Absolute photoionization cross sections of ${\mathrm{Xe}}^{3+}$ to ${\mathrm{Xe}}^{6+}$ ions have been measured over an extended photon energy range from the first ionization threshold to $160\mathrm{eV}$. Single and double photoionization cross sections have been obtained. In the case of ${\mathrm{Xe}}^{3+}$ and ${\mathrm{Xe}}^{6+}$ ions, two completely independent setups, both based on the merged beam technique, have been used. Multiconfiguration Dirac-Fock calculations were performed to interpret the spectra in the region of $4d$ electron excitations. The partition of the oscillator strength between the discrete photoexcitation and the direct photoionization channels in the $4d$ subshell has allowed us to follow precisely the collapse of the $4f$ wave function for these ions of the isonuclear series of Xe.

We study the cross section for inelastic collisions of atoms and small molecules under the influence of highly relativistic charged ions as they occur in storage rings and accelerators. Starting from fully time-resolved calculations of the electronic dynamics, we study a set of complementing approximations, a sudden approximation for the extremely fast collision at low impact parameter combined with time-dependent perturbation theory for the weak and slow collisions at large impact parameters. This leads eventually to a simple, yet reliable, estimate in terms of the ion-hydrogen cross section and the variance of the dipole moment for the actual system. We discuss, in particular, the systems H, He, C, N, O, Ne, ${\mathrm{H}}_2$, and ${\mathrm{N}}_2$.

We describe the potential energy landscapes of elemental ${\mathrm{S}}_8$, ${\mathrm{Se}}_8$, and ${\mathrm{Te}}_8$ clusters using disconnectivity graphs. Inherent structures include both ring and chain configurations, with rings especially dominant in ${\mathrm{Se}}_8$. We also map the potential energy landscapes of heterogeneous ${\mathrm{Se}}_n(\mathrm{S},\mathrm{Te}{)}_{8-n}$ clusters, which offer insights into the structure of heterogeneous chalcogen glasses.

We have employed an electromagnetically induced transparency resonance to demonstrate a technique to perform cold-atom velocimetry. It relies on the fact that the linewidth of this coherent resonance presents a residual Doppler broadening which is very sensitive to the angle between the coupling and probe beams. The method is demonstrated using a degenerate two-level system of laser-cooled cesium atoms.

We present theoretical predictions for the equation of state of a harmonically trapped Fermi gas in the unitary limit. Our calculations compare Monte Carlo results with the equation of state of a uniform gas using three distinct perturbation schemes. We show that in experiments the temperature can be usefully calibrated by making use of the entropy, which is invariant during an adiabatic conversion into the weakly interacting limit of molecular BEC. We predict the entropy dependence of the equation of state.

We propose a method for efficient storage and recall of arbitrary nonstationary light fields, such as, for instance, single photon time-bin qubits or intense fields, in optically dense atomic ensembles. Our approach to quantum memory is based on controlled, reversible, inhomogeneous broadening and relies on a hidden time-reversal symmetry of the optical Bloch equations describing the propagation of the light field. We briefly discuss experimental realizations of our proposal.

The existence of long-lived (metastable) molecular hydrogen anions $\mathrm{H}_2{}^{-}$, $\mathrm{D}_2{}^{-}$, $\mathrm{H}_3{}^{-}$, and $\mathrm{D}_3{}^{-}$ is demonstrated. These anion species were produced by sputtering of $\mathrm{Ti}{\mathrm{D}}_2$ or $\mathrm{Hf}{\mathrm{H}}_2$ targets with ${\mathrm{Cs}}^{+}$ ions and were identified by secondary-ion mass spectrometry (SIMS). From the respective flight times through the spectrometer, minimum lifetimes for $\mathrm{H}_2{}^{-}$ and $\mathrm{D}_2{}^{-}$ of a few microseconds can be inferred. Correspondingly longer lifetimes can be expected for the triatomic hydrogen molecules. From the SIMS intensities, the following apparent abundance ratios are derived: $\mathrm{H}_2{}^{-}∕{\mathrm{H}}^{-}=3.5\times {10}^{-6}$, $\mathrm{H}_3{}^{-}∕{\mathrm{H}}^{-}=1.7\times {10}^{-8}$, $\mathrm{D}_2{}^{-}∕{\mathrm{D}}^{-}=3.1\times {10}^{-5}$, and $\mathrm{D}_3{}^{-}∕{\mathrm{D}}^{-}=7\times {10}^{-9}$. The kinetic energy spectra of $\mathrm{D}_2{}^{-}$ and ${\mathrm{D}}^{-}$ demonstrate that these hydrogen anions are indeed formed by sputtering at the sample surface. According to recent theoretical calculations it is proposed that in the emission event molecular hydrogen species with high rotational and vibrational excitation are formed that are stable on the time scale of the experiment.

