Nonreciprocal transport, where the left-to-right-flowing current differs from the right-to-left-flowing one, is an unexpected phenomenon in bulk crystals. BiTeBr is a noncentrosymmetric material, with a giant Rashba spin-orbit coupling which presents this unusual effect when placed in an in-plane magnetic field. It has been shown that this effect depends strongly on the carrier density; however, in situ tuning has not yet been demonstrated. We developed a method where thin BiTeBr flakes are gate tuned via ionic-liquid gating through a thin protective hexagonal boron nitride layer. Tuning the carrier density allows a more than $400\%$ variation of the nonreciprocal response in our sample. Our study demonstrates how a few-atomic-layer-thick van der Waals protection layer allows ionic gating of chemically sensitive, exotic nanocrystals.

Optical Kerr resonators support a variety of stable nonlinear phenomena in a simple and compact design. The generation of ultrashort pulses and frequency combs has been shown to benefit several applications, including spectroscopy and telecommunications. The most common anomalous dispersion Kerr resonators can be accurately described by a well-studied mean-field Lugiato-Lefever equation (LLE). Recently observed highly chirped pulses in normal dispersion resonators with a spectral filter, however, cannot. Here, we examine the LLE in the normal dispersion regime modified with a Gaussian spectral filter (LLE-F). In addition to solutions associated with the LLE, we find stable highly chirped pulses. Solutions are strongly dependent on the filter bandwidth. Because of the large changes per round trip, the validity of the LLE-F fails over a large range of experimentally relevant parameters. While the mean-field approach leads to accurate predictions with respect to the nonlinearity coefficient and the dispersion, the dependence of drive power on loss deviates significantly from an experimentally accurate model, which leads to opportunities for Kerr resonators including frequency comb generation from low-Q-factor cavities.

Variational quantum algorithms (VQAs) that estimate values of widely used physical quantities such as the rank, the quantum entropies, the Bures fidelity, and the quantum Fisher information of mixed quantum states are developed. In addition, variations of these VQAs are also adapted to perform other useful functions such as quantum state learning and approximate fractional inverses. The common theme shared by the proposed algorithms is that their cost functions are all based on minimizing the quantum purity of a quantum state. Strategies to mitigate or avoid the problem of exponentially vanishing cost function gradients are also discussed.

Two-photon absorption (TPA) is of fundamental importance in super-resolution imaging and spectroscopy. Its nonlinear character allows for the prospect of using quantum resources, such as entanglement, to improve measurement precision or to gain new information on, e.g., ultrafast molecular dynamics. Here, we establish the metrological properties of nonclassical squeezed light sources for precision measurements of TPA cross sections. We find that the Cramér-Rao bound does not provide a fundamental limit for the precision achievable with squeezed states in the limit of very small cross sections. Considering the most relevant measurement strategies—namely, photon-counting and quadrature measurements—we determine the quantum advantage provided by squeezed states as compared to coherent states. We find that squeezed states outperform the precision achievable by coherent states when performing quadrature measurements, which provide improved scaling of the Fisher information with respect to the mean photon number $\sim n^4$. Due to the interplay of the incoherent nature and the nonlinearity of the TPA process, unusual scaling can also be obtained with coherent states, which feature an $\sim n^3$ scaling in both quadrature and photon-counting measurements.

Magnetic skyrmions are topological spin textures that exhibit classical or quantum quasiparticle behavior. A substantial amount of research has occurred in this field, both because of their unique electromagnetic properties and potential application for future nonvolatile memory storage applications, as well as fundamental questions on their topology and unique magnetic phases. Here, we investigate the fluctuation properties of a magnetic Fe/Gd skyrmion lattice, using short-pulsed x rays. We first measure spontaneous fluctuations of the skyrmion lattice phase and find an inherent, collective mode showing an underdamped oscillation with a relaxation of a couple of nanoseconds. Further observations track the response towards the continuous phase transition and a “critical-like” slowing down of fluctuations is observed well before the critical point. These results suggest that the skyrmion lattice phase never fully freezes into a static crystal. This constant state of fluctuation indicates that the physics of topological magnetic phases may have more in common with high-temperature superconductors with disorder.

