We study single ionization of helium by the impact of the ${\mathrm{H}}_2^{+}$ molecule which in the collision undergoes a transition from the ground state to its first dissociative state. We show that two types of interference effects arise in the spectra of the emitted electrons. One of them is due to the interaction of the atomic electron with the nuclei of the molecule. In the other one the interference is caused by the interaction of this electron with the electron of the molecule. Interference effects of the first type dominate at the low collision velocities due to the effective threshold for the electron-electron interactions.

The radiative electron capture of a target electron into the projectile continuum has been studied for the collision system ${\text{U}}^{88+}+{\text{N}}_2\to {\text{U}}^{88+}+{\left[{\text{N}}_2^{+}\right]}^{*}+e^{-}+\gamma$ at 90 MeV/$\mathrm{u}$. Using a magnetic electron spectrometer, the energy distribution of cusp electrons emitted under an angle of $0^{\circ }$ with respect to the projectile beam and with a velocity close to the projectile velocity has been measured in coincidence with the emitted photons under various observation angles. The experimental results provide a stringent test for the corresponding process in inverse kinematics, namely, the theory of electron-nucleus bremsstrahlung at the high-energy endpoint. For comparison this process is calculated using fully relativistic Dirac wave functions and using semirelativistic Sommerfeld-Maue wave functions.

We use the classical ensemble method to analyze the nonsequential double ionization of Ar atoms at a lower laser intensity. The correlated-electron momentum distribution is shown. The electron trajectories from recollision-induced ionization (RII) and from recollision-induced excitation with subsequent ionization (RESI) are studied separately. We find that the momentum of RESI electrons distributes uniformly in the four quadrants of the momentum distribution diagram, while that of RII electrons distributes mainly in the second and fourth quadrants of the momentum distribution diagram. The Coulomb repulsion between the two electrons is responsible for the anticorrelation emission in RII. The momentum distributions of the fast and the slow electrons observed by Liu et al. [Phys. Rev. Lett. 112, 013003 (2014)] are well reproduced in our simulations.

We theoretically investigate the evaporative cooling of cold rubidium atoms that are brought close to a solid surface. The dynamics of the atom cloud are described by coupling a dissipative Gross-Pitaevskii equation for the condensate with a quantum Boltzmann description of the thermal cloud (the Zaremba-Nikuni-Griffin method). We have also performed experiments to allow for a detailed comparison with this model and find that it can capture the key physics of this system provided the full collisional dynamics of the thermal cloud are included. In addition, we suggest how to optimize surface cooling to obtain the purest and largest condensates.

We present a theoretical analysis of spatial correlations in a one-dimensional driven-dissipative nonequilibrium condensate. Starting from a stochastic generalized Gross-Pitaevskii equation, we derive a noisy Kuramoto-Sivashinsky equation for the phase dynamics. For sufficiently strong interactions, the coherence decays exponentially in close analogy to the equilibrium Bose gas. When interactions are small on a scale set by the nonequilibrium condition, we find through numerical simulations a crossover between a Gaussian and exponential decay with peculiar scaling of the coherence length on the fluid density and noise strength.

Using a variational approach, we present the full solutions of the simplified one-dimensional two-fluid hydrodynamic equations for a unitary Fermi gas trapped in a highly elongated harmonic potential, which was recently derived by G. Bertaina and coworkers [Phys. Rev. Lett. 105, 150402 (2010)]. We calculate the discretized mode frequencies of first and second sound along the weak axial trapping potential, as a function of the temperature and the form of superfluid density. We show that the density fluctuations in second-sound modes, due to their coupling to first-sound modes, are large enough to be measured in current experimental setups such as that exploited by M. K. Tey et al. at the University of Innsbruck [Phys. Rev. Lett. 110, 055303 (2013)]. Owing to the sensitivity of second sounds on the form of superfluid density, the high precision of the measured second-sound frequencies may provide us a promising way to accurately determine the superfluid density of a unitary Fermi gas, which so far has remained elusive.

