Many-body Green's functions encode all the properties and excitations of interacting electrons. While these are challenging to be evaluated accurately on a classical computer, recent efforts have been directed toward finding quantum algorithms that may provide a quantum advantage for this task, exploiting architectures that will become available in the near future. In this work we introduce a novel near-term quantum algorithm for computing one-particle Green's functions via their Lehmann representation. The method is based on a generalization of the quantum equation of motion algorithm that gives access to the charged excitations of the system. We demonstrate the validity of the present proposal by computing the Green's function of a two-site Fermi-Hubbard model on a IBM quantum processor.

We present a de Haas–van Alphen quantum oscillation study of the Dirac nodal-line semimetal HfSiS up to 32 T to unravel the structure of the high-frequency magnetic breakdown spectrum that was previously obscured in transport experiments. Despite a threefold enhanced gap between adjacent electron and hole pockets relative to the sister compound ZrSiS, a large number of large-area magnetic breakdown orbits enclosing the nodal-loop are identified. All breakdown orbits are assigned by extracting their cyclotron masses. Moreover, one additional low-frequency magnetic breakdown orbit, previously absent in ZrSiS, is observed and attributed to the larger spin-orbit interaction in HfSiS.

We develop an exact map of all states and operators from two-dimensional lattices of spins-$1/2$ into lattices of fermions and bosons with mutual semionic statistical interaction that goes beyond previous dualities of ${\mathbb{Z}}_2$ lattice gauge theories because it does not rely on imposing local conservation laws and captures the motion of “charges” and “fluxes” on equal footing. This map allows to explicitly compute the Berry phases for the transport of fluxes in a large class of symmetry-enriched topologically ordered states with emergent ${\mathbb{Z}}_2$ gauge fields that includes chiral, nonchiral, Abelian or non-Abelian, that can be perturbatively connected to models where the visons are static and the emergent fermionic spinons have a noninteracting dispersion. The numerical complexity of computing such vison phases reduces, therefore, to computing overlaps of ground states of free-fermion Hamiltonians. Among other results, we establish numerically the conditions under which the Majorana-carrying flux excitation in Ising-topologically ordered states enriched by translations acquires the 0 or $\pi$ phase when moving around a single plaquette.

The two-dimensional nature of engineered transition-metal ultrathin oxide films offers a large playground of yet to be fully understood physics. Here, we study pristine ${\mathrm{SrVO}}_3$ monolayers that have recently been predicted to display a variety of magnetic and orbital orders. We find that nonlocal magnetic (orbital) fluctuations lead to a strong (weak to moderate) momentum differentiation in the self-energy, particularly in the scattering rate. In the one-band 2D Hubbard model, momentum selectivity on the Fermi surface (“$k=k_F$”) is known to lead to pseudogap physics. Here instead, in the multiorbital case, we evidence a differentiation between momenta on the occupied (“$k<k_F$”) and the unoccupied side (“$k>k_F$”) of the Fermi surface. Based on the dynamical vertex approximation, and introducing a “binaural fluctuation diagnostics” tool, we advance the understanding of spectral signatures of nonlocal fluctuations. Our work calls to (re)examine ultrathin oxide films and interfaces with methods beyond dynamical mean-field theory and may point to correlation-enhanced thermoelectric effects.

We study a vast family of continuum Rokhsar-Kivelson (RK) states, which have their ground state encoded by a local quantum field theory. These describe certain quantum magnets, and are also important in quantum information. We prove the separability of the reduced density matrix of two disconnected subsystems, implying the absence of entanglement between the two subsystems—a stronger statement than the vanishing of logarithmic negativity. As a particular instance, we investigate the case where the ground state is described by a relativistic boson, which is relevant for certain magnets or Lifshitz critical points with dynamical exponent $z=2$, and we propose nontrivial deformations that preserve their RK structure. Specializing to 1D systems, we study a deformation that maps the ground state to the quantum harmonic oscillator, leading to a gap for the boson. We study the resulting correlation functions, and find that cluster decomposition is restored. We analytically compute the $c$ function for the entanglement entropy along a renormalization group flow for the wavefunction, which is found to be strictly decreasing as in CFTs. Finally, we comment on the relations to certain stoquastic quantum spin chains. We show that the Motzkin and Fredkin chains possess unusual entanglement properties not properly captured by previous studies.