We present a method to identify distinct tunneling modes in a tunable superconducting tunnel junction composed of a superconducting tip and a sample in a scanning tunneling microscope. Combining the relative decay constant of tunneling current extracted from $I-V-z$ spectroscopy with its statistical analysis over the atomic disorders in the sample surface, we identified the crossover of dominant tunneling modes between single charge tunneling, Andreev reflection (AR), and Josephson tunneling with respect to the bias voltage at a measurement temperature nearly half of the critical temperature. The method enables one to determine the specific tunneling regime independently of the spectral shapes and to reveal intrinsic modulation of AR and Josephson current by disorder that will be crucial for superconducting quantum information processing.

The beyond-mean-field Lee-Huang-Yang (LHY) correction is ubiquitous in dilute ultracold quantum gases. However, its effects are often elusive due to the typically much larger influence of the mean-field (MF) energy. In this work, we study an ultracold mixture of ${}^{23}\mathrm{Na}$ and ${}^{87}\mathrm{Rb}$ with tunable attractive interspecies interactions. The LHY effects manifest in the formation of self-bound quantum liquid droplets and the expansion dynamics of the gas-phase sample. A liquid-to-gas-phase diagram is obtained by measuring the critical atom numbers below which the self-bound behavior disappears. In stark contrast to trapped gas-phase condensates, the gas-phase mixture formed following the liquid-to-gas-phase transition shows an anomalous expansion featuring a larger release energy for increasing MF attractions.

A scheme is proposed for generating a polarized positron beam via multiphoton Breit-Wheeler process during the collision of a 10-GeV-level, 20-pC seeding electron beam with a second 10-GeV, multi-nC driving electron beam. The high-density driving beam with the length and transverse size less than 1 $\mu \mathrm{m}$ provides a strong self-generated field. The field experienced by the seeding beam can be unipolar by incorporating a proper impact parameter, which is crucial to realizing the positron polarization. We employ semiclassical Monte Carlo simulations to calculate the spin- and polarization-resolved photon emission and electron-positron pair production in the locally constant-field approximation. The simulation results show that a highly polarized positron beam with a polarization degree above $40\%$ can be generated in several femtoseconds, which is robust against the beam parameters. The positron polarization degree can be further increased to $60\%$ provided the seeding beam is initially polarized.

We generalize solid-state tight-binding techniques for the spectral analysis of large superconducting circuits. We find that tight-binding states can be better suited for approximating the low-energy excitations than charge basis states, as illustrated for the interesting example of the current-mirror circuit. The use of tight binding can dramatically lower the Hilbert-space dimension required for convergence to the true spectrum, and allows for the accurate simulation of larger circuits that are out of reach of charge basis diagonalization.

Spin waves propagating through a stripe domain structure and reservoir computing with their spin dynamics have been numerically studied, focusing on the relation between physical phenomena and computing capabilities. Our system utilizes a spin-wave-based device that has a continuous magnetic garnet film and one-input/72-output electrodes on top. To control spatially distributed spin dynamics, a stripe magnetic domain structure and amplitude-modulated triangular input waves were used. The spatially arranged electrodes detected spin vector outputs with various nonlinear characteristics that were leveraged for reservoir computing. By moderately suppressing nonlinear phenomena, our system achieves $100\%$ prediction accuracy in temporal exclusive-OR problems with a delay step up to 5. At the same time, it shows perfect inference in delay tasks with a delay step more than 7 and its memory capacity has a maximum value of 21. This study demonstrated that our spin-wave-based reservoir computing has a high potential for edge-computing applications and also can offer a rich opportunity for further understanding the underlying nonlinear physics.

We study the effect of strong disorder on topology and entanglement in quench dynamics. Although disorder-induced topological phases have been well studied in equilibrium, the disorder-induced topology in quench dynamics has not been explored. In this paper, we predict a disorder-induced topology of postquench states characterized by the quantized dynamical Chern number and the crossings in the entanglement spectrum in $(1+1)$ dimensions. The dynamical Chern number undergoes transitions from zero to unity and back to zero when increasing the disorder strength. The boundaries between different dynamical Chern numbers are determined by delocalized critical points in the postquench Hamiltonian with the strong disorder. An experimental realization in quantum walks is discussed.