In this paper we consider the optical coupling of two four-level atoms via cooperative Raman scattering of correlated photons of the nondegenerate spontaneous parametric radiation. The Raman scattering of twin photons incident on the atoms is additionally stimulated by coherent classical radiation in such a way that it is slightly off resonant for each atom considered independently but resonant for the joint system. We discuss the special excitation conditions when the amplitude of the cooperative stimulated Raman scattering from quantum to classical modes becomes greater than the normal transition rates associated with independent Raman scattering. In certain conditions weak entanglement between the Zeeman sublevels in the ground states of these atoms can appear. The small admixture of entanglement in the system of two atoms leads to strong entanglement between transverse macroscopic spin fluctuations of two spatially separated atomic ensembles coupled by such quantum light. The proposed mechanism of entanglement is relevant to the ground state spin subsystem of two distant macroscopic and optically thin atomic ensembles. Such an entangled state can be stored for a long time and used as a basic Einstein-Podolsky-Rosen system in different applications pursuing the goal of spreading quantum information schemes in atomic subsystems.

We investigate the Casimir force acting between real metals at nonzero temperature. It is shown that the zero-frequency term of the Lifshitz formula is difficult to interpret in the case of a real metal described by the Drude model. This is because the scattering theory underlying the Lifshitz formula is not well formulated when the dielectric permittivity takes into account dissipation. To give the zeroth term of the Lifshitz formula a definite meaning, different prescriptions have been used recently by different authors with diverse results. These results are shown to be improper and in disagreement with experiment and the general physical requirements. We propose a prescription that is a generalization of the Schwinger, DeRaad, and Milton recipe formulated earlier for ideal metals. On this basis, detailed numerical and analytical computations of the temperature Casimir force are performed in the configurations of two plane plates and of a spherical lens (sphere) above a plate. The corrections due to nonzero temperature and finite conductivity found here are in agreement with the limiting case of a perfect metal and fit all experimental and theoretical requirements. Among other facts, previous results obtained in the framework of the plasma model are confirmed. It appears that they are the limiting case of Drude model computations when the relaxation parameter goes to zero. A comparison with the Casimir force acting between dielectric test bodies is made.

Complete calculations of the vacuum-polarization corrections to the hyperfine splitting of the $1s$ and $2s$ states in heavy ions are presented. The magnetic-loop Wichmann-Kroll correction is evaluated for the point-dipole model of the nuclear magnetization as well as for the single-particle nuclear model. For the latter case the related correction to the nuclear magnetic moments is also evaluated. The results of the calculations are compared with previous evaluations of this effect.

High-resolution $4d$ and $5p$ photoelectron spectra of free barium atoms were measured using narrow-band-width synchrotron radiation of 131.2 eV photon energy. These measurements were necessary to disentangle the complex structure of the electron spectra. We also performed a Hartree-Fock calculation with relativistic corrections. A very good agreement between theory and experiment was found, which allowed the assignment of all but the weakest lines, in addition to pointing out the significant contribution of configuration interaction and shake processes.

All-electron calculations in large basis sets of excitation energies, oscillator strengths, and polarizabilities of small alkali-metal clusters of Li, Na, and K, with up to eight atoms, are performed using time-dependent density-functional theory. It is shown that the use of the recently developed statistical average of orbital potentials (SAOP) exchange-correlation (xc) potential [P.R.T. Schipper et al., J. Chem. Phys. 112, 1344 (2000)] leads to polarizabilities of these alkali-metal clusters which are 10–15 % larger than polarizabilities calculated with the xc potential of the local-density approximation (LDA). The lower LDA polarizabilities (in comparison to the SAOP) are shown to originate from differences in the low-lying excitation energies, which are determined by the xc potential in the molecular inner and valence region. In spite of such differences, both SAOP and LDA results are shown to provide reliable assignments of the experimental absorption spectra, with typical errors in peak positions of only 0.1– 0.2 eV, or even less.

