I show that a closed four-level system pumped by a single laser exhibits the following features: lasing without population inversion in any state basis, lasing with population inversion in the dressed-state basis, and lasing with population inversion in the bare-state basis. The system can also exhibit a large refractive index with either zero or negative absorption.

The nonlinear interaction between transverse modes bifurcating from the trivial solution is studied for a ${\mathrm{CO}}_2$ laser with ‘‘imperfect’’ O(2) symmetry. The modes considered in this work are the Gaussian mode and the ones with angular momentum ±1. Solutions qualitatively different than the ones present in the perfect case are found, which are in agreement with the ones observed [E.J. D’Angelo et al., Phys. Rev. Lett. 68, 3702 (1992)] in recent experiments.

We study the photon correlations between the output of two spectral filters set within the fluorescence triplet of a two-state atom. The time uncertainty arising from the spectral resolution of the filters implies a possible interference between opposite orders of emission, contributing to the same detection order. Furthermore, the fluorescent emission is a quantum-mechanical process, and successive emissions in different components do not commute. The correlation functions are affected both by the memory time of the filters and by the noncommutativity of successive emissions. When the two filter bandwidths are larger than the widths of the components, this only modifies the short-time behavior of the correlation functions between two photons from different spectral components. For narrow filters, the entire correlation function is dominated by memory-time effects. Positive (bunching) correlations arise when the two filters are set at the same frequency, and also when they are positioned symmetrically at opposite sides of the driving frequency.

We study the cascade three-level selective-reflection spectroscopy for normal incidence at a dielectric–atomic-vapor interface in a pump-probe scheme. Taking into account the effects of spatial dispersion arising from deexcitation by collisions with the interface and the transient response of atoms leaving the surface, we evaluate the modification of the probe-field reflection induced by the presence of the pump field in terms of a nonlocal nonlinear susceptibility. The dependence of this susceptibility on the probe frequency for various given pump frequencies is calculated. Sub-Doppler structures in the selective reflection spectrum are obtained and analyzed. Potential applications to the study of the interaction of highly excited atomic states with the surface are indicated.

Light-induced drift arises in a binary mixture when velocity-selective optical excitation is combined with a state-dependent kinetic collision frequency (or cross section). This phenomenon is studied experimentally for rovibrationally excited ${\mathrm{C}}_2$${\mathrm{H}}_4$ immersed in the atomic buffer gases Ne, Ar, Kr, or Xe at temperatures between 275 and 500 K. In several cases, the laser-frequency dependence of the drift is observed to be anomalous and to change dramatically with temperature. This means that the relative change in collision frequency upon excitation depends strongly on velocity. To evaluate the data, an analytic model description is presented and applied to the experimental data. It is found that for each particular buffer gas, all data are well described with a relative difference in collision rate depending on a single parameter only, i.e., the average relative velocity. This dependence is found to be different for the various noble gases. This suggests that the attractive part of the interaction potential is crucial for the occurrence of anomalous light-induced drift.

Calculations of the width of avoided crossings (pseudocrossings) in the quadratic Zeeman spectrum of hydrogen are presented. By using a noncompact Lie group that reflects the approximate symmetry of the Hamiltonian, semiquantitative agreement with accurate calculations is obtained over a wide dynamic range of splittings.

Cramer’s theorem is formulated in the context of quantum optics. A physical meaning for the theorem is given and is illustrated by the generation of thermal noise from a pure quantum state.

Diffraction is shown to play a dramatic role in the frequency characteristics of ring-laser gyroscopes. It is proved theoretically and experimentally that a small misalignment of the cavity leads to a nonreciprocity of the resonant diffraction mechanism. The resonant diffraction losses of the two counterpropagating modes being then different, the asymmetries of their output-power versus cavity-frequency profiles are different. This leads to the existence of a frequency bias between the two modes. Diffraction is also shown to play a fundamental role in the lock-in mechanism. It is indeed experimentally demonstrated that the asymmetric evolution of the lock-in threshold with the laser frequency is governed by resonant diffraction mechanisms.

Nondegenerate oscillation is studied in the limit of a fast-decaying pump mode. The study is facilitated by analytic solutions of the positive P representation. In the absence of all individual signal and idler losses, states of definite signal-idler photon-number difference are generated. Determination of this photon number enables preparation of a macroscopic superposition state.

We calculate a neutron’s transition amplitudes to various energy and spin states when it interacts with a magnetic field oscillating in time. Two field configurations are analyzed: the oscillating component normal to the static component when the rotating-wave approximation is inapplicable and the oscillating and static components parallel to each other. The two cases require different mechanisms of angular momentum conservation. Experimental tests are proposed including neutron interferometry and double crystal diffraction, the latter permitting observation of quantized energy transfer from fields oscillating at only a few hundred kilohertz.

We consider the motion of a ‘‘slow’’ atom through a single-mode quantized radiation field; for example, a high-Q microwave cavity. It is shown that under some circumstances different photon-number states may be resolved by observation of the atomic passage time. Moreover, measurement of one or more passage times may prepare a photon-number state that can be detected in a nondestructive manner.

