general strategy to design topological moiré materials.

Kirigami, the art of introducing cuts in thin sheets to enable articulation and deployment, has become an inspiration for a novel class of mechanical metamaterials with unusual properties. Here we complement the use of periodic tiling patterns for kirigami designs by showing that quasicrystals can also serve as the basis for designing deployable kirigami structures, and analyze the geometrical, topological, and mechanical properties of these aperiodic kirigami structures.

It was hypothesized already more than 40 years ago that photoelectron spectroscopy should in principle be able to image atomic orbitals. If this can be made to work for orbitals in crystalline solids, one would have literally a different view on the electronic structure of a wide range of quantum materials. Here, we demonstrate how hard x-ray photoelectron spectroscopy can make direct images of the orbitals making up the band structure of our model system, ${\mathrm{ReO}}_3$. The images are energy specific and enable us to unveil the role of each of those orbitals for the chemical bonding and the Fermi surface topology. The orbital image information is complementary to that from angle-resolved photoemission and thus completes the determination of the electronic structure of materials.

Here, we demonstrate an experimental proof of concept for nanofocused x-ray photon correlation spectroscopy, a technique sensitive to nanoscale fluctuations present in a broad range of systems. The experiment, performed at the NanoMAX beamline at MAX IV, uses a novel event-based x-ray detector to capture nanoparticle structural dynamics with microsecond resolution. By varying the nanobeam size from $\sigma =88$ nm to $\sigma =2.5\mu \mathrm{m}$, we quantify the effect of the nanofocus on the small-angle scattering lineshape and on the diffusion coefficients obtained from nano-XPCS. We observe that the use of nanobeams leads to a multifold increase in speckle contrast, which greatly improves the experimental signal-to-noise ratio, quantified from the two-time intensity correlation functions. We conclude that it is possible to account for influence of the high beam divergence on the lineshape and measured dynamics by including a convolution with the nanobeam profile in the model.

Using a nonperturbative classical model, we numerically investigate the dynamics of mobile particles interacting with an infinite chain of harmonic oscillators, an abstraction of ionic conduction through solid-state materials. We show that coupling between the mobile particles and a single mass of the chain is sufficient to induce dissipation of the mobile particles' energy over a wide range of system parameters. When we introduce thermal fluctuations in the position of the chain mass, the mobile particles exhibit thermalization, eventually reaching the same temperature scale as the chain. This model demonstrates how a minimal set of ingredients can exhibit a link between microscopic motion and macroscopic observables, with computationally efficient simulations. Finally, we suggest some experimental platforms that could realize such a model.

Entanglement is the fundamental difference between classical and quantum systems and has become one of the guiding principles in the exploration of high- and low-energy physics. The calculation of entanglement entropies in interacting quantum field theories, however, remains challenging. Here, we present the first method for the explicit computation of reduced density matrices of interacting quantum field theories using truncated Hamiltonian methods. The method is based on constructing an isomorphism between the Hilbert space of the full system and the tensor product of Hilbert spaces of subintervals. This naturally enables the computation of the von Neumann and arbitrary Rényi entanglement entropies as well as mutual information, logarithmic negativity, and other measures of entanglement. Our method is applicable to equilibrium states and nonequilibrium evolution in real time. It is model independent and can be applied to any Hamiltonian truncation method that uses a free basis expansion. We benchmark the method on the free Klein-Gordon theory finding excellent agreement with the analytic results. We further demonstrate its potential on the interacting sine-Gordon model, studying the scaling of von Neumann entropy in ground states and real-time dynamics following quenches of the model.

Intercalation is an established technique for tailoring the electronic structure of epitaxial graphene. Moreover, it enables the synthesis of otherwise unstable two-dimensional (2D) layers of elements with unique physical properties compared to their bulk versions due to interfacial quantum confinement. In this work, we present uniformly Pb-intercalated quasifreestanding monolayer graphene on SiC, which turns out to be essentially charge neutral with an unprecedented $p$-type carrier density of only $(5.5\pm 2.5)\times {10}^9$ ${\mathrm{cm}}^{-2}$. Probing the low-energy electronic structure throughout the entire first surface Brillouin zone by means of momentum microscopy, we clearly discern additional bands related to metallic 2D Pb at the interface. Low-energy electron diffraction further reveals a $10\times 10$ Moiré superperiodicity relative to graphene, counterparts of which cannot be directly identified in the available band structure data. Our experiments demonstrate 2D interlayer confinement and associated band structure formation of a heavy-element superconductor, paving the way towards strong spin-orbit coupling effects or even 2D superconductivity at the graphene-SiC interface.

We demonstrate theoretically and numerically that a warm fluid model of a plasma supports space-time quasicrystalline structures. These structures are highly nonlinear, two-phase, ion acoustic waves that are excited autoresonantly when the plasma is driven by two small amplitude chirped-frequency ponderomotive drives. The waves exhibit density excursions that substantially exceed the equilibrium plasma density. Remarkably, these extremely nonlinear waves persist even when the small amplitude drives are turned off. We derive the weakly nonlinear analytical theory by applying Whitham's averaged variational principle to the Lagrangian formulation of the fluid equations. The resulting system of coupled weakly nonlinear equations is shown to be in good agreement with fully nonlinear simulations of the warm fluid model. The analytical conditions and thresholds required for autoresonant excitation to occur are derived and compared to simulations. The weakly nonlinear theory guides and informs numerical study of how the two-phase quasicrystalline structure “melts” into a single phase traveling wave when one drive is below a threshold. These nonlinear structures may have applications to plasma photonics for extremely intense laser pulses, which are limited by the smallness of density perturbations of linear waves.