We present a random-phase-approximation formalism for the collective spectrum of two hyperfine species of dilute ${}^{40}\mathrm{K}$ atoms, magnetically trapped at zero temperature and subjected to a repulsive s-wave interaction between atoms with different spin projections. We examine the density and the spin oscillation spectra, as well as the transition density profiles created by external multipolar fields. The zero sound spectrum is always fragmented and the density and spin channels become clearly distinguishable if the trapping potentials acting on the species are identical. Although this distinction is lost when these confining fields are different, at selected excitation frequencies the transition densities may display the signature of the channel.

A model of interacting one-dimensional fermions confined to a harmonic trap is proposed. The model is treated analytically to all orders of the coupling constant by a method analogous to that used for the Luttinger model. As a first application, the particle density is evaluated and the behavior of Friedel oscillations under the influence of interactions is studied. It is found that attractive interactions tend to suppress the Friedel oscillations, while strong repulsive interactions enhance the Friedel oscillations significantly. The momentum distribution function and the relation of the model interaction to realistic pair interactions are also discussed.

Generalized Einstein B coefficients for coherently driven closed three-level systems are introduced by means of the quantum-jump technique. The nonreciprocity between stimulated emission and absorption for both one-photon and two-photon gain and loss processes has been studied and quantified in terms of the rates of the particular incoherent processes present in each three-level system. Some general properties of these generalized Einstein B coefficients have been found. In particular, whenever the generalized Einstein B coefficient for one-photon gain overcomes that for one-photon loss then the generalized Einstein B coefficient for two-photon loss overcomes that for two-photon gain, and vice versa. Finally, we have obtained simple analytical expressions indicating the way to maximize the asymmetry between stimulated emission and absorption coefficients either for one-photon or for two-photon processes.

Alternative lower bounds for the binding energy of a quantum-mechanical system of interacting particles are presented. These bounds are expressed in terms of two-particle quantities and improve the conventional bounds of the Hall-Post type. They are constructed by considering not only the energy in the two-particle system, but also the structure of the pair wave function. We apply the formal results to various numerical examples, and show that in some cases dramatic improvement over the existing bounds is reached.

Results are presented for simulations of electron-positron pair production in relativistic heavy-ion collisions leading to electron capture and positron ejection. We apply a two-center relativistic continuum distorted-wave model to represent the electron or positron dynamics during the collision process. The results are compared with experimental cross-section data for ${\mathrm{La}}^{57+}$ and ${\mathrm{Au}}^{79+}$ impact on gold, silver, and copper targets. The theory is in good agreement with experiment for ${\mathrm{La}}^{57+}$ impact, verifying the result that the process increases in importance with both collision energy and target atomic number, and improves upon previous simulations of this process.

We present an analysis of the characteristics of nonadiabatic couplings due to the existence of conical intersections between potential energy surfaces of triatomic systems in collinear configurations. We discuss the relative merits and performance of four techniques that we tested to remove the singularities, and illustrate our findings for a conical point of two molecular surfaces involved in ${\mathrm{N}}^{5+}+{\mathrm{H}}_2$ collisions.

Two developments in scattering theory are reported. We show, in a practical way, how one can calculate the scattering amplitude without invoking a partial-wave expansion. First, the integral expression for the scattering amplitude $f\left(\theta \right)$ is simplified by an analytic integration over the azimuthal angle. Second, the full scattering wave function that appears in the integral expression for $f\left(\theta \right)$ is obtained by solving the Schrödinger equation with the finite element method. As an example, we calculate electron scattering from static hydrogen (the Hartree potential). With minimal computational effort, we obtain accurate and stable results for the scattering amplitude.