We report a gravity gradiometer composed of two atom interferometers which are simultaneously locked to the respective fringe center by feedback control of the Raman laser phase and magnetic field gradient. Compared with the conventional full-fringe recording method, this dual-fringe-locking strategy is capable of increasing the sampling rate and improving the sensitivity for gravity gradient measurements. Meanwhile, it retains the intrinsic advantage of rejecting common mode noises for gravity gradiometers. A short-term sensitivity of $670 \mathrm{E}/{\mathrm{Hz}}^{1/2}$ with a 0.25 Hz sampling rate is achieved in our gravity gradiometer. This approach may also find possible applications in other atom-interferometry-based experiments utilizing dual atomic clouds.

Traveling unidirectional localized edge states in optical honeycomb lattices are analytically constructed. They are found in honeycomb arrays of helical waveguides designed to induce a periodic pseudomagnetic field varying in the direction of propagation. Conditions on whether a given pseudofield supports a traveling edge mode are discussed; a special case of the pseudofields studied agrees with recent experiments. Interesting classes of dispersion relations are obtained. Envelopes of nonlinear edge modes are described by the classical one-dimensional nonlinear Schrödinger equation along the edge. Nonlinear states termed edge solitons are predicted analytically and are found numerically.

An experimentally feasible configuration of a prism coupler with an electromagnetically-induced-transparency (EIT) medium layer, e.g., a semiconductor-quantum-dot (SQD) medium, deposited upon its prism base is suggested for generating tunable surface-plasmon-polariton resonance. Such surface-plasmon-polariton resonance and optical excitation of a surface plasmon wave can be manipulated by switchable quantum interference among SQD multilevel transitions driven by two external control fields. When an incident probe field is coupled into a surface plasmon wave excitation mode, the surface-plasmon-polariton (SPP) resonance at the interface between the SQD medium layer and the substrate will arise, and the quantum-coherently controllable reflection spectrum of the probe field on the prism base can be achieved. In this process, destructive and constructive quantum interference (determined by the intensity ratio of the two external control fields) in the SQD multilevel system plays a key role for achieving the tunable reflection spectrum. The EIT-based surface-plasmon-polariton resonance presented here will have three characteristics (some of them would be attractive): (i) switchable quantum interference exhibited by surface plasmon wave excitation, (ii) quantum-coherently controllable surface plasmon polaritons by external optical fields, (iii) surface wave sensitive to dispersion of the SQD quantum coherent medium. Such an effect of controllable optical response based on the quantum-interference switchable surface-plasmon-polariton resonance in the EIT-prism coupler may find some potential applications in design of new photonic and quantum optical devices.

We investigate the effects of a low-frequency (acoustic) phonon bath on the dynamics of a quantum dot modeled as a cascade three-level system. We show that the phonon bath appears to the upper transition of the cascade system as a broadband reservoir of inverted rather than conventional harmonic oscillators. The action of the annihilation and creation operators of the inverted oscillator are interchanges relative to those of the usual harmonic oscillator that it serves as a linear amplifier to the system, and thereby gives rise to unusual features in the dynamics of the quantum dot. We find that the phonon bath, although being in a thermal state, affects the quantum dot as a correlated-type reservoir which results in the decay of the system to a correlated two-photon state with the population distribution no longer obeying a Boltzmann distribution. It is particularly interesting that even for a zero-temperature phonon reservoir, the steady state is a correlated state which, under appropriate conditions on the Rabi frequencies and the damping rates, can reduce to a strongly correlated pure state. It is shown that the two-photon correlations result in a significant squeezing and strong two-photon correlations in the radiation field emitted by the quantum dot. The presence of the correlations in the system is manifest in the presence of quantum beats in the time evolution of the populations and the radiation intensity. The effect of the ordinary spontaneous emission on the features induced by the phonon bath is also discussed.