We have extended the unified-colored-noise approximation proposed by Jung and Hänggi [Phys. Rev. A 35, 4464 (1987)] to the multidimensional case. The method has been used to obtain the Fokker-Planck equation of two dye-laser models with saturation terms: the loss-noise model (model A) and the gain-noise model (model B). The approximate formulas of the mean, variance, and skewness of first-passage-time distribution (FPTD) for the models are derived. The Fokker-Planck equation we obtained does not suffer from the problem of negative diffusion functions. They are valid for the whole range of the variable x. Based on the feature of the Fokker-Planck equation obtained in this paper, our results of the mean, variance, and skewness of the FPTD for models A and B are not restricted to the small correlation time of the pump fluctuation. It is worth noting that the prefactor of the pump fluctuation term in the model B seems to play a curb role on the increase of the variance and skewness.

Two laser pulses that overlap in time have been successfully used for coherent excitation of a molecule or atom to a desired state. An intermediate state that is at or near resonance with the laser frequencies has been used in past experiments and calculations. Here we replace this intermediate state by a continuum of intermediate states. An analytic solution of a simple model suggests that the continuum can be used in such an excitation scheme, provided that the laser pulses are arranged in the so-called counterintuitive order. We use a special case of this solution to relate this work to ionization suppression and coherent population trapping.

The concept of the classical effective Rabi field, which connects quantum-mechanical vacuum Rabi oscillations with semiclassical Rabi oscillations is introduced. The effective Rabi field associated with the vacuum Rabi oscillation expected in the spontaneous emission in a high-Q cavity is shown to have contributions not only from the vacuum fluctuation but also from the emitted field from the atom. If the cavity has finite loss, the latter field decays with time while the vacuum fluctuation maintains its magnitude.

A Fox and Li analysis [Bell Syst. Tech J. 40, 453 (1961); Proc. IEEE 51, 80 (1963); IEEE J. Quantum Electron QE-4, 460 (1968)] for a laser resonator with a radially varying gain yields a picture of the mode evolution dynamics and a steady-state mode distribution which are different from the constant-gain case. When an axially maximal gain is sharply peaked, the mode-settling time is markedly reduced and the final mode distribution is more tightly focused. When the gain distribution is sharply annular, an annular or ‘‘doughnut’’-shaped mode develops. In some intermediate cases, which may be relevant to some gas lasers, a stable transverse mode never definitively evolves. Misalignments of the gain with the cavity yield distortions to the mode shape and these are also explored.

We examine the quantum noise characteristics of the field produced by a phase-conjugate resonator formed from two mirrors, one of which is a phase-conjugate mirror. We show that the output field on the side of the phase-conjugate mirror can be completely squeezed. Numerical results for the quantum-statistical properties of the forward and backward output fields are given as functions of the reflectivities of the two mirrors.

We show that a closed four-level system can exhibit lasing without population inversion in any state basis. The same system can be also made to exhibit lasing without population inversion in the bare-state basis but with population inversion in the dressed-state basis. We derive the conditions of lasing without inversion and examine the dependence of the gain on various system parameters. The lasing threshold and the steady-state photon numbers are calculated. Comparison with a ssV-type, three-level model is presented.

Using the Wigner function formalism in phase space, we analyze the decay of quantum coherences in phase-sensitive reservoirs. We show that the decay rate of quantum coherences in phase-sensitive reservoirs can be significantly modified compared to the decay rate in ordinary (phase-insensitive) thermal reservoirs. Depending on the phases of the quantum system (field mode) and the squeezed reservoir, the decay rate of the quantum coherence can be either enhanced or significantly suppressed, which is in agreement with the results obtained recently by other methods [T. A. B. Kennedy and D. F. Walls, Phys. Rev. A 37, 152 (1988)]. We show that in an ideally squeezed reservoir with a high degree of squeezing, the decay rate of the quantum coherence (i.e., the decay rate of off-diagonal terms of the density matrix in the coherent-state basis) can be equal to the decay rate of the energy of the system (i.e., the decay rate of diagonal terms of the density matrix). Suppression of the decay rate of the quantum coherence leads to preservation of nonclassical effects such as the oscillations in the photon number distribution. Moreover, we find that some initial superposition states of light exhibiting super-Poissonian photon statistics can be transformed into intermediate sub-Poissonian states under the influence of phase-sensitive reservoirs.

We study the effect of finite atomic lifetimes on a micromaser from temporal evolution. A stream of finite-lifetime two-level atoms is injected inside a cavity in a coherent superposition of their upper and lower levels. We study the effect of the atomic decay on the generation of quasipure states of the inner electromagnetic field. First, we study the stationary photon number distribution when the atoms are injected in their upper level. We compare our results with previous calculations by Zhu, Wang, and Fearn [Phys. Rev. A 44, 737 (1991)]. We find that quasinumber states can be generated for a long time, in a range of physical parameters that allows their experimental realization. Second, we investigate the effect of finite atomic lifetimes on the generation of cotangent states, and we study their squeezing properties.

In this paper, we investigate the reflection and transmission of an atomic wave by an optical ripple mirror which may be realized by a standing-wave laser field. We show that the ripple mirror can operate as a separator of the incident atomic waves corresponding to different internal states by reflection and transmission. For different laser parameters, the atomic ripple mirror can reflect one linear combination of two ground states and transmit the orthogonal linear combination. The transmitted superposition state corresponds to the well-known ‘‘dark’’ state. The atomic wave function is a coherent superposition of reflected and transmitted waves entangled with different internal states.