A combined experimental and theoretical investigation of time- and alignment-dependent propagation of light in an ultracold atomic gas of atomic ${}^{85}\mathrm{Rb}$ is reported. Coherences among the scattering amplitudes for light scattering off excited hyperfine levels produce strong variations of the light polarization in the vicinity of atomic resonance. Measurements are in excellent agreement with Monte Carlo simulations of the multiple scattering process.

We study the properties of no-signaling correlations that cannot be reproduced by local measurements on entangled quantum states. We say that such correlations violate Tsirelson bounds. We show that if these correlations are obtained by some reversible unitary quantum evolution $U$, then $U$ cannot be written in the product form $U_A\otimes U_B$. This implies that $U$ can be used for signaling and for entanglement generation. This result is completely general and in fact can be viewed as a characterization of Tsirelson bounds. We then show how this result can be used as a tool to study Tsirelson bounds and we illustrate this by rederiving the Tsirelson bound of $2\sqrt{2}$ for the Clauser-Horn-Shimony-Holt inequality, and by deriving a new Tsirelson bound for qutrits.

We investigate quantum electrodynamic effects under the influence of an external, time-dependent electromagnetic field, which mediates dynamic modifications of the radiative corrections. Specifically, we consider the quantum electrodynamic vacuum-polarization tensor under the influence of two external background fields: a strong laser field and a nuclear Coulomb field. We calculate the charge and current densities induced by a nuclear Coulomb field in the presence of a laser field. We find the corresponding induced scalar and vector potentials. The induced potential, in first-order perturbation theory, leads to a correction to atomic energy levels. The external laser field breaks the rotational symmetry of the system. Consequently, the induced charge density is not spherically symmetric, and the energy correction therefore leads to a “polarized Lamb shift.” In particular, the laser generates an additional potential with a quadrupole moment. The corresponding laser-dressed vacuum-polarization potential behaves like $1∕r^3$ at large distances, unlike the Uehling potential, which vanishes exponentially for large $r$. The energy corrections are of the same order of magnitude for hydrogenic levels, irrespective of the angular momentum quantum number. The induced current leads to a transition dipole moment which oscillates at the second harmonic of the laser frequency and is mediated by second-order harmonic generation in the vacuum-polarization loop. In the far field, at distances $r⪢1∕\omega$ from the nucleus ($\omega$ is the laser frequency), the laser induces mutually perpendicular electric and magnetic fields, which give rise to an energy flux that corresponds to photon fusion leading to the generation of real photons, again at the second harmonic of the laser. Our investigation might be useful for other situations where quantum field theoretic phenomena are subjected to external fields of a rather involved structure.

We consider the problem of optimally discriminating two Pauli channels in the minimax strategy, maximizing the smallest of the probabilities of correct identification of the channel. We find the optimal input state at the channel and show the conditions under which using entanglement strictly enhances distinguishability. We finally compare the minimax strategy with the Bayesian one.

Both direct and indirect weak nonresonant interactions are shown to produce entanglement between two initially disentangled systems prepared as a tensor product of thermal states, provided the initial temperature is sufficiently low. Entanglement is determined by the Peres-Horodecki criterion, which establishes that a composite state is entangled if its partial transpose is not positive. If the initial temperature of the thermal states is higher than an upper critical value $T_{\mathit{uc}}$ the minimal eigenvalue of the partially transposed density matrix of the composite state remains positive in the course of the evolution. If the initial temperature of the thermal states is lower than a lower critical value $T_{\mathit{lc}}⩽T_{\mathit{uc}}$ the minimal eigenvalue of the partially transposed density matrix of the composite state becomes negative, which means that entanglement develops. We calculate the lower bound $T_{\mathit{lb}}$ for $T_{\mathit{lc}}$ and show that the negativity of the composite state is negligibly small in the interval $T_{\mathit{lb}}<T<T_{\mathit{uc}}$. Therefore the lower-bound temperature $T_{\mathit{lb}}$ can be considered as the critical temperature for the generation of entanglement. It is conjectured that above this critical temperature a composite quantum system could be simulated using classical computers.

