We analyze the effects of a reservoir having a generic spectrum (colored noise) on the index of refraction of a system of strongly driven two-level atoms (gas) probed by a weak field. We show that for high Rabi frequencies, $\Omega ,$ a simple analytic expression results for the susceptibility function (with respect to the probe) when $\nu \approx \Omega ,$ where $\nu$ is the detuning between driving and probe fields frequencies. Several distinct features are revealed in that function when compared to the one resulting from a reservoir with a white-noise spectrum (atom-vacuum coupling). We also show that the Mach-Zehnder interferometry could permit distinguishing between the two kinds of spectra and we point out the sensitive dependence of the index of refraction of the atomic gas on $\Omega$ and find a signature for the non-Markovian evolution.

The dynamics of a solid-state laser sustaining the oscillation of two orthogonally polarized eigenstates and passively Q switched by a ${\mathrm{Cr}}^{4+}:\mathrm{Y}\mathrm{A}\mathrm{G}$ (YAG denotes yttrium aluminum garnet) saturable absorber is experimentally investigated. A set of rate equations in which the orientation-dependent interactions between the intensities and the saturable absorber have been taken into account is developed to describe theoretically the evolution of such a laser. When a [001]-cut ${\mathrm{Cr}}^{4+}:\mathrm{Y}\mathrm{A}\mathrm{G}$ plate is used, orthogonally polarized pulsed eigenstates are emitted either successively or simultaneously, depending on the orientations of the eigenstates with respect to the crystallographic axes. Namely, in the latter case, beat note carrying pulses are emitted, whose beat frequency is continuously tunable. On the other hand, when a [111]-cut ${\mathrm{Cr}}^{4+}:\mathrm{Y}\mathrm{A}\mathrm{G}$ plate is used, the simultaneity regime is never observed. All experimental results, obtained using a diode pumped Nd:YAG laser, are well confirmed by the theoretical model.

A general theory is presented for the photodetection statistics of coherent radiation that has been amplified by a disordered medium. The beating of the coherent radiation with the spontaneous emission increases the noise above the shot-noise level. The excess noise is expressed in terms of the transmission and reflection matrices of the medium, and evaluated using the methods of random-matrix theory. Intermode scattering between N propagating modes increases the noise figure by up to a factor of N, as one approaches the laser threshold. Results are contrasted with those for an absorbing medium.

A three-dimensional quantum nonperturbative theory of spontaneous emission in a planar microcavity is developed on the basis of a complete set of orthogonal standing-wave mode functions. Earlier results on the spontaneous decay rate and far-field emission pattern which have been obtained in a perturbative fashion are shown to be contained in our theory. Time evolution of the initially excited atomic state is considered and comparison with perturbative results is given. It is found that though the Fabry-Perot cavity is of an open type, vacuum-field Rabi oscillations are still possible. Conditions that are favorable for this strong coupling regime are discussed. The near-field spontaneous-emission pattern and the transition from the near field to the far field are investigated. For moderate mirror reflectivities, the spread in the near-field emission pattern is found to be in good agreement with the effective mode radius concept. When the mirror reflectivity increases, however, one has a strong coupling regime and together with it, multiple reabsorptions and reemissions of the photon may occur, leading to a better localization of the photon around the atomic position. These considerations may be useful in designing microlasers of the planar type.

The standard quantum limit to the linewidth of a laser for which the gain medium can be adiabatically eliminated is $l_0=\kappa /2\mathrm{}\mathrm{n̄}.$ Here $\kappa$ is the intensity damping rate and $\mathrm{n̄}$ the mean photon number. This contains equal contributions from the loss and gain processes, so that simple arguments which attribute the linewidth wholly to phase noise from spontaneous gain are wrong. I show that an unstimulated gain process actually introduces no phase noise, so that the ultimate quantum limit to the linewidth comes from the loss alone and is equal to $l_{\mathrm{ult}}=\kappa /4\mathrm{}\mathrm{n̄}.$ I investigate a number of physical gain mechanisms which attempt to achieve gain without phase noise: a linear atom-field coupling with a finite interaction time, a nonlinear atom-field coupling, and adiabatic photon transfer using a counterintuitive pulse sequence. The first at best reaches the standard limit $l_0,$ the second reaches $\frac{3}{4}{l}_0,$ and the third reaches the ultimate limit of $l_{\mathrm{ult}}=\frac{1}{2}{l}_0.$

