We have measured fully differential cross sections (FDCS) for ionization in $75\text{−}\mathrm{keV}p+{\mathrm{H}}_2$ collisions for ejected electron speeds close to the projectile speed. The data were analyzed in dependence on both the electron emission angle and the projectile scattering angle. Pronounced postcollisional effects between the projectile and the ejected electrons were observed. Significant differences between experiment and theory and between two conceptually very similar theoretical models were found. This shows that in the region of electron-projectile velocity-matching the FDCS is very sensitive to the details of the underlying few-body dynamics.

We investigate the propagation of ultrashort laser pulses in atomic rubidium vapor. The pulses are intensive enough to ionize the atoms and are directly resonant with the 780-nm $D_2$ line. We derive a relatively simple theory for computing the nonlinear optical response of atoms and investigate the competing effects of strong resonant nonlinearity and ionization in the medium using computer simulations. A nonlinear self-channeling of pulse energy is found to produce a continuous plasma channel with complete ionization. We evaluate the length, width, and homogeneity of the resulting plasma channel for various values of pulse energy and initial focusing to identify regimes optimal for applications in plasma-wave accelerator devices such as that being built by the AWAKE collaboration at CERN. Similarities and differences with laser pulse filamentation in atmospheric gases are discussed.

Supersolids are theoretically predicted quantum states that break the continuous rotational and translational symmetries of liquids while preserving superfluid transport properties. Over the last decade, much progress has been made in understanding and characterizing supersolid phases through numerical simulations for specific interaction potentials. The formulation of an analytically tractable framework for generic interactions still poses theoretical challenges. By going beyond the usually considered quadratic truncations, we derive a systematic higher-order generalization of the Gross-Pitaevskii mean-field model in conceptual similarity with the Swift-Hohenberg theory of pattern formation. We demonstrate the tractability of this broadly applicable approach by determining the ground-state phase diagram and the dispersion relations for the supersolid lattice vibrations in terms of the potential parameters. Our analytical predictions agree well with numerical results from direct hydrodynamic simulations and earlier quantum Monte Carlo studies. The underlying framework is universal and can be extended to anisotropic pair potentials with a complex Fourier-space structure.

Nonthermal fixed points in the evolution of a quantum many-body system quenched far out of equilibrium manifest themselves in a scaling evolution of correlations in space and time. We develop a low-energy effective theory of nonthermal fixed points in a bosonic quantum many-body system by integrating out long-wavelength density fluctuations. The system consists of $N$ distinguishable spatially uniform Bose gases with $\text{U}\left(N\right)$-symmetric interactions. The effective theory describes interacting Goldstone modes of the total and relative-phase excitations. It is similar in character to the nonlinear Luttinger-liquid description of low-energy phonons in a single dilute Bose gas, with the markable difference of a universal nonlocal coupling function depending, in the large-$N$ limit, only on momentum, single-particle mass, and density of the gas. Our theory provides a perturbative description of the nonthermal fixed point, technically easy to apply to experimentally relevant cases with a small number of fields $N$. Numerical results for $N=3$ allow us to characterize the analytical form of the scaling function and confirm the analytically predicted scaling exponents. The predicted and observed exponentially suppressed coherence at short distances takes the form of that of a quasicondensate in low-dimensional equilibrium systems. The fixed point which is dominated by the relative phases is found to be Gaussian, while a non-Gaussian fixed point is anticipated to require scaling evolution with a distinctly lower power of time.

High-dimensional quantum entangled states are a highly desired resource for their extended possibilities in quantum information processing. In this paper, we introduce on-chip engineering of a variety of path-entangled states based on the simultaneous enabling of spontaneous parametric down-conversion and efficient coupling between waveguides in a quadratic coupled-waveguide system. By varying the properties of pump beams, the phase-matching conditions, and the coupling coefficients in a three-coupled-waveguide system, it is possible to generate the deterministic and robust three-path biphoton entangled states without any postprocessing, as well as to transform the output photons among different types of path entanglement. This integrated implementation of the entanglement source will become an important building block for practical applications of on-chip quantum technologies.

Nonequilibrium quantum thermometry is a central topic for both fundamental and technological purposes. It shows advantages against its equilibrium counterpart by reducing the uncertainty associated to the temperature of the bath, and a negligible invasiveness. Here, we analyze how quantum features influence the dynamical speed of single- and two-qubit nonequilibrium thermometers interacting with a bosonic thermal bath. Our investigations exploit Riemannian geometric tools to show that the Riemannian speed of the nonequilibrium thermometer is only indirectly influenced by its quantum features, thus highlighting how the quantum fingerprint seems to be concealed by the classical aspects of thermalization dynamics.

