The fluorescence spectra and lasing behavior of dye molecules adsorbed on a droplet levitated in an electrodynamic trap have been studied. A simple classical model describes the dye–cavity-mode interaction quantitatively well when the homogeneous linewidth of the dye is wider than the free spectral range of the cavity. When the free spectral range becomes comparable with the homogeneous linewidth, the prediction of the model systematically deviates, indicating that stochastic fluctuations of the radiating dipoles need to be taken into account.

The density dependence of the distribution of collective dipole coupling strengths realized by an atomic beam interacting with a ${\mathrm{TEM}}_{00}$ mode of an optical cavity is analyzed. An analytic expression for the characteristic function is derived, and low- and high-density limits are taken to define the one-atom and many-atom regimes of cavity QED, respectively. Intermediate densities $(N_{\mathrm{eff}}\sim 0.5)$ lie in a few-atom $(\sim 4-)$ regime.

We present exact trajectory solutions for a relativistic electron in the presence of a superintense linearly polarized laser pulse and a uniform magnetic field. To be concrete, the pulse envelope is taken in the form ${\sin }^2\left[\right(\kappa /2\left)\eta \right]$, where $\eta$ is the phase and $0<\mathrm{}\kappa <1\mathrm{}$. Using the analytical trajectories, we study numerically the spectra of radiation, emitted by the accelerated electron, along various observation directions. In particular, we find that light in the forward direction can be emitted essentially at two frequencies ${\omega }_0$ and $r{\omega }_0$, where ${\omega }_0$ is the laser frequency and r is related to the ratio of cyclotron frequency to the laser frequency. Spectra emitted in the directions of the electric and the magnetic components of the laser field are, in contrast, very rich in structure with highly irregular distributions of positions and strengths.

We analyze the conditions under which the Risken-Nummedal-Graham-Haken multimode instability can be observed in ring-cavity class-B three- and four-level lasers. Relevant examples of these are rare-earth doped fiber lasers. We show that the standard two-level laser model can be applied to three- and four-level laser systems under quite general conditions. We take advantage of this isomorphism to derive the conditions for the observation of the instability. The analysis is extended to the case in which the uniform field limit does not apply in order to cover more realistic situations.

We analyze the excess noise in the framework of the conventional quantum theory of laserlike systems. Our calculation is conceptually simple and our result also shows a correction to the semiclassical result derived earlier.

Spontaneous emission and Lamb shift of atoms in absorbing dielectrics and dense atomic gases are discussed using a microscopic Green’s-function approach. Uncorrelated and random atomic positions are assumed, and the associated unphysical interactions between different atoms at the same location are eliminated (local field correction). For the case of an atom in a purely dispersive medium, the spontaneous-emission rate is altered by the well-known Lorentz local-field factor. When the mean distance between atoms becomes less than the resonance wavelength, results different from previously suggested expressions are found. In particular, it is shown that nearest-neighbor interactions become important. The results suggest that, for large densities, absorbing disordered dielectrics cannot accurately be described by a macroscopic approach that neglects correlations between atomic positions.

Using electron-spin-resonance transitions within the nitrogen-vacancy center in diamond, we have obtained a sharp electromagnetically induced transparency (EIT) feature when the coupling and probing fields satisfy a two-photon resonance condition. In this paper it is shown that by the application of an additional driving field the sharp EIT feature can be split into a doublet or a triplet depending on the experimental configuration. The results are interpreted in terms of dynamic Stark splitting of the EIT feature, and the spectra are shown to be related to Autler-Townes and Mollow spectra.

A numerical method is introduced that solves the optical Bloch equations describing a two-level atom interacting with a strong polychromatic pump field with an equidistant spectrum and an arbitrarily intense monochromatic probe field. The method involves a transformation of the optical Bloch equations into a system of equations with time-independent coefficients at steady state via double harmonic expansion of the density-matrix elements, which is then solved by the method of matrix inversion. The solutions so obtained lead immediately to the determination of the polarization of the atomic medium and of the absorption and dispersion spectra. The method is applied to the case when the pump field is bichromatic and trichromatic, and the physical interpretation of the numerically computed spectra is given.