Starting from the quantized version of Maxwell’s equations for the electromagnetic field in an arbitrary linear Kramers-Kronig dielectric, the Heisenberg equations of motion for a two-level atom resonantly coupled to the radiation field in the presence of dispersive and absorbing dielectric bodies are derived. The theory is applied to the problem of spontaneous decay of excited atoms in absorbing media. The decay rate is calculated for the (Glauber-Lewenstein) real-cavity model, and a comparison with the recently studied (Clausius-Mosotti) virtual-cavity model [S. Scheel, L. Knöll, D.-G. Welsch, and S. M. Barnett, Phys. Rev. A 60, 1590 (1999)] is given. It is shown that owing to nonradiative decay associated with absorption, the rate of spontaneous decay sensitively depends on the cavity radius, particularly when the atomic transition frequency approaches an absorption band of the medium. Only when the effect of absorption is fully disregarded, is the familiar local-field correction factor recovered.

The correlated intensity fluctuations in the two polarization modes $(x$ and $y)$ of a ${\mathrm{TEM}}_{00}$ vertical-cavity semiconductor laser are studied experimentally and theoretically. We show that the dynamics of the laser polarization and total output power are almost completely decoupled, and demonstrate how the frequency dependence of the correlation function $C_{\mathrm{xy}}$ is related to dichroism and relaxation oscillations.

The Bogoliubov dispersion relation for the elementary excitations of the weakly interacting Bose gas is shown to hold for the case of the weakly interacting photon gas (the “photon fluid”) in a nonlinear Fabry-Perot cavity. The chemical potential of a photon in the two-dimensional photon fluid does not vanish. The Bogoliubov relation, which is also derived by means of a linearized fluctuation analysis in classical nonlinear optics, implies the possibility of a new, superfluid state of light. The theory underlying an experiment in progress to observe sound waves in the photon fluid is described, and another experiment to measure the critical velocity of this superfluid is proposed.

This paper investigates the generation of quadrature-squeezed states of light using a degenerate optical parametric amplifier (DOPA) that is pumped by a focused Gaussian beam. The formulation that is presented facilitates the calculation of squeezing for an arbitrary local oscillator beam. This formulation also establishes a formal equivalence between the classical parametric gain and the measured level of squeezing. The maximum squeezing that can be achieved using a Gaussian-beam local oscillator is determined to be limited to 13.4 dB, as a consequence of gain-induced diffraction. The phase lag of the maximally squeezed quadrature is shown to be significantly different from the plane-wave theoretic value of $\pi /2,$ unless the focusing is very weak. The use of a second DOPA for generating a local oscillator beam that is matched to the squeezed field is also investigated. In this case, squeezing is limited only by the available pump power.

An analysis of the Colle-Salvetti (CS) wave-function functional of the density as applied to the He atom shows: (i) it is not normalized; (ii) the corresponding Coulomb hole structure is inaccurate; (iii) the Coulomb hole sum rule is violated; (iv) the Coulomb component of the Kohn-Sham (KS) correlation potential is inaccurate; (v) the KS correlation potential is erroneous; (vi) the Coulomb correlation and correlation–kinetic-energy components of the KS correlation energy are in error. Thus, the description by this wave function of the physics of electron correlation is inaccurate. As such the results obtained via the CS wave function and those based on it are not well founded.

The two-center Coulomb problem (with opposite signs of charges) for the Dirac equation is solved by the method of matching the logarithmic derivatives of the asymptotic solutions. The formulas for the near continuum-state energy term of a relativistic electric-dipole system are obtained analytically. Two cases are considered: $Z<\mathrm{}137\mathrm{}$ and $Z>\mathrm{}137.\mathrm{}$ The Dirac equation for $Z>\mathrm{}137\mathrm{}$ is solved by the usual method of a cutoff potential at small distances.