We present an efficient and flexible method for solving the non-linear lasing equations of the steady-state ab initio laser theory. Our strategy is to solve the underlying system of partial differential equations directly, without the need of setting up a parametrized basis of constant flux states. We validate this approach in one-dimensional as well as in cylindrical systems, and demonstrate its scalability to full-vector three-dimensional calculations in photonic-crystal slabs. Our method paves the way for efficient and accurate simulations of microlasers which were previously inaccessible.

Some optomechanical systems can be transparent to a probe field when a strong driving field is applied. These systems can provide an optomechanical analog of electromagnetically induced transparency (EIT). We study the transmission of a probe field through a hybrid optomechanical system consisting of a cavity and a mechanical resonator with a two-level system (qubit). The qubit might be an intrinsic defect inside the mechanical resonator, a superconducting artificial atom, or another two-level system. The mechanical resonator is coupled to the cavity field via radiation pressure and to the qubit via the Jaynes-Cummings interaction. We find that the dressed two-level system and mechanical phonon can form two sets of three-level systems. Thus, there are two transparency windows in the discussed system. We interpret this effect as an optomechanical analog of two-color EIT (or double EIT). We demonstrate how to switch between one and two EIT windows by changing the transition frequency of the qubit. We show that the absorption and dispersion of the system are mainly affected by the qubit-phonon coupling strength and the transition frequency of the qubit.

A controlled-phase gate was demonstrated in superconducting Xmon transmon qubits with fidelity reaching 99.4%, relying on the adiabatic interaction between the $|11\rangle$ and $|02\rangle$ states. Here we explain the theoretical concepts behind this protocol, which achieves fast gate times with only ${\sigma }_z$ control of the Hamiltonian, based on a theory of nonlinear mapping of state errors to a power spectral density and use of optimal window functions. With a solution given in the Fourier basis, optimization is shown to be straightforward for practical cases of an arbitrary state change and finite bandwidth of control signals. We find that errors below ${10}^{-4}$ are readily achievable for realistic control wave forms.

The Wigner regime of a system of electrons in an external field is characterized by a low electron density and a high electron-interaction energy relative to the kinetic energy. The low-correlation regime is in turn described by a high electron density and an electron-interaction energy smaller than the kinetic energy. The Wigner regime of a nonuniform-electron-density system is investigated via quantal density functional theory (QDFT). Within QDFT, the contributions of electron correlations due to the Pauli exclusion principle, Coulomb repulsion, and correlation-kinetic effects are separately delineated and explicitly defined. The nonuniform-electron-density system studied is that of the Hooke's atom in the Wigner regime, for which the exact wave function is derived. As such, the results of the QDFT analysis are exact. It is observed that in comparison to the low-correlation case, not only is the electron-interaction energy greater than the kinetic energy as a fraction of the total energy, but so are its individual Hartree, Pauli, and Coulomb components. The ionization potential as a fraction of the total energy too is greater. But most significantly, in the Wigner regime, the correlation-kinetic energy as a fraction of both the electron-interaction and the total energy is substantially greater than in the low-correlation case. Hence, we propose that the Wigner regime now also be characterized by a high correlation-kinetic energy. The kinetic energy as a fraction of the total energy, however, is less than in the low-correlation case. This fact and the high correlation-kinetic energy value in the Wigner regime is explained by the concept of “quantal compression” of the kinetic energy density derived from QDFT. The corresponding results for the low-correlation case are in turn a consequence of a “quantal decompression” of the kinetic energy density. From the QDFT analysis, the exact values for the Kohn-Sham theory “exchange-correlation” and “correlation” energy functionals of the density and their respective functional derivatives are also obtained. These results ought to be of value in the construction and testing of approximate energy functionals valid for the Wigner regime.

The three-body dynamics of the ionization of the atomic hydrogen by 30-keV antiproton impact has been investigated by calculation of fully differential cross sections (FDCSs) using the classical-trajectory Monte Carlo (CTMC) method. The results of the calculations are compared with the predictions of quantum mechanical descriptions: The semiclassical time-dependent close-coupling theory; the fully quantal, time-independent close-coupling theory; and the continuum-distorted-wave-eikonal-initial-state model. In the analysis particular emphasis was put on the role of the nucleus-nucleus ($NN$) interaction played in the ionization process. For low-energy electron ejection the CTMC method predicts a large $NN$ interaction effect on FDCSs, in agreement with the quantum mechanical descriptions. By examining individual particle trajectories it was found that the relative motion between the electron and the nuclei is coupled very weakly with that between the nuclei, consequently the two motions can be treated independently. A simple procedure is presented by which the $NN$ interaction effect can be included in the calculations carried out without it.

