We study a Raman-driven two-quantum interaction in the motion of a trapped atom, which in the Lamb-Dicke limit corresponds to a classically driven parametric interaction known from nonlinear optics. When the Lamb-Dicke approximation fails, interference effects between the driving laser waves and the spatially extended motional wave function of the atom lead to nonlinear modifications of the motional dynamics. Destructive interferences occur for particular values of the motional amplitude. They lead to a partitioning of the motional phase space accompanied by interesting dynamical effects. Starting the dynamics in the motional ground state of the atom, a quadrature-squeezed state emerges for short interaction times, in agreement with recent experiments [D. M. Meekhof et al., Phys. Rev. Lett. 76, 1796 (1996)]. For longer interaction times the phase-space partitioning gives rise to a reduction of the variance of the motional excitation number and to phase-locking effects leading to a partial revival of the state towards the origin of the phase space.

We present physical reasoning for the missing interference [Zhu, Narducci, and Scully, Phys. Rev. A 52, 4791 (1995)] in emission in a laser-driven V system. We demonstrate how the interference effects can be restored by considering an additional channel of spontaneous emission. Analytical results are given to identify the various pathways contributing to this interference. The interference terms show up in the form of dispersive contributions to the line shape.

The Lindblad approach to continuous quantum measurements is applied to a system composed of a two-level atom interacting with a stationary quantized electromagnetic field through a dispersive coupling, fulfilling quantum nondemolition criteria. Two schemes of measurements are examined. The first one consists of measuring the atomic electric dipole, which indirectly allows one to infer the photon distribution inside the cavity. The second one schematizes a measurement of photon momentum, which permits one to describe the atomic level distribution. Decoherence of the corresponding reduced density matrices is studied in detail for both cases, and its relationship to recent experiments is finally discussed.

The absorption and stimulated emission of coherent electromagnetic radiation tuned close to an autoionizing state is investigated with the aim of clarifying some of the aspects behind the proposals of inversionless gain involving such atomic configurations. This is carried out within the framework of individual atomic evolution rather than by the use of density matrix equations, thus providing better insight into the fundamental processes of absorption and emission. In particular, we find that stimulated emission does not overcome absorption in spite of quantum interference of the Fano type being present. It is also shown that gain, in this configuration, although it can take place without population inversion between the involved levels, requires in any case a strong departure from thermal equilibrium conditions. Thereby a strong pumping mechanism, or equivalently a strong excitation power, is needed to reach the lasing threshold. This raises some questions on the relevance of the population inversion indicator of efficient gain implicitly assumed in similar configurations and in many related works.

Recently a quantization scheme for the phenomenological Maxwell theory of the full electromagnetic field in an inhomogeneous three-dimensional, dispersive, and absorbing dielectric medium has been developed and applied to a system consisting of two infinite half-spaces with a common planar interface (H.T. Dung, L. Knöll, and D.-G. Welsch, Phys. Rev. A 57, 3931 (1998)). Here we show that the scheme, which is based on the classical Green-tensor integral representation of the electromagnetic field, applies to any inhomogeneous medium. For this purpose we prove that the fundamental equal-time commutation relations of QED are preserved for an arbitrarily space-dependent, Kramers-Kronig consistent permittivity. Further, an extension of the quantization scheme to linear media with bounded regions of amplification is given, and the problem of anisotropic media is briefly addressed.

A perturbation treatment for Raman generation with a combination of long, short, and delayed laser pulses is presented. When the coupling and probe lasers are applied in a counterintuitive sequence, the fast oscillatory contributions to populations and coherence are eliminated by robust adiabatic passage, allowing a much simpler solution to the problem. Such counterintuitive and on-resonance operation allows effective electromagnetically induced transparency to evolve so that the probe laser photons will experience no absorption yet still fully participate in the nonlinear frequency conversion. Consequently, better conversion efficiency should be possible.

Based on the logarithmic perturbation method proposed by Aharonov and Au [Phys. Rev. Lett. 42, 1582 (1979)], we calculate explicitly up to fourth order and arrive at different results for the energy corrections and nodal point shifts of the first excited state.

A potential whose ground state is the even coherent state is constructed. The first excited state of this potential is shown to be a good approximation of the odd coherent state. The nonclassical features of this state are studied and compared with those of the odd coherent state.

The effect of multiple scattering on the broadening and the shift of the spectral lines of light propagating in a medium is investigated within Wolf’s framework. The condition for no blueshift is derived. It is shown that under the same hypothesis the property $\Delta {\omega }_{N+1\mathrm{}}\gg {\delta }_N$ is satisfied. Moreover, the no blueshift condition has been found to be closely related to the no blurring condition. These results might be of relevance in the astronomical domain.

