Results of numerical simulations on a $\mathrm{Ni}$-like silver x-ray laser pumped by a single picosecond laser pulse are presented. Since the mechanisms responsible for the significant reduction in the pump energy are not well understood, the results of theoretical simulations with emphasis on the plasma kinetics and dynamics in a $\mathrm{Ni}$-like $\mathrm{Ag}$ x-ray laser are presented and referred to the experimental data. Special attention has been paid to the influence of the pump pulse shape and length on the gain and its duration. It was found that a low-level pulse pedestal being an integral part of the leading edge of the pump pulse is very beneficial to the pump energy reduction. The thermal cooling process has been identified as the mechanism strongly contributing to gain termination if a low-energy single-profile laser pulse with the width of a few picoseconds is used in the pump process.

High-order harmonic generation (HHG) is limited by the probability of recombination of the returning electron with the parent ion. We show here that the internuclear distance of simplest diatomic molecules, determining differently delocalized initial electronic states, can be used to enhance the HHG conversion efficiency exceeding the atomic one. We propose an experiment based on a double-pulse pump-drive scheme where a strong transient enhancement of the HHG yield is predicted.

We study the limits imposed by intrinsic fluctuations in system-control parameters on the ability of two-qubit (exchange) Hamiltonians to generate entanglement, starting from arbitrary initial states. We find three classes for Gaussian and Laplacian fluctuations. For the Ising and $XYZ$ models there are qualitatively distinct, sharp entanglement-generation transitions, while the class of Heisenberg, $XY$, and $XXZ$ Hamiltonians is capable of generating entanglement for any finite noise level. Our findings imply that exchange Hamiltonians are surprisingly robust in their ability to generate entanglement in the presence of noise, thus potentially reducing the need for quantum error correction.

Excitation of Faraday patterns in the weak confinement space of low-dimensional Bose-Einstein condensates (BEC) by a periodic modulation of the trap frequency, ${\Omega }_{\text{tight}}$, in the tight confinement space is shown. For slow modulation the low-dimensional dynamics of the BEC in the weak confinement space is described by a Gross-Pitaevskii equation with time modulated nonlinearity coefficient. For increasing modulation frequencies a noticeable reduction of the pattern formation threshold is observed near $2{\Omega }_{\text{tight}}$, which is related to the parametric excitation of the internal breathing mode in the tight confinement space. These predictions could be relevant for the experimental excitation of Faraday patterns in BEC.

We apply the Hermitian phase operator method (HPOM), as discussed by Pegg and Barnett [D. T. Pegg and S. M. Barnett, Phys. Rev. A 39, 1665 (1989)] to a two-well Bose-Hubbard model. Application of the HPOM formalism yields an approximate quantum phase model, or a Schrödinger-like differential equation for the phase variable $\theta$, which is a conjugate variable to the half of the number difference between the wells, $m=(N_1-N_2)∕2$. In the construction of the model we take care so that the Hermiticity of the original Bose-Hubbard model in number representation is inherited. We demonstrate that the quantum phase model supersedes two theoretical models suggested for the group of so-called “normal” states, for which $⟨\theta ⟩=⟨m⟩=0$: a non-Hermitian “exact quantum phase model” by Anglin, Drummond, and Smerzi [J. R. Anglin, P. Drummond, and A. Smerzi, Phys. Rev. A 64, 063605 (2001)], and the “phonon model” by Javanainen and Ivanov [J. Javanainen and M. Yu. Ivanov, Phys. Rev. A 60, 2351 (1999)]. We apply the HPOM to the so called “superfluid” states, for which $⟨m⟩=0$ and $⟨\theta ⟩=\pi$, and find their superfluid phonon model. We find that the superfluid phonon model has a threshold point $\mathcal{M}=\delta ∕\left(4gN\right)=1$, at which the model diverges. Here, $\delta$ is the tunneling rate between the wells and $g$ is the self-interaction, while $N$ is the total number of particles. The above threshold phonon model is quite accurate in the description of the number and phase statistics of the superfluid states. We show that at the threshold point HPOM itself can be exactly solved. However, the agreement between the exact solution and the HPOM solution is poor: $⟨m^2⟩$ close to the threshold assumes values $\sim N^2$, thus voiding the second order expansion the HPOM is based on, $⟨m^2⟩⪡N^2$.