We find a relationship between unitary transformations of the dynamics of quantum systems with time-dependent Hamiltonians and gauge theories. In particular, we show that the nonrelativistic dynamics of spin-$\frac{1}{2}$ particles in a magnetic field $B^i\mathrm{}\left(t\right)\mathrm{}$ can be formulated in a natural way as an SU(2) gauge theory, with the magnetic field $B^i\mathrm{}\left(t\right)\mathrm{}$ playing the role of the gauge potential $A^i$. The present approach can also be applied to systems of n levels with time-dependent potentials, $U\left(n\right)\mathrm{}$ being the gauge group. This geometric interpretation provides a powerful method to find exact solutions of the Schrödinger equation. The root of the present approach rests in the Hermiticity property of the Hamiltonian operators involved. In addition, the relationship with true gauge symmetries of n-level quantum systems is discussed.

Relativistic configuration-interaction calculations, including valence and some shallow core-valence correlation indicate that ${\mathrm{La}}^{-}$ has 11 bound states. The seven odd states arising from 6p attachment have electron affinities (in meV) of 462 ${(}^1{\mathrm{D}}_2),\hspace{0.5em}282\hspace{0.5em}{(}^3{\mathrm{F}}_2),\hspace{0.5em}247\hspace{0.5em}{(}^3{\mathrm{F}}_3),$ 235 ${(}^3{\mathrm{D}}_1),\hspace{0.5em}145\hspace{0.5em}{(}^3{\mathrm{D}}_2),\hspace{0.5em}84\hspace{0.5em}{(}^3{\mathrm{F}}_4),$ and 56 ${(}^3{\mathrm{D}}_3).\mathrm{}$ The remaining four bound states are even $5d$ attachments with electron affinities (EA’s) of 434 ${(}^3{\mathrm{F}}_2),\hspace{0.5em}375\hspace{0.5em}{(}^3{\mathrm{F}}_3),\hspace{0.5em}312\hspace{0.5em}{(}^3{\mathrm{F}}_4),$ and 62 ${(}^1{\mathrm{D}}_2).\mathrm{}$ The majority of these levels are reported here for the first time. Two of these EA’s are in good agreement with the recent experimental values of Covington et al. [J. Phys. B 31, L855 (1998)]. The largest $5d-6p$ f value is $\sim 0.00565.$

Our previous result on the correction of the Bethe stopping power theory for heavy target elements is amended, with the application of a more consistent version of the semirelativistic Bethe sum rule worked out recently [Phys. Rev. A 57, 4994 (1998)]. This correction is found to be significant for high-Z target atoms and relatively high-energy incident particles.

An extended variational method is used to study doubly excited ${}^1{D}^e$ ${\mathrm{H}}^{-}$ and ${}_2(-{1,0)}_n^2$ $\mathrm{}(n=2,3\mathrm{})\mathrm{}$ ${}^1{S}^e;$ ${}_2{\mathrm{}(0,1)\mathrm{}}_n^{+}$ $\mathrm{}(n=2,3\mathrm{}),$ ${}_2{\mathrm{}(1,0)\mathrm{}}_4^{-}$, and ${}_2(-{1,0)}_3^{-}$ ${}^1{P}^o$; and ${}_2{\mathrm{}(1,0)\mathrm{}}_n^{+}$ $\mathrm{}(n=2,3\mathrm{}),$ ${}_2{\mathrm{}(1,0)\mathrm{}}_4^{+}$, and ${}_2{\mathrm{}(0,1)\mathrm{}}_3^0$ ${}^1{D}^e$ He, in which the electron-electron correlations are strong. Based on the flexibility of B-spline functions, we regrouped the saddle-point wave functions, obtained in the first saddle-point variational calculation, to fewer terms, and included more partial waves to achieve more accurate wave functions. More converged resonant energies and widths are obtained by the present saddle-point variational and complex-rotation method. The relation of convergence to electron-electron correlations is discussed. The present improved results are compared well with other theoretical results.

Fine-structure energy levels and oscillator strengths (OS’s) for some allowed transitions in Fe X are studied using extensive configuration-interaction (CI) wave functions, with relativistic effects incorporated through the Breit-Pauli Hamiltonian. The inclusion of crucial correlation effects arising from the two-electron excitation configurations ${3s}^2{3p}^3{3d}^2$ and ${3s3p}^4{3d}^2$ in the CI expansion resolves all but two of the energy discrepancies between a recent calculation and experiment. Our OS’s agree excellently with the NIST data, but significant disagreements with other calculations call for further investigations.

An effect of a wave-packet scattering off an attractive one-dimensional well is found numerically and analytically. For a wave packet narrower than the width of the well, the scattering proceeds through a quasibound state of almost zero energy. The wave reflected from the well is a polychotomous (multiple-peak) monochromatic and coherent train. The transmitted wave is a spreading smooth wave packet. The effect is strong for low average speeds of the packet, and it disappears for wide packets.