We study theoretically the photoelectron angular distributions (PADs) from two-color two-photon near-threshold ionization of hydrogen and noble gas (He, Ne, and Ar) atoms by a combined action of femtosecond extreme ultraviolet (EUV) and near-infrared (IR) laser pulses. By using second-order time-dependent perturbation theory, we clarify how the two-photon ionization process depends on the EUV-IR pulse delay and how it is connected to the interplay between resonant and nonresonant ionization paths. Furthermore, by solving the time-dependent Schrödinger equation, we calculate the anisotropy parameters ${\beta }_2$ and ${\beta }_4$ as well as the amplitude ratio and relative phase between partial waves characterizing the PADs. We show that, in general, these parameters notably depend on the time delay between the EUV and IR pulses, except for He. This dependence is related to the varying relative role of resonant and nonresonant paths of photoionization. Our numerical results for H, He, Ne, and Ar show that the pulse-delay effect is more pronounced for $p$-shell ionization than for $s$-shell ionization.

We investigate the generation of Rydberg states of hydrogen atoms with intense laser pulses by solving the time-dependent Schrödinger equation and by means of classical-trajectory Monte Carlo simulations. Both linearly polarized multicycle pulses and pairs of optical half-cycle pulses are used. Comparisons between these methods show that both the Coulomb force and initial lateral momentum, which have effects on the $n$ distribution and $l$ distribution of the population of excited states, are important in the generation of Rydberg states.

We show that the presence of a soliton in a single-species condensate, at zero temperature, enhances the quantum depletion sufficiently enough to induce dynamical instability of the system. We also predict that for two-species condensates, two Goldstone modes emerge in the excitation spectrum at phase separation. Of these, one is due to the presence of the soliton. We use Hartree-Fock-Bogoliubov theory with Popov approximation to examine the mode evolution, and demonstrate that when the anomalous mode collides with a higher energy mode it renders the solitonic state oscillatory unstable. We also report a soliton-induced change in the topology of the density profiles of the two-species condensates at phase separation.

We theoretically investigate a scheme to enhance relative number squeezing and spin squeezing in a two-component Bose-Einstein condensate (BEC) by utilizing the inherent mean-field dynamics of the condensate. Due to the asymmetry in the scattering lengths, the two components exhibit large density oscillations where they spatially separate and recombine. The effective nonlinearity responsible for the squeezing is increased by up to 3 orders of magnitude when the two components spatially separate. We perform a multimode simulation of the system using the truncated Wigner method and show that this method can be used to create significant squeezing in systems where the effective nonlinearity would ordinarily be too small to produce any significant squeezing in sensible time frames, and we show that strong spatial dynamics resulting from large particle numbers aren't necessarily detrimental to generating squeezing. We develop a simplified semianalytic model that gives good agreement with our multimode simulation and will be useful for predicting squeezing in a range of different systems.

We investigate the spectral and temporal properties of light scattered near resonantly by a single quantum dot when the incident laser field is a frequency comb consisting of a superposition of monochromatic waves equidistant in frequency. Such fields encompass those generated by, e.g., a periodically pulsed laser. A general theoretical treatment for the calculation of first- and second-order correlation functions is given which takes account of spectral diffusion through a slowly varying detuning from resonance, permitting accurate comparison with experiments. We explore the two distinct regimes in which the frequency-comb separation is either larger or smaller than the radiative decay rate. We verify the validity of our calculations by a comparison with experimental data for the case of a bichromatic field and discuss the manifestation of phase coherence between the incoming field and the scattered single-photon wave packet.