A large class of non-Markovian quantum processes in open systems can be formulated through time-local master equations which are not in Lindblad form. It is shown that such processes can be embedded in a Markovian dynamics which involves a time-dependent Lindblad generator on an extended state space. If the state space of the open system is given by some Hilbert space $\mathcal{H}$, the extended state space is the triple Hilbert space $\mathcal{H}\otimes {\mathbb{C}}^3$ which is obtained by combining the open system with a three-state system. This embedding is used to derive an unraveling for non-Markovian time evolution by means of a stochastic process in the extended state space. The process is defined through a stochastic Schrödinger equation which generates genuine quantum trajectories for the state vector conditioned on a continuous monitoring of an environment. The construction leads to a continuous measurement interpretation for non-Markovian dynamics within the framework of the theory of quantum measurement.

We consider the Casimir-Polder interaction between two atoms, one in the ground state and the other in its excited state. The interaction is time dependent for this system, because of the dynamical self-dressing and the spontaneous decay of the excited atom. We calculate the dynamical Casimir-Polder potential between the two atoms using an effective Hamiltonian approach. The results obtained and their physical meaning are discussed and compared with previous results based on a time-independent approach, which uses a nonnormalizable dressed state for the excited atom.

We develop a scheme for quantum computation with neutral atoms, based on the concept of “marker” atoms, i.e., auxiliary atoms that can be efficiently transported in state-independent periodic external traps to operate quantum gates between physically distant qubits. This allows for relaxing a number of experimental constraints for quantum computation with neutral atoms in microscopic potential, including single-atom laser addressability. We discuss the advantages of this approach in a concrete physical scenario involving molecular interactions.

Fluorescence polarimetry has been used to determine the relative partial-wave Auger decay widths for transitions to states of the $\mathrm{Xe}\mathrm{II}$ $5p^{4}6p$ multiplet after photoexcitation of the ${\mathrm{Xe}}^{*}4d_{5∕2}^{-1}6p(J^{*}=1)$ resonance by linearly and circularly polarized synchrotron radiation. Combination with data on the angular distribution and spin polarization of the Auger electrons, providing information on the relative phases of the amplitudes, constitutes the complete experiment on the Auger decay. Multiconfiguration relativistic calculations of the amplitudes have been performed and compared to the measurements.

The decay in the hyperfine coherence of optically trapped atoms due to interactions with the environment, is only partly suppressed by “echo” spectroscopy [M. F. Andersen et al. Phys. Rev. Lett. 90, 023001 (2003)], primarily due to dynamical (time-dependent) dephasing mechanisms. We demonstrate here an improved pulse sequence, for which the decay of coherence is reduced by a factor of $2.5$ beyond the reduction offered by the “echo” spectroscopy. This reduction occurs when each dark period in-between pulses is shorter than the time scale over which substantial dephasing develops. The coherence time is then limited by mixing to other vibrational levels in the trap and, to a lesser extent, the lifetime of the internal states of the atoms.

The possibility of generating vortices during matter-wave propagation through microstructures is examined. Vortices can arise solely due to wave interference in low-density ultracold atom clouds, and do not require any atom–atom (nonlinear) interactions. The properties of these “interference vortices” are understood from a simple two-mode model in a straight waveguide. This model is then applied to vortex creation in a circular bend since a circular waveguide bend is one of the simplest atom optical elements that can induce mode excitations. Time-independent and time-dependent analyses are used to investigate vortex creation and dynamics in these systems.

We report a joint experimental and theoretical study of circular dichroism in the valence photoelectron spectra of a free chiral molecule. The circular dichroism in photoelectron spectroscopy is measured at the magic angle for various valence states of $R(+)$ and $S(-)$ methyl-oxirane enantiomers in the vapor phase. The maximum dichroism measured is about $5\times {10}^{-2}$. Experimental and theoretical results are in agreement. The ab initio calculation employs a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian.

We connect the theory of a Lyapunov functional for an open system with the thermophysical concept of the relative entropy or equivalently with the entropy production rate. In order to do this we consider the environment of the open system as a thermal reservoir, which introduces the temperature as a thermophysical quantity. The general theory of the Lyapunov functional of an open system constructs a metric operator which contains an element of ambiguity in its scaling. This ambiguity can be removed by assuming a thermostatistic environment which fixes the scaling of the metric operator in a unique manner. The Lyapunov functional thus acquires a thermophysical meaning and can be connected with a relative entropy. In this case the metric operator is related to the inverse of the stationary density operator of the system. We illustrate the theory on a two-level atomic system and a degenerate three-level atom which are exposed to a thermal electromagnetic environment.