Crossing a quantum critical point in finite time challenges the adiabatic condition due to the closing of the energy gap, which ultimately results in the formation of excitations. Such nonadiabatic excitations are typically deemed detrimental in many scenarios, and consequently several strategies have been put forward to circumvent their formation. Here, however, we show how these nonadiabatic excitations—originated from the failure to meet the adiabatic condition due to the presence of a quantum critical point—can be controlled and thus harnessed to perform certain tasks advantageously. We focus on closed cycles reaching the quantum critical point of fully connected models analyzing two examples. First, a quantum battery that is loaded by approaching a quantum critical point, whose stored and extractable work increases exponentially via repeating cycles. Second, a scheme for the fast preparation of spin squeezed states containing multipartite entanglement that offer a metrological advantage, analogous to a two-axis twisting scheme. The corresponding figure of merit in both examples crucially depends on the universal critical exponents and the scaling of the protocol in the vicinity of the transition. Our results highlight the rich interplay between quantum thermodynamics and metrology with critical nonequilibrium dynamics.

We analyze the dynamics of a passive colloidal probe immersed in an active bath using an optical trap to study three physical processes: (1) the nonequilibrium fluctuations transferred to the probe by the active bath, (2) the friction experienced by the probe as it is driven through the active bath, and (3) the force relaxation of the probe returning to its equilibrium position. We measure the local force dynamics where all of the following characteristics are of $\mathcal{O}\left(1\right)$: the size of the probe colloid relative to the active bath particle; the size of the probe colloid relative to the characteristic run-length of an active particle; and the timescale of probe movement to the persistence time of an active particle. We find at Péclet (Pe) $\ll 1$ the active suspension exhibits shear thinning down to the solvent viscosity (but not below); at $0.85<$ Pe $\le 5.1$, the active bath shear thickens; and at Pe $\ge 8.5$, the effective viscosity of the active bath shows a decreased effect of thickening and plateaus. These results are in agreement with recent modeling and simulations of the nonlinear rheology of an isotropic active bath, providing experimental verification, and suggesting the model predictions extends to moderately dense suspensions. Further, we observe that the distribution of force fluctuations depends on Pe, unlike in passive equilibrium baths. Lastly, we measure the energy transfer rate from the active bath to the probe to be $\langle J\rangle \approx {10}^3{k}_{B}T/\mathrm{s}$, which leads to an increase in the effective diffusion of the probe by a factor of $\sim 2$.

A powerful result of topological band theory is that nontrivial phases manifest obstructions to constructing localized Wannier functions. In Chern insulators, it is impossible to construct Wannier functions that respect translational symmetry in both directions. Similarly, Wannier functions that respect time-reversal symmetry cannot be formed in quantum spin Hall insulators. This molecular orbital interpretation of topology has been enlightening and was recently extended to topological crystalline insulators which include obstructions tied to space-group symmetries. In this paper, we introduce a class of two-dimensional topological materials known as optical $N$-insulators that possess obstructions to constructing localized molecular polarizabilities. The optical $N$-invariant $N\in \mathbb{Z}$ is the winding number of the atomistic susceptibility tensor $\chi$ and counts the number of singularities in the electromagnetic linear response theory. We decipher these singularities by analyzing the optical band structure of the material—the eigenvectors of the susceptibility tensor—which constitutes the collection of optical Bloch functions. The localized basis of these eigenvectors are optical Wannier functions which characterize the molecular polarizabilities at different lattice sites. We prove that in a nontrivial optical phase $N\ne 0$, such a localized polarization basis is impossible to construct. Utilizing the mathematical machinery of $K$ theory, these optical $N$-phases are refined further to account for the underlying crystalline symmetries of the material, generating a complete classification of the topological electromagnetic phase of matter.

Robust cooling and nondestructive imaging are prerequisites for many emerging applications of neutral atoms trapped in optical tweezers, such as their use in quantum information science and analog quantum simulation. The tasks of cooling and imaging can be challenged, however, by the presence of large trap-induced shifts of their respective optical transitions. Here, we explore a system of ${}^{39}\mathrm{K}$ atoms trapped in a near-detuned $\left(780\mathrm{nm}\right)$ optical tweezer, which leads to relatively minor differential (ground versus excited state) Stark shifts. We demonstrate that simple and robust loading, cooling, and imaging can be achieved through combined addressing of the ${\mathrm{D}}_{\text{1}}$ and ${\mathrm{D}}_{\text{2}}$ transitions. While imaging on the ${\mathrm{D}}_{\text{2}}$ transition, we can simultaneously apply $\mathrm{\Lambda }$-enhanced gray molasses (GM) on the ${\mathrm{D}}_{\text{1}}$ transition, preserving low backgrounds for single-atom imaging through spectral filtering. Using ${\mathrm{D}}_{\text{1}}$ cooling during and after trap loading, we demonstrate enhanced loading efficiencies as well as cooling to low temperatures. These results suggest a simple and robust path for loading and cooling large arrays of potassium atoms in optical tweezers through the use of resource-efficient near-detuned optical tweezers and GM cooling.