Resonant formation of the muonic molecule $dt\mu$ in $t\mu$ atom collision with condensed H-D-T targets is considered. A specific resonance correlation function, which is a generalization of the Van Hove single-particle correlation function, is introduced to calculate the resonant-formation rate in such targets. This function is derived in the case of a polycrystalline harmonic solid. Also, a general asymptotic form of the resonance correlation function for high momentum transfers is found, which is valid for any solid or dense-fluid hydrogen-isotope target. Numerical calculations of the rates are performed for solid hydrogen isotopes at zero pressure, using the isotropic Debye model of a solid. It is shown that condensed-matter effects in resonant formation are strong, which explains some unexpected experimental results. In particular, the resonance profiles are affected by large zero-point vibrations of the hydrogen-isotope molecules bound in the considered crystals, even for high $(\sim 1\mathrm{eV})$ collision energies. This is important for explaining the time-of-flight measurements of the $dt\mu$-formation rate, carried out at TRIUMF. The calculated mean values of the formation rate in solid D-T targets, for fixed target temperatures and steady-state conditions, are in good agreement with the PSI and RIKEN-RAL experiments.

$K$-shell radiation of fast heavy ions penetrating solid matter was used to analyze the stopping dynamics of ions over more than 80% of the stopping path. The most important advantage of this method is that the data is obtained with a high spatial resolution directly from the interaction volume. In experiments $11.4\mathrm{MeV}∕\mathrm{u}$ Ca projectile were slowed down in solid quartz and low-density $\mathrm{Si}{\mathrm{O}}_2$ aerogel targets. Characteristic projectile and target spectra in the photon energy range of 1.5–4 keV were registered by means of spherically bent crystal spectrometers with high spectral and spatial resolution in the direction of the ion beam propagation. $K$-shell spectra of heavy ions induced by close collisions with target atoms provided information about the projectile charge state and velocity dynamics. The line intensity distribution of the $K$-shell transitions arising from ions with different ion charges represents the charge state distribution along the ion beam track. The variation of the line Doppler shift due to the ion deceleration in the target material was used to determine the ion velocity dynamics. The spectroscopic analysis of the stopping process was complemented by measurements of the energy loss and ion charge state distribution after the ion beam emerged from the target using a standard time-of-flight method and magnet spectrometer.

We present calculations showing that ionization of ${\mathrm{Xe}}_N$ $(N=108–1524)$ clusters by intense femtosecond lasers is governed by two mechanisms that have their origin in molecular and plasma physics, respectively. Optimal-control calculations show that a three-pulse sequence optimizes cluster ionization and involves both enhanced ionization and resonant driving of collective electron oscillations. As a result we can understand how ionization in intense laser fields evolves from small molecules to large clusters.

Using a density-functional calculation within the generalized gradient approximation, we have investigated the structural and electronic properties of antimony clusters. The lowest-energy structures of neutral and anionic ${\mathrm{Sb}}_n$ clusters up to $n=10$ were determined and compared with those of light-element group-V clusters $({\mathrm{As}}_n,{\mathrm{P}}_n)$. The size dependence of binding energies, gaps between lowest-unoccupied and highest-occupied molecular orbitals, second-order difference of total energies, and adiabatic electron affinities (EAs) of ${\mathrm{Sb}}_n$ clusters were discussed. Even-odd oscillation as well as magic-number behavior at ${\mathrm{Sb}}_4$ were observed. Qualitative agreement was found between theoretical and experimental EAs.

We investigate within mean-field theory the influence of a one-dimensional optical lattice and of trapped degenerate fermions on the critical rotational frequency for vortex line creation in a Bose-Einstein condensate. We consider laser intensities of the lattice such that quantum coherence across the condensate is ensured. We find a sizable decrease of the thermodynamic critical frequency for vortex nucleation with increasing applied laser strength and suggest suitable parameters for experimental observation. Since ${}^{87}\mathrm{Rb}-{}^{40}\mathrm{K}$ mixtures may undergo collapse, we analyze the related question of how the optical lattice affects the mechanical stability of the system.

We perform a comprehensive study of stability of a pumped atom laser in the presence of pumping, damping, and outcoupling. We also introduce a realistic feedback scheme to improve stability by extracting energy from the condensate and determine its effectiveness. We find that while the feedback scheme is highly efficient in reducing condensate fluctuations, it usually does not alter the stability class of a particular set of pumping, damping, and outcoupling parameters.

We propose that by exciting ultracold atoms from the zeroth to the first Bloch band in an optical lattice, multiflavor bosonic Hubbard Hamiltonians can be realized in a different way. In these systems, each flavor hops in a separate direction and on-site exchange terms allow pairwise conversion between different flavors. Using band-structure calculations, we determine the parameters entering these Hamiltonians and derive the mean-field ground-state phase diagram for two effective Hamiltonians (two dimensional, two flavors, and three dimensional, three flavors). Further, we estimate the stability of atoms in the first band using second-order perturbation theory and find lifetimes that can be considerably (10–100 times) longer than the relevant time scale associated with intersite hopping dynamics, suggesting that quasiequilibrium can be achieved in these metastable states.