Based on space-time-resolved solutions to relativistic quantum field theory we illustrate interpretational difficulties in associating field-theoretical quantities with properties of particles. These difficulties are related to the fact that the definition of the spatial probability density of particles depends on the choice of the Hilbert subspace on which the field operator is projected. We illustrate these problems by analyzing pair-production probabilities and spatial densities for the electron-positron dynamics associated with a spatially localized subcritical potential that is turned on and off in time.

We study the dynamics of entanglement for a one-dimensional spin system, where spins are coupled through nearest-neighbor exchange interaction and subject to different external magnetic fields. First we examine the system size effect on the entanglement with three different external magnetic fields changing with time $t$: an exponential function $e^{[-Kt]}$ and two periodic $\sin \left[Kt\right]$ and $\cos \left[Kt\right]$ functions, where $K$ is a control parameter. We have found that the entanglement fluctuates shortly after a disturbance by an external magnetic field when the system size is small. For larger system size, the entanglement reaches a stable state for a long time before it fluctuates. However, this fluctuation of entanglement disappears when the system size goes to infinity. We also show that in a periodic external magnetic field, the nearest-neighbor entanglement displays a periodic structure with a period related to that of the magnetic field. Moreover, changing the direction of the magnetic field will destroy the concurrence in the system.

Known quantum pure states of a qudit can be remotely prepared onto a group of particles of qubits exactly or probabilistically with the aid of two-level Einstein–Podolsky–Rosen states. We present a protocol for such kind of remote state preparation. We are mainly focused on the remote preparation of the ensembles of equatorial states and those of states in real Hilbert space. In particular, a kind of states of qudits in real Hilbert space have been shown to be remotely prepared in faith without the limitation of the input space dimension.

Measurements and calculations of a contribution of the nondipole terms in the angular distribution of photoelectrons from the C $K$ shell of randomly oriented CO molecules are reported. In two sets of measurements, the angular distribution in the plane containing the photon polarization and the photon momentum vectors of linearly polarized radiation and the full three-dimensional photoelectron momentum distribution after absorption of circularly polarized light have been measured. Calculations have been performed in the relaxed core Hartree-Fock approximation with a fractional charge. Both theory and experiment show that the nondipole terms are very small in the photon energy region from the ionization threshold of the $K$ shell up to about $70\mathrm{eV}$ above it.

When a frequency of the circularly polarized laser field approaches zero the above threshold ionization rate should approach the well-known static-field limit of tunneling ionization. In the high-intensity limit of the laser field the Keldysh-Faisal-Reiss (KFR) theory is expected to be valid. For the ground state of a hydrogen atom we study various forms of the KFR theory when both conditions: $\omega ⪡1\mathrm{a}.\mathrm{u}.$ and $\gamma ⪡1(\omega$ is the frequency and $\gamma$ the Keldysh parameter) are satisfied. For the circularly polarized laser field ionization rate in the Keldysh theory [which utilizes the length gauge $(\vec{d}\bullet \vec{E})$ form of the matrix element] is calculated analytically. We show numerically that if the WKB Coulomb correction in the final state of the ionized electron is included, the Keldysh theory gives the correct result in the tunneling domain. In the barrier-suppression regime the Keldysh theory without this correction gives ionization rates close to the exact static-field results. The Reiss theory [which utilizes the velocity gauge $(\vec{p}\bullet \vec{A})$ form of the matrix element] leads to too small ionization rates in the limit $\omega \to 0$, $\gamma \to 0$.