We study an electron distribution under a quasiperiodic potential in light of hyperuniformity, aiming to establish a classification and analysis method for aperiodic but orderly density distributions realized in, e.g., quasicrystals. Using the Aubry-André-Harper model, we first reveal that the electron-charge distribution changes its character as the increased quasiperiodic potential alters the eigenstates from extended to localized ones. While these changes of the charge distribution are characterized by neither multifractality nor translational-symmetry breaking, they are characterized by hyperuniformity class and its order metric. We find a nontrivial relationship between the density of states at the Fermi level, a charge-distribution histogram, and the hyperuniformity class. The change to a different hyperuniformity class occurs as a first-order phase transition except for an electron-hole symmetric point, where the transition is of the third order. Moreover, we generalize the hyperuniformity order metric to a function, to capture more detailed features of the density distribution, in some analogy with a generalization of the fractal dimension to a multifractal one.

In condensed matter, scattering processes determine the transport of charge carriers. In case of heterostructures, interfaces determine many dynamic properties such as charge transfer and transport, and spin current dynamics. Here, we discuss optically excited electron dynamics and their propagation across a lattice-matched, metal-metal interface of single crystal quality. Using femtosecond time-resolved linear photoelectron spectroscopy upon optically pumping different constituents of the heterostructure, we establish a technique that probes the electron propagation and its energy relaxation simultaneously. In our approach, a near-infrared pump pulse excites electrons directly either in the Au layer or in the Fe layer of epitaxial Au/Fe/MgO(001) heterostructures while the transient photoemission spectrum is measured by an ultraviolet probe pulse on the Au surface. Upon femtosecond laser excitation, we analyze the relative changes in the electron distribution close to the Fermi energy and assign characteristic features of the time-dependent electron distribution to transport of hot and nonthermalized electrons from the Fe layer to the Au surface and vice versa. From the measured transient electron distribution, we determine the excess energy, which we compare with a calculation based on the two-temperature model that takes diffusive electron transport into account. On this basis, we identify a transition with increasing Au layer thickness from a superdiffusive to a diffusive transport regime at 20–30 nm.

To simulate the dynamics of massless Dirac fermions in curved space-times with one, two, and three spatial dimensions, we construct tight-binding Hamiltonians with spatially varying hoppings. These models represent tilted Weyl semimetals where the tilting varies with position, in a manner similar to the light cones near the horizon of a black hole. We illustrate the gravitational analogies in these models by numerically evaluating the propagation of wave packets on the lattice and then comparing them to the geodesics of the corresponding curved space-time. We also show that the motion of electrons in these spatially varying systems can be understood through the conservation of energy and the quasiconservation of quasimomentum. This picture is confirmed by calculations of the scattering matrix, which indicate an exponential suppression of any noncontinuous change in the quasimomentum. Finally, we show that horizons in the lattice models can be constructed also at finite energies using specially designed tilting profiles.

We report electron transport studies of a thin InAs-Al hybrid semiconductor-superconductor nanowire device using a four-terminal design. Compared to previous studies, thinner InAs nanowire (diameter less than 40 nm) is expected to reach fewer subband regime. The four-terminal device design excludes electrode contact resistance, an unknown value, which has inevitably affected previously reported device conductance. Using tunneling spectroscopy, we find large zero bias peaks (ZBPs) in differential conductance on the order of $2e^2/h$. At specific gate voltage settings, we find a magnetic-field-driven transition between a zero bias peak and a zero bias dip while the zero-bias conductance sticks close to $2e^2/h$. We discuss a topologically trivial interpretation involving disorder, smooth potential variation and quasi-Majorana zero modes.