The tensor-virial method is applied for a study of oscillation modes of uniformly rotating Bose-Einstein condensed gases, whose rigid-body rotation is supported by an vortex array. The second-order virial equations are derived in the hydrodynamic regime for an arbitrary external harmonic trapping potential assuming that the condensate is a superfluid at zero temperature. The axisymmetric equilibrium shape of the condensate is determined as a function of the deformation of the trap; its domain of stability is bounded by the constraint Ω<1 on the rotation rate (measured in units of the trap frequency ${\omega }_0).\mathrm{}$ The oscillations of the axisymmetric condensate are stable with respect to the transverse-shear and toroidal modes of oscillations, corresponding to the $l=2,|m|=1,2\mathrm{}$ surface deformations. The eigenfrequencies of the modes are real and represent undamped oscillations. The condensate is also stable against quasiradial pulsation modes $(l=2,m=0),$ and its oscillations are undamped, if the superflow is assumed incompressible. In the compressible case we find that for a polytropic equation of state, the quasiradial oscillations are unstable when $\gamma \left(3\mathrm{}-{\Omega }^2\right)<\mathrm{}1\mathrm{}-3{\Omega }^2,$ and are stable otherwise. Thus, a dilute Bose gas, whose equation of state is polytropic with γ=2 to leading order in the diluteness parameter, is stable irrespective of the rotation rate. In nonaxisymmetric traps, the equilibrium constrains the (dimensionless) deformation in the plane orthogonal to the rotation to the domain $A_2>{\Omega }^2$ with Ω<1. The second-harmonic-oscillation modes in nonaxisymmetric traps separate into two classes that have even or odd parity with respect to the direction of the rotation axis. Numerical solutions show that these modes are stable in the parameter domain where equilibrium figures exist.

The dynamics of a solid-state laser sustaining the oscillation of the Laguerre–Gaussian ${\mathrm{TEM}}_{0,l}^{*}$ mode is theoretically and experimentally studied. The results of investigations of the existence conditions of self-modulation, chaotic, frequency locking, and self-pulsing regimes are given. The experimental results, obtained using a diode-pumped solid-state laser, are well confirmed by the theoretical model. From the observations of the locking phenomena of the first-order family, we also confirm the theoretical predictions that the locking occurs as a subcritical bifurcation in a solid-state laser.

We demonstrate that it is possible to use the balanced homodyning with array detectors to measure the quantum state of a correlated two-mode signal field. We show the applicability of the method to fields with complex mode functions, thus generalizing the work of Beck [Phys. Rev. Lett. 84, 5748 (2000)] in several important ways. We further establish that, under suitable conditions, array detector measurements from one of the two ouputs are sufficient to determine the quantum state of signal. We show the power of the method by reconstructing a truncated Perelomov state, which exhibits complicated structure in the joint probability density for the quadratures.

We present an experimental study of the dynamics of a polymer laser upon excitation with an ultrashort pump pulse. The laser is unusual in the sense that the cavity length is such that the cavity round-trip time is intermediate between the duration of the pump pulse and the spontaneous-emission time of the gain medium. The cavity is only partly filled with gain medium. The unusual time scales, in combination with the spatial gain distribution, result in the emission of a neat and regular train of short pulses. Its generation can be viewed as a special case of gain switching. A one-dimensional rate-equation model that takes the nonuniformity of the gain explicitly into account is presented. The model describes both the evolution of the inversion, which is also measured experimentally, and the dynamics of the output in great detail, and yields excellent agreement with the experimental results. A heuristic description of the arisal of the pulse train in terms of amplified spontaneous emission and gain depletion is given.

We have analyzed the Hanle effect on the closed $F_{\mathrm{g}}=\overrightarrow{3}{F}_{\mathrm{e}}=4\mathrm{}$ $D_2$-line transition of ${}^{85}\mathrm{Rb}.$ Exciting the rubidium atoms by circularly polarized laser light, and scanning an applied transverse magnetic field, a bright resonance Hanle signal is obtained at different values of an applied longitudinal magnetic field. We report experimental and numerical evidence of this bright resonance.

We show that a three-level $\Lambda$-type atom interacting with a classical standing-wave field resonantly coupling one transition and a weak probe laser field resonantly coupling the second transition can be localized provided the population of the upper state is observed.