The issue of whether non-Markovianity (NM) could be considered as a resource in quantum information has been a subject of intense debate in recent years. Recently, a simple mechanism was proposed in which one of the main features of NM, the backflow of information from the environment to the system, represents a fundamental and quantifiable resource for generating entanglement within an open quantum system coupled to a finite and small environment [N. Mirkin, P. Poggi, and D. Wisniacki, Phys. Rev. A 99, 020301(R) (2019)]. In this work, we extend the validity of this resource mechanism by studying a completely different and more general scheme where the system is coupled to an infinite structured reservoir. Under both setups, we show that the degree of NM univocally determines the optimal degree of entanglement reachable by controlling the open system. This result strongly suggests the universality of a quantitative relation between entanglement and NM by using quantum optimal control.

We find a simple solution to the problem of probe laser light shifts in two-photon optical atomic clocks. We show that there exists a magic polarization at which the light shifts of the two atomic states involved in the clock transition are identical. We calculate the differential polarizability as a function of laser polarization for two-photon optical clocks based on neutral calcium and strontium, estimate the magic polarization angle for these clocks, and determine the extent to which probe laser light shifts can be suppressed. We show that the light shift and the two-photon excitation rate can be independently controlled using the probe laser polarization.

Most investigations of rare-earth ions in solids for quantum information have used crystals where the rare-earth ion is a dopant. Here, we analyze the conversion of quantum information from microwave photons to optical frequencies using crystals where the rare-earth ions, rather than being dopants, are part of the host crystal. These concentrated crystals are attractive for frequency conversion because of their large ion densities and small linewidths. We show that conversion with both high efficiency and large bandwidth is possible in these crystals. In fact, the collective coupling between the rare-earth ions and the optical and microwave cavities is large enough that the limitation on the bandwidth of the devices will instead be the spacing between magnon modes in the crystal.

We study a two-state non-Hermitian waveguide system that carries an exceptional point (EP). It is commonly believed that dynamically encircling an EP exhibits a chiral behavior when the starting point of the loop lies in the branch cut with eigenmodes being symmetric and antisymmetric modes. We show here that such statement is conditional; i.e., the dynamics can in fact be nonchiral for specially designed loops with the starting point in the branch cut. In particular, we find that for two homotopic loops (i.e., loops that can be transformed continuously from one to another without crossing any EP), the outcomes can be completely different even if the two loops share the same starting state, enclose the same EP, and encircle the EP in the same direction. Our findings greatly enrich the understanding of the physics in dynamical processes of EP encircling in non-Hermitian systems.

The geometric spin Hall effect of light (GSHEL), similar to the spin Hall effect of light, is also a spin-dependent shift in the centroid of light intensity (energy flux), but it is a purely geometric effect and independent of the light-matter interaction. In this paper, we discuss the GSHEL with respect to momentum flux instead of energy flux. Interestingly, for the symmetric energy-momentum (E-M) tensor, its centroid shift of momentum flux is double that of the energy flux; however, for the canonical E-M tensor, its centroid shift of momentum flux agrees with that of the energy flux. Furthermore, when considering the effect produced by orbital angular momentum, for these two conventional E-M tensors the centroid shifts of the momentum fluxes are twice those of the energy fluxes. To tell which E-M tensor of the electromagnetic field would be more “correct,” we propose a possible experimental scheme to test the GSHEL of momentum flux through the mechanical effect of light.

Manipulation of qudits in optical tables is a difficult and nonscalable task. The use of integrated optical circuits opens new possibilities for the generation, manipulation, and characterization of high dimensional states besides the ease of transmission of these states through an optical fiber. In this work we propose photonic circuits to perform minimum quantum state tomography of path qudits and show how to determine all the constituents parameters of these circuits (beam splitters and phase shifters). Our strategies were based on the symmetries of the involved POVMs (positive operator-valued measures) suggested for minimum tomography and allowed us to obtain interferometers smaller than those obtained by other already known methods. The calculations of the transmittances and reflectivities of the beam splitters were made using the definition of probability operators in extended Hilbert spaces and the application of Naimark's theorem. The employment of equidistant states for the definition of the POVM elements allowed us to develop a recipe applicable to the tomography of qudits of any dimension, generalizing our scheme.