The momentum-shell renormalization group (RG) is used to study the condensation of interacting Bose gases without and with disorder. First of all, for the homogeneous disorder-free Bose gas the interaction-induced shifts in the critical temperature and chemical potential are determined up to second order in the scattering length. The approach does not make use of dimensional reduction and is thus independent of previous derivations. Secondly, the RG is used together with the replica method to study the interacting Bose gas with delta-correlated disorder. The flow equations are derived and found to reduce, in the high-temperature limit, to the RG equations of the classical Landau-Ginzburg model with random-exchange defects. The random fixed point is used to calculate the condensation temperature under the combined influence of particle interactions and disorder.

Optical second-harmonic (SH) generation from the surface of spherical particles of radii much smaller than the optical wavelength has been investigated experimentally. Angle- and polarization-resolved measurements from dye-coated polystyrene spheres have been obtained for radii of 55 and $85\mathrm{nm}$. The SH signals obey general axial- and polarization-selection rules. The polarization-resolved angular patterns agree with the electromagnetic theory of SH Rayleigh scattering that describes the process in terms of locally excited quadrupolar and nonlocally excited dipolar emission. The experimental results differ significantly from those predicted for a local dipole-allowed SH scattering process.

Theoretical investigations of the positronium formation in positron-magnesium collision for energies ranging from the positronium threshold to $60.0\mathrm{eV}$ using the optical-model approach are reported in this paper. A complex equivalent-local polarization potential is obtained to describe the rearrangement process, which produces the Ps formation cross section. We report the $\mathrm{Ps}\left(1s\right)$, $\mathrm{Ps}\left(2s\right)$, $\mathrm{Ps}\left(2p\right)$, and the total $\mathrm{Ps}(n=1+2)$ formation cross sections and compare with available experimental measurements and other theoretical results.

We investigate the dynamics of dark-state polaritons in an atomic ensemble with ground-state degeneracy. A signal light pulse may be stored and retrieved from the atomic sample by adiabatic variation of the amplitude of a control field. During the storage process, a magnetic field causes rotation of the atomic hyperfine coherences, leading to collapses and revivals of the dark-state polariton number. These collapses and revivals should be observable in measurements of the retrieved signal field, as a function of storage time and magnetic field orientation.

Heterodyne-detected transient grating experiments on the OH-stretch mode of HDO dissolved in ${\mathrm{D}}_2\mathrm{O}$ resolve two distinctly different contributions originating from the initially excited OH stretch and the OD stretch which is thermally activated during the OH population relaxation. It is demonstrated that interference of both contributions greatly affects the outcome of IR photon echo experiments.

A system of traps is considered, each containing a large number of Bose-condensed atoms. This ensemble of traps is subject to the action of an external modulating field generating nonequilibrium non-ground-state condensates. When the frequency of the modulating field is in resonance with the transition frequency between two different topological coherent modes, each trap becomes an analog of a finite-level resonant atom. Similarly to the case of atoms in an electromagnetic resonant field, one can create entanglement between atomic traps subject to a common resonant modulating field generating higher coherent modes in each of the traps. A method is suggested for regulating entanglement production in such a system of multitrap and multimode Bose-Einstein condensates coupled through a common resonant modulating field. Several regimes of evolutional entanglement production, regulated by manipulating the external field, are illustrated by numerical calculations.

We investigate the influence of random errors in external control parameters on the stability of holonomic quantum computation in the case of arbitrary loops and adiabatic connections. A simple expression is obtained for the case of small random uncorrelated errors. Due to universality of mathematical description our results are valid for any physical system which can be described in terms of holonomies. Theoretical results are confirmed by numerical simulations.