It is argued that the nonlocality without entanglement discovered quite recently by Bennett et al. [Phys. Rev. A 59, 1070 (1999)] should rather be interpreted as the Einstein-Podolsky-Rosen paradox without entanglement. It would be a true demonstration of nonlocality without entanglement if one knew that quantum mechanics provides the best possible means for extracting information from a physical system, i.e., that it is “informationally complete.”

We propose a quantum computer structure based on coupled asymmetric single-electron quantum dots. Adjacent dots are strongly coupled by means of electric dipole-dipole interactions enabling rapid computation rates. Further, the asymmetric structures can be tailored for a long coherence time. The result maximizes the number of computation cycles prior to loss of coherence.

The lifetimes of 16 levels in Tm II have been measured using the fast-beam-laser technique to resolve the disagreement between time-resolved laser-induced fluorescence measurements recently published by Anderson, Den Hartog, and Lawler [J. Opt. Soc. Am. B 13, 2382 (1996)] and a new relativistic Hartree-Fock calculation by Quinet, Palmeri, and Biémont [J. Quant. Spectrosc. Radiat. Transf. 62, 625 (1999)]. In all cases except one, our results support the previous measurement, but with a higher precision.

We show that employing more stringent inequalities in the derivation of uncertaintylike relationships can improve their accuracy. In particular, Eq. (23) due to Romera et al. [Phys. Rev. A 59, 4064 (1999)] can be further improved using the Faris inequalities rather than using the Hölder inequalities.

The x-ray spectrum of heliumlike ${\mathrm{Ti}}^{20+}$ excited by a monoenergetic electron beam was measured at an energy just above the electron-impact excitation threshold (4800 eV) with a pair of polarization-sensitive spectrometers. Values for the linear polarization were inferred for the $1s2p{}^1{P}_1\to {1s}^2{}^1{S}_0$ resonance line, the $1s2p{}^3{P}_1\to {1s}^2{}^1{S}_0$ intercombination line, the $1s2p{}^3{P}_2\to {1s}^2{}^1{S}_0$ magnetic quadrupole line, and the $1s2s{}^3{S}_1\to {1s}^2{}^1{S}_0$ forbidden line, as well as for the $1s2s2p{}^2{P}_{3/2}\to {1s}^{2}2s{}^2{S}_{1/2}$ resonance line in ${\mathrm{Ti}}^{19+}.$ Relativistic distorted-wave calculations of the magnetic sublevel-specific electron-impact excitation cross sections are presented for comparison. Combined with earlier measurements of iron, the results confirm the predicted Z dependence of the polarization.

Wannier theory is critically reexamined and compared with semiclassical S-matrix theory, which serves as a reference calculation. We find that for certain situations classical half-collision calculations with a fixed reaction zone radius strongly differ from the results of a full S-matrix treatment even at energies very close to threshold. By giving a parameter-free description of an energy-dependent reaction zone radius we take the initial state more accurately into account in the half-collision approach. This extension produces results for ionization probabilities which are in excellent agreement with the full-collision semiclassical S-matrix theory.

We report on a substantial light-force-induced line-center shift of the sub-Doppler dip in a saturation spectroscopy configuration where mechanical effects are expected to be negligible. We observe the shift on the $2{}^3{S}_1\to 2{}^3{P}_1$ open transition of ${}^4\mathrm{He}$ that is relevant for a determination of the fine-structure constant $\alpha$ [Phys. Rev. Lett. 82, 1112 (1999)]. We discuss the physical origin of the shift and we provide a numerical analysis to support our interpretation. The shift can be quite important for various other high-precision spectroscopy measurements.

Bose-Einstein condensation of an ideal Bose gas trapped in any-dimensional space is studied for the finite particle number effects. The corrections of statistical properties due to the finite particle number effects are obtained. We find that these corrections are more significant for the low-energy excited states which are the focus of our interest. We also find that the asymmetry of the external potential intensifies these effects.

We study the Bose-Einstein condensate in the MIT double-well trap. We calculate the ground-state density profile of ${}^{23}$Na atoms and the Bogoliubov spectrum of the elementary excitations as a function of the strength of the double-well barrier. In particular, we analyze the behavior of quantum-mechanical collective excitations. Finally, we discuss the observability criteria for macroscopic quantum tunneling and macroscopic quantum self-trapping.