Quantum coherence was recently formalized as a physical resource to measure the strength of superposition. Based on the resource theory, we present a systematic framework that connects a coherence measure to the security of quantum key distribution. By considering a generic entanglement-based key distribution protocol under the collective attack scenario, we show that the key rate can be quantified by the coherence of the shared bipartite states. This framework allows us to derive the key rate of the BB84 and six-state protocols. By utilizing fine-grained parameters, we also derive the improved key rates of both protocols within the coherence-based framework. Furthermore, we apply it to a practical issue, detection efficiency mismatch, and obtain an improved result. In conclusion, this framework demonstrates the interplay among coherence, entanglement, and quantum key distribution at a fundamental level.

Phase-randomized optical homodyne detection is a well-known technique for performing quantum state tomography. So far, it has been mainly considered a sophisticated tool for laboratory experiments but unsuitable for practical applications. In this work, we change the perspective and employ this technique to set up a practical continuous-variable quantum random number generator. We exploit a phase-randomized local oscillator realized with a gain-switched laser to bound the min-entropy and extract true randomness from a completely uncharacterized input, potentially controlled by a malicious adversary. Our proof-of-principle implementation achieves an equivalent rate of 270 Mbit/s. In contrast to other source-device-independent quantum random number generators, the one presented herein does not require additional active optical components, thus representing a viable solution for future compact, modulator-free, certified generators of randomness.

We report an angle-resolved electron-energy-loss spectroscopy study on the valence excitations of $\mathrm{S}{\mathrm{F}}_6$. Momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles of the low-lying valence excitations have been derived from electron-energy-loss spectra measured at an incident electron energy of 3 keV. We have also performed theoretical calculations of the GOS profiles involving the influence of molecular vibration. Comparisons between experiment and theory have revealed that coupling between electronic states through molecular vibration plays a significant role in the $1{}^{1}T_{1g}$ dipole-forbidden transition.

We theoretically study high-harmonic generation (HHG) from transition-metal elements Mn and ${\mathrm{Mn}}^{+}$ using full-dimensional, all-electron, first-principles simulations. The HHG spectra calculated with the time-dependent complete-active-space self-consistent-field (TD-CASSCF) and occupation-restricted multiple-active-space (TD-ORMAS) methods exhibit a prominent peak at $\sim 50$ eV, successfully reproducing resonant enhancement observed in previous experiments [Opt. Express 20, 25239 (2012)]. Artificially freezing $3p$ orbitals in simulations results in its disappearance, which shows the essential role played by $3p$ electrons in the resonant harmonics (RH). Further transition-resolved analysis unambiguously identifies constructively interfering $3p\text{−}3d$ ($m=0,\pm 1$) giant resonance transitions as the origin of the RH, as also implied by its position in the spectra. Time-frequency analysis indicates that the recolliding electron combines with the parent ion to form the upper state of the transitions. In addition, this study shows that the TD-CASSCF and TD-ORMAS methods can be applied to open-shell atoms with many unpaired inner electrons.

We improve the efficiency of sawtooth-wave adiabatic-passage cooling for strontium atoms in three dimensions and combine it with standard narrow-line laser cooling. With this technique, we create strontium magneto-optical traps with $6\times {10}^7$ bosonic ${}^{88}\mathrm{Sr}$ ($1\times {10}^7$ fermionic ${}^{87}\mathrm{Sr}$) atoms at phase-space densities of $2\times {10}^{-3}$ ($1.4\times {10}^{-4}$). Our method is simple to implement and is faster and more robust than traditional cooling methods.

Recent experimental and theoretical results show that weakly interacting atomic Bose-Bose mixtures with attractive interspecies interaction are stabilized by beyond-mean-field effects. Here we consider the peculiar properties of these systems in a strictly one-dimensional configuration, taking also into account the nontrivial role of spin-orbit and Rabi couplings. We show that when the value of inter- and intraspecies interaction strengths are such that mean-field contributions to the energy cancel, a self-bound bright soliton fully governed by quantum fluctuations exists. We derive the phase diagram of the phase transition between a single-peak soliton and a multipeak (striped) soliton, produced by the interplay between spin-orbit, Rabi couplings and beyond-mean-field effects, which also affect the breathing mode frequency of the atomic cloud. Finally, we prove that a phase imprinting of the single-peak soliton leads to a self-confined propagating solitary wave even in the presence of spin-orbit coupling.