An analytic expression valid at any temperature T is derived for the interaction energy $V\left(R\right)\mathrm{}$ of two polarizable electric dipoles separated by distance R immersed in the thermal quantum field. The expression is a sum that reduces to the Casimir-Polder vacuum interaction integral for $\mathrm{KR}\ll 1$ and to the classical limit for $\mathrm{KR}\gg 1,$ where $K\equiv 2\pi k_{B}T/ħc.\mathrm{}$ For atomic dipoles with dominant absorption frequency ${\omega }_0{=k}_{0}c,$ this $V\left(R\right)\mathrm{}$ and its logarithmic derivative are evaluated numerically and plotted vs R for several values of ${\omega }_0$ and T. Also, simple approximate expressions for $V\left(R\right)\mathrm{}$ and their ranges of validity are derived. Both the numerical results and the simple approximate expressions scale with $\mathrm{KR}$ for $R\gtrsim R_2{=110/k}_0$ and with $k_{0}R$ for $R\lesssim R_3=0.74/K.\mathrm{}$ They also exhibit the London limit $\propto R^{-6}$ for $R\lesssim {0.023/k}_0,$ the Casimir-Polder limit $\propto R^{-7}$ for $R_2\lesssim R\lesssim R_3,$ if this range exists, and the classical limit $\propto R^{-6}$ for $R\gtrsim 7.4/K.$ If $k_{B}T/ħ{\omega }_0\gtrsim {10}^3,$ then $R_2\gtrsim R_3,$ and true $R^{-7}$ behavior does not occur. A physical analysis of these results and suggestions for crucial experiments are presented.

The interaction potential at finite temperature between two atoms in the asymptotic limit of large separations, $R\gg ħ{c/k}_{B}T,$ is $V\left(R\right)=-{3k}_{B}T{\alpha }^2{\mathrm{}\left(0\right)/R}^6,$ identical to the expression for two classical polarizable particles. This is in contrast to the zero-temperature, large-separation Casimir-Polder retarded potential $V\left(R\right)=-23ħc{\alpha }^2\mathrm{}\left(0\right)/4\mathrm{}\pi R^7.$ The result is demonstrated by two independent methods, so as to facilitate a reinterpretation of retardation effects in interatomic interactions. Here, we argue that the conventional mechanistic interpretation of retardation at large separations in terms of the loss of interatomic correlations due to the finite velocity of light is too simplistic, and relies on nonquantum, time-dependent concepts to explain an equilibrium, quantum effect. We offer an alternative picture in terms of the nature of the thermal excitations of the electromagnetic field, which results in a remarkable manifestation of the correspondence principle at sufficiently large interatomic distances.

The selection rules for the harmonic generation within the two-color crossed-beam scheme proposed by Tong and Chu [Phys. Rev. A 58, R2656 (1998)] can be readily proved on the basis of the dynamical symmetry of the Floquet Hamiltonian by Alon, Averbukh, and Moiseyev [Phys. Rev. Lett. 80, 3743 (1998)], represented within the dipole approximation.

The generation and propagation of attosecond pulses in He gas by using intense sub-10-fs driver pulse are numerically investigated. It is shown that a single attosecond pulse can be generated by Fourier synthesis of selected successive harmonics either in the cutoff or in the plateau regions of spectrum. In both cases the maximal electric field intensity and the duration of the attosecond pulse can be controlled by appropriately choosing the propagation length.

The spatial ring-shaped distribution of atoms in a magneto-optical trap, produced by misaligned Gaussian laser beams, is investigated with respect to the increase in the number of trapped atoms. Experimental observation of the bifurcation of a single ring into a double-ring structure is reported and explained using a model that includes radiation force due to reabsorption of photons together with the conventional radiative force.

A recent proof [H. P. Stapp, Am. J. Phys. 65, 300 (1997)], formulated in the symbolic language of modal logic, claims to show that contemporary quantum theory, viewed as a set of rules that allow us to calculate statistical predictions among certain kinds of observations, cannot be imbedded in any rational framework that conforms to the principles that (1) the experimenters’ choices of which experiments they will perform can be considered to be free choices, (2) outcomes of measurements are unique, and (3) the free choices just mentioned have no backward-in-time effects of any kind. This claim is similar to Bell’s theorem, but much stronger, because no reality assumption alien to quantum philosophy is used. The paper being commented on [W. Unruh, Phys. Rev. A 59, 126 (1999)] argues that some such reality assumption has been “smuggled” in. That argument is examined here and shown, I believe, to be defective.