A new implementation of quantum gates by adiabatic passage with an optical cavity is proposed. This implementation allows one to perform not only elementary gates, such as one-qubit gates and a controlled-NOT gate, but also multiqubit controlled unitary gates. Some quantum gates are numerically simulated. From the simulation results, it is concluded that this implementation of the three-qubit controlled gates is more efficient than decomposing into the elementary gates.

We report the results of variational calculations of elastic electron scattering by tetrafluoroethene $\left({\mathrm{C}}_2{\mathrm{F}}_4\right)$ with incident electron energies ranging from $0.5\text{to}20\mathrm{eV}$, using the complex Kohn method and effective core potentials. These are the first fully ab initio calculations to reproduce experimental angular differential cross sections at energies below $10\mathrm{eV}$. Low-energy electron scattering by ${\mathrm{C}}_2{\mathrm{F}}_4$ is sensitive to the inclusion of electronic correlation and target-distortion effects. We therefore present results that describe the dynamic polarization of the target by the incident electron. The calculated cross sections are compared with recent experimental measurements.

The screening of a ${\mathrm{He}}^{+}$ ion embedded in a paramagnetic electron gas is studied using density functional theory within the local spin density approximation. We calculate the induced electron density and the induced density of states for each spin orientation, parallel and antiparallel to that of the electron bound to the ${\mathrm{He}}^{+}$ ion. Our results show that the screening is preferably due to parallel spin electrons, especially for low electron densities of the medium. In a second step, the rates for Auger neutralization of a ${\mathrm{He}}^{+}$ ion in an electron gas are calculated, paying special attention to their dependence on the spin of the electron excited in the Auger process. The results obtained are used to interpret experiments in which the spin polarization of the emitted yield is measured when a ${\mathrm{He}}^{+}$ projectile is neutralized in front of a metal surface.

We perform accurate calculations of the dependence of transition frequencies in two-valence-electron atoms and ions on a variation of the fine-structure constant, $\alpha =e^2∕\hslash c$. The relativistic Hartree-Fock method is used with many-body perturbation theory and configuration interaction methods to calculate transition frequencies. The results are to be used in atomic-clock-type laboratory experiments designed to test whether $\alpha$ varies in time.

We derive a collection of separability conditions for bipartite systems of dimension $d\times d$ which is based on the entropic version of the uncertainty relations. A detailed analysis of the two-qubit case is given by comparing the new separability conditions with existing criteria.

Interferometry of single particles with internal degrees of freedom is investigated. We discuss the interference patterns obtained when an internal state evolution device is inserted into one or both the paths of the interferometer. The interference pattern obtained is not uniquely determined by the completely positive maps (CPMs) that describe how the devices evolve the internal state of a particle. By using the concept of gluing of CPMs, we investigate the structure of all possible interference patterns obtainable for given trace preserving internal state CPMs. We discuss what can be inferred about the gluing, given a sufficiently rich set of interference experiments. It is shown that the standard interferometric setup is limited in its abilities to distinguish different gluings. A generalized interferometric setup is introduced with the capacity to distinguish all gluings. We also connect to another approach using the well known fact that channels can be realized using a joint unitary evolution of the system and an ancillary system. We deduce the set of all such unitary “representations” and relate the structure of this set to gluings and interference phenomena.

A systematic study of the decay rates of antiprotonic helium (${\overline{p}}^4{\mathrm{He}}^{+}$ and ${\overline{p}}^3{\mathrm{He}}^{+}$) at CERN Antiproton Decelerator has been made by a laser spectroscopic method. The decay rates of some of its short-lived states, namely those for which the Auger rates ${\gamma }_A$ are much larger than their radiative decay rates $({\gamma }_{\mathrm{rad}}\sim 1\mu s^{-1})$, were determined from the time distributions of the antiproton annihilation signals induced by laser beams, and the widths of the atomic resonance lines. The magnitude of the decay rates, especially their relation with the transition multipolarity, is discussed and compared with theoretical calculations.