We propose and develop a method for theoretical description of circularly (elliptically) polarized optical pulse resonant coherent interactions with two-level atoms. The method is based on the time-evolution equations of a two-level quantum system in the presence of a time-dependent dipole perturbation for electric dipole transitions between states with total angular-momentum projection difference $(\Delta J_z=\pm 1)$ excited by a circularly polarized electromagnetic field [Feynman et al., J. Appl. Phys. 28, 49 (1957)]. The adopted real-vector representation approach allows for coupling with the vectorial Maxwell’s equations for the optical wave propagation and thus the resulting Maxwell pseudospin equations can be numerically solved in the time domain without any approximations. The model permits a more exact study of the ultrafast coherent pulse propagation effects taking into account the vector nature of the electromagnetic field and hence the polarization state of the optical excitation. We demonstrate self-induced transparency effects and formation of polarized solitons. The model represents a qualitative extension of the well-known optical Maxwell-Bloch equations valid for linearly polarized light and a tool for studying coherent quantum control mechanisms.

The formation of ${\mathrm{S}}^{+}$, ${\mathrm{S}}^{2+}$, ${\mathrm{S}}^{3+}$, and ${\mathrm{S}}^{4+}$ is observed following inner-shell photodetachment of ${\mathrm{S}}^{-}$. The photodetachment spectra for all possible ionic products are obtained over a large region of photon energies covering both the $2p$ and $2s$ thresholds (${\mathrm{S}}^{5+}$, is energetically allowed at the higher photon energies, but not observed), and are placed on an absolute scale. The $2s$ threshold energy is measured to be 224.6(5) eV, allowing the determination of the neutral atomic S $2s^{-1}3s^{2}3p^5$ inner-shell excited state energy for the first time. The ${\mathrm{S}}^{-}2s^{-1}3s^{2}3p^6{}^2\mathrm{S}_{1∕2}$ state is observed as a Feshbach resonance 2.3(5) eV below the $2s$ threshold in the ${\mathrm{S}}^{+}$, ${\mathrm{S}}^{2+}$, and ${\mathrm{S}}^{3+}$ product channels.

An analytical model for a dipole-bound anion (DBA) is proposed based on the exactly solvable three-dimensional Schrödinger equation for the excess electron bound by dipole potential of the parent neutral molecule (NM) in the Born–Oppenheimer approximation. The model gives reasonable analytical approximation for the dependence of the DBA binding energy on the NM dipole moment previously found numerically by many authors. The cross section of one-photon photodetachment of DBA is calculated in explicit analytical form. In the limit of high photon frequency, $\omega$, the calculated cross-section displays $\sim {\omega }^{-2}$ behavior, which agrees perfectly with the experimental data [Bailey et al., J. Chem. Phys 104, 6976 (1996)]. At the threshold, the cross section demonstrates Gailitis–Damburg oscillations. Numerical dependence is provided for the maximal value of the cross section as a function of the NM dipole moment and the binding energy of the excess electron.

A cluster consisting of many atoms or molecules may be considered, in some circumstances, to be a single large molecule with a well-defined polarizability. Once the polarizability of such a cluster is known, one can evaluate certain properties—e.g. the cluster’s van der Waals interactions, using expressions derived for atoms or molecules. In the present work, we evaluate the static polarizability of a cluster using a microscopic method that is exact within the linear and dipolar approximations. Numerical examples are presented for various shapes and sizes of clusters composed of identical atoms, where the term “atom” actually refers to a generic constituent, which could be any polarizable entity. The results for the clusters’ polarizabilities are compared with those obtained by assuming simple additivity of the constituents’ atomic polarizabilities; in many cases, the difference is large, demonstrating the inadequacy of the additivity approximation. Comparison is made (for symmetrical geometries) with results obtained from continuum models of the polarizability. Also, the surface effects due to the nonuniform local field near a surface or edge are shown to be significant.

We have systematically studied the method proposed by Patnaik et al. [Phys. Rev. A 69, 035803 (2004)] that manipulates the retrieval of stored light pulses. The measured probe pulse width of the retrieval is inversely proportional to the intensity of the reading field. We also show that the method does not introduce any phase shift or jump into the retrieved pulses. Our study demonstrates that the distortion at the output of the light storage can be corrected by manipulating the retrieval process and the phase information of the stored pulses can remain intact during the process.

We study the dynamics of a single excitation in an infinite $XXZ$ spin chain, which is launched from the origin. We study the time evolution of the spread of entanglement in the spin chain and obtain an expression for the second-order spatial moment of concurrence, about the origin, for both ordered and disordered chains. In this way, we show that a finite central disordered region can lead to sustained superballistic growth in the second-order spatial moment of entanglement within the chain.