We derive the fully time-dependent solution to a run-and-tumble model for a particle which has tumbling restricted to the boundaries of a one-dimensional interval. This is achieved through a field-theoretic perturbative framework by exploiting an elegant underlying structure of the perturbation theory. We calculate the particle densities, currents and variance as well as characteristics of the boundary tumbling. The analytical findings, in agreement with Monte Carlo simulations, show how the particle densities are linked to the scale of diffusive fluctuations at the boundaries. The generality of our approach suggests it could be readily applied to similar problems described by Fokker-Planck equations containing localized reaction terms.

Hybrid devices based on the superconducting qubits have emerged as a promising platform for controlling the quantum states of macroscopic resonators. The nonlinearity added by a qubit can be a valuable resource for such control. Here we study a hybrid system consisting of a mechanical resonator longitudinally coupled to a transmon qubit. The qubit readout can be done by coupling to a readout mode like in c-QED setup. The coupling between the mechanical resonator and transmon qubit can be implemented by the modulation of the superconducting quantum interference device inductance. In such a tripartite system, we analyze the steady-state occupation of the mechanical mode when all three modes are dispersively coupled. We use the quantum noise and the Lindblad formalism to show that the sideband cooling of the mechanical mode to its ground state is achievable. We further experimentally demonstrate that measurements of the thermomechanical motion are possible in the dispersive limit, while maintaining a large coupling between qubit and mechanical mode. Our theoretical calculations suggest that single-photon strong coupling is within the experimental reach in such hybrid devices.

X-ray photon correlation spectroscopy (XPCS) provides an understanding of complex dynamics in materials that are tied to their synthesis, properties, and behaviors. Analysis of XPCS data for dynamics that are far from equilibrium is labor intense and often can impede the discovery process, especially in experiments with high collection rates. Moreover, binning and averaging, involved in the analysis for alleviating poor signal-to-noise ratio, leads to a loss of temporal resolution and the accumulation of systematic error for the parameters quantifying the dynamics. Here, we integrate a denoising autoencoder model into workflows for the analysis of nonequilibrium two-time intensity-intensity correlation functions. Noise reduction allows for extracting the parameters that characterize the sample's dynamics with the temporal resolution limited only by frame rates. Not only does it improve the quantitative usage of the data, but it also creates the potential for automating the analytical workflow, which is a key to high-throughput or autonomous XPCS experiments. Various approaches for the uncertainty quantification and extension of the model for anomalies' detection are discussed.

We study the transport properties of a Josephson junction consisting of two identical $s$-wave superconductors separated by an even-layer ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ (MBT). Using recursive Green's function method, we calculate the supercurrent in the presence of a perpendicular magnetic field and find that its quantum interference exhibits distinct patterns when the MBT is in different magnetic states. In the antiferromagnetic state, the MBT is an axion insulator supporting an extended “hinge” supercurrent, which leads to a sinusoidal interference pattern decaying with the field strength. In the ferromagnetic state, the MBT is a Chern insulator and the unbalanced chiral supercurrents on opposite edges give rise to a highly asymmetric interference pattern. If the MBT turns into a metal as the Fermi level is tuned into the conduction band, the interference exhibits a Fraunhofer pattern due to the uniformly distributed bulk supercurrent. Our work unravels a strong indicator to identify different phases in the MBT and can be verified directly by experiments.

Temperatures below 1 mK on-chip hold great potential for quantum physics but present a great challenge due to the lack of suitable thermometry and the detrimental pulse-tube vibrations of cryogen-free refrigerators. Here, we solve the pulse-tube problem using a rigidly wired metallic sample holder, which provides a microkelvin environment with low heat leaks despite the vibrations. Further, we demonstrate an improved type of temperature sensor, the gate Coulomb blockade thermometer (gCBT), employing a gate metallization covering the entire device. This immunizes against nanofabrication imperfections and uncontrollable offset charges, and extends the range to lower temperatures compared to a junction CBT with the same island capacitance, here down to $\approx$160 $\mu$K for a 10% accuracy. Using on- and off-chip cooling, we demonstrate electronic temperatures as low as 224 $\pm$ 7 $\mu$K, remaining below 300 $\mu$K for 27 hours, thus providing time for experiments. Finally, we give an outlook for cooling below 50 $\mu$K for a future generation of microkelvin transport experiments.