Triple differential cross sections have been calculated for electron-impact ionization of ${\mathrm{He}}^{+}$ in an electron incident energy range 75–3000 eV at an angle of $45°$ in a symmetric coplanar energy-sharing geometry. In the present calculation the three Coulomb two-body interactions, the dynamic screening modification of these interactions due to the presence of the third particle and the correlation in the final channel, and the distortion of Coulomb force without the bound electron screening effect in the initial channel have been studied. The results have been compared with the existing theoretical ones.

We have developed a one-parameter, model wave function for the positronium ion $ē\mathrm{ee}$, which incorporates the correct cusp and coalescence properties when two particles are close to each other, and the asymptotic property when one of the electrons is far away. The predicted values for the energy and other properties are close to the exact values and generally superior to the values from a 13-parameter wave function.

Using the beam-foil spectroscopy technique, we have measured the decay time of the $3p{}^2{P}_{3/2}$ level in sodiumlike bromine. The results, using an arbitrarily normalized decay curves analysis, are in good agreement with the most recent theoretical calculations. They also confirm an earlier experimental result in sodiumlike krypton, suggesting that some previous measurements of the lifetimes of $3p{}^2{P}_{3/2}$ states of nearby isoelectronic ions are too high.

A screened hydrogenic model with the effective nuclear charge fitted to empirical or measured binding energy is used to study the Ps-formation cross section from lithium and sodium. It is found that the cross section depends crucially on the choice of the screening and the Coulomb-distorted results for differential cross section $d\sigma /d\Omega$ always fall below the corresponding first-Born-approximation values. A possible physical explanation is sought to visualize these facts. The values for total cross section computed at different projectile energies for sodium are found to be in good agreement with those obtained from other sophisticated approaches. For lithium our model gives satisfactory results only at high energies.

Molecular-dynamics (MD) simulations have been used to calculate the many-body time-correlation functions and interaction-induced absorption spectra for the small clusters composed of binary argon (Ar)-krypton (Kr) mixtures. The liquid state phase of ${\mathrm{Ar}}_k{\mathrm{Kr}}_{13-k}$ $\mathrm{}(0<k<\mathrm{}13\mathrm{})\mathrm{}$ clusters has been studied, and considerable differences between the interaction-induced absorption of the clusters and bulk matter are observed.

Theoretical electronic structure studies have been carried out to show that the recently observed thermal isomerization in ${\mathrm{Cs}}_4{\mathrm{Cl}}_3{\mathrm{}}^{\mathrm{-}}$ clusters is due to three energetically close structures with a cuboid, a ladder, and an octagonal ring arrangement. It is found that the electronic excited states can serve as a fingerprint for identification of these structures. By calculating total energies and electronic excited states, we identify the cuboid as the most stable geometry. The vibrational spectra of the isomers are shown to be marked by low-energy modes which enable them to isomerize at ordinary temperatures.

In a recent paper Meyer and Yeoman [Phys. Rev. Lett. 79, 2650 (1997)] have shown that the resonance fluorescence from two atoms placed in a cavity and driven by an incoherent field can produce an interference pattern with a dark center. We study the fluorescence from two coherently driven atoms in free space and show that this system can also produce an interference pattern with a dark center. This happens when the atoms are in nonequivalent positions in the driving field, i.e., the atoms experience different intensities and phases of the driving field. We discuss the role of the interatomic interactions in this process and find that the interference pattern with a dark center results from the participation of the antisymmetric state in the dynamics of the driven two-atom system.

A graphical method is used to obtain the quasiphoton number and hence the photon-number distribution (PND) for squeezed states of real parameters. The existing results on the PND of a squeezed state are compared with the results obtained by the present method. The recent experiment on the PND agrees well with the giant oscillation reported in the present paper.

The question of whether the central-field approximation is a suitable vehicle for accounting for core-excitation effects is revisited. It is concluded that the errors that this model makes in inner-shell excitation energies can be smaller than suggested by Crance and Aymar [J. Phys. B 20, L155 (1987)].

We investigate the interaction of a two-level atom with a short femtosecond optical pulse without the rotating-wave approximation. We have derived the population inversion due to the counterrotating terms. It is shown that due to the counterrotating terms the population inversion oscillates with high frequency and there is the effective time-dependent addition to the Rabi frequency that varies inversely proportionally to the length of an optical pulse and this effect is actual for a short femtosecond optical pulse. It is shown that there is the transient cubic nonlinearity due to the counterrotating terms.

We propose a scheme for generating macroscopic quantum-interference (Schrödinger-cat) states of the motion of a trapped ion. In the scheme, a trapped ion is driven by two laser beams tuned to the first lower and upper vibrational sidebands, respectively. Under certain conditions, the detection of the internal state of the ion leaves the vibrational motion in a coherent state superposition. The scheme can be easily generalized to prepare other kinds of cat states.