Molecular magnetic toroidal moments are molecule-based structures of quantum spins that are expected to boost magnetic storage technology and quantum computing. We study selected fictitious but typical examples of single-molecule toroidal magnet behavior, discuss the essence of the concept, and clarify inappropriate or even wrong assignments of physical properties. We provide an outlook that discusses necessary ingredients to the concept of toroidicity.

The equilibration of electronic carriers in metals after excitation by an ultrashort laser pulse provides an important class of nonequilibrium phenomena in metals and allows measuring the effective electron-phonon coupling parameter. Since the observed decay of the electronic distribution is governed by the interplay of both electron-electron and electron-phonon scattering, the interpretation of experimental data must rely on models that ideally should be easy to handle, yet accurate. In this paper, an extended rate-equation model is proposed that explicitly includes nonthermal electronic carriers while at the same time incorporating data from first-principles calculations of the electron-phonon coupling via Eliashberg-Migdal theory. The model is verified against experimental data for thin Pb films grown on Si(111). Improved agreement between theory and experiment at short times ($<0.3\mathrm{ps}$) due to nonthermal electron contributions is found. Moreover, the rate equations allow for widely different coupling strength to different phonon subsystems. Consequently, an indirect, electron-mediated energy transfer between strongly and weakly coupled groups of phonons can be observed in the simulations that leads to a retarded equilibration of the subsystems only after several picoseconds.

We investigate the coherent energy transfer between two quantum systems mediated by a quantum bus. In particular, we consider the energy-transfer process between two qubits and how it can be influenced by using a third qubit or photons in a resonant cavity as mediators. Inspecting different figures of merit and considering both on- and off-resonance configurations, we characterize the energy-transfer performances. We show that while the qubit-mediated transfer shows no advantages with respect to a direct-coupling case, the cavity-mediated one is progressively more and more efficient as function of the number of photons stored in the cavity that acts as a quantum bus. The speeding-up of the energy-transfer time, due to a quantum mediator, paves the way for new architecture designs in quantum technologies and energy-based quantum logics.

Quasielastic neutron scattering (QENS) and neutron powder diffraction of the complex transition metal hydrides ${\mathrm{Li}}_5{\mathrm{MoH}}_{11}$ and ${\mathrm{Li}}_6{\mathrm{NbH}}_{11}$ were measured in a temperature range of 10–300 K to study their structures and dynamics, especially the dynamics of the hydrogen atoms. These hydrides contain unusual ninefold H-coordinated complex ions (${\mathrm{MoH}}_9^{3-}$ or ${\mathrm{NbH}}_9^{4-}$) and hydride ions $\left({\mathrm{H}}^{-}\right)$. A QENS signal appeared >150 K due to the relaxation of H atoms. The intermediate scattering functions derived from the QENS spectra are well fitted by a stretched exponential function called the Kohlrausch-Williams-Watts functions with a small stretching exponent β ≈ 0.3–0.4, suggesting a wide relaxation time distribution. The $Q$ dependence of the elastic incoherent structure factor is reproduced by the rotational diffusion of $M{\mathrm{H}}_9$ ($M=\mathrm{Mo}$ or Nb) anions. The results are well supported by a van Hove analysis for the motion of H atoms obtained using first-principles molecular dynamics calculations. We conclude that the wide relaxation time distribution of the $M{\mathrm{H}}_9$ rotation is due to the positional disorder of the surrounding Li ions and a unique rotation with $M{\mathrm{H}}_9$ anion deformation (pseudorotation).