We investigate the quantum noise gained by a weak probe laser interacting with a driven two-level system in the Doppler-broadening limit in which the so-called ‘‘dead zone’’ of the absorption spectrum is formed. We find that the plot of the noise as a function of the pump-probe detuning (which we shall refer to as the ‘‘noise spectrum’’) is hardly affected by the Doppler broadening for the case of a resonant pump. It consists of a large central peak flanked by two smaller peaks at the edges of the dead zone, which are at the Rabi sideband frequencies. Thus the dead zone, which is produced when absorption and gain regions of the absorption spectrum are averaged by Doppler broadening, is a rather noisy region, indicating the occurrence of many competing absorption and emission processes. The probe noise spectrum for a detuned pump is quite different in the presence and absence of Doppler broadening as is the absorption spectrum. On Doppler broadening the absorption spectrum consists of a dead zone centered at the Rayleigh resonance frequency with asymmetric absorption peaks on either side. The noise spectrum has a peak at the center with two sidebands at the edges of the dead zone. The sidebands are of almost the same intensity for short path lengths, but the peak on the side of the three-photon scattering becomes larger than the other peak as the path length increases.

We discuss the conditions which give rise to anomalous features in the resonance fluorescence of a two-level atom interacting with a resonant classical field and a squeezed vacuum. A simple expression is obtained, which is shown to coincide closely with the conditions for amplification of a probe beam and with the collapse of the atomic system into a pure state. We thereby illustrate striking observable consequences of the evolution of a driven system into a pure state. This situation also provides a method for preparing an atom in a pure state whose nature depends on the squeezing phase and Rabi frequency. The conditions for hole burning are obtained analytically, and the sensitivity of these effects to the two-photon correlations produced by the squeezed vacuum is demonstrated.

We discuss the theory of extracting an interaction Hamiltonian from a preassigned unitary transformation of quantum states. Such a procedure is of significance in quantum computations and other optical information processing tasks. We particularize the problem to the construction of totally symmetric 2N ports as introduced by Zeilinger and his collaborators [A. Zeilinger, M. Zukowski, M. A. Horne, H. J. Bernstein, and D. M. Greenberger, in Fundamental Aspects of Quantum Theory, edited by J. Anandan and J. J. Safko (World Scientific, Singapore, 1994)]. These are realized by the discrete Fourier transform, which simplifies the construction of the Hamiltonian by known methods of linear algebra. The Hamiltonians found are discussed and alternative realizations of the Zeilinger class transformations are presented. We briefly discuss the applicability of the method to more general devices.

We have demonstrated the triple-pulse pumping of a Ne-like Ge soft-x-ray laser at 19.6 nm in amplified spontaneous emission (ASE) and amplifier modes. Two-dimensional beam characteristics have been investigated with a curved slab target. In comparison with the double-pulse pumping scheme, the increase of output intensity of the soft-x-ray laser has been obtained with the triple-pulse pumping scheme in the ASE mode. The amplifier mode of the Ne-like Ge soft-x-ray laser has been achieved with the triple-pulse pumping scheme. Significantly different beam intensity distributions between the horizontal (x) and vertical (y) directions have been observed. A hydrodynamic simulation coupled with ray-trace calculations has been made with the curved slab targets. The ray-trace calculations show different beam propagation in the x and y directions due to different electron density distributions. The beam divergence has also been calculated in the ASE and amplifier modes and compared to the measured beam divergence. Based on the experimental and simulation results, we conclude that there are different beam characteristics in the two directions. There is beam guiding in the x direction but not in the y direction.

Based on the polarization interference between one-photon and two-photon processes, we demonstrated an ultrafast modulation spectroscopy in a cascade three-level system (UMSCTS) in Na vapor, which showed a modulation of the four-wave mixing signal intensity with a 55-fs period, corresponding to the beating between the resonant frequencies of the transitions from 3${\mathit{S}}_{1/2}$ to 3${\mathit{P}}_{3/2}$ and from 3${\mathit{P}}_{3/2}$ to 4${\mathit{D}}_{3/2,5/2}$. We have also discussed the possibility of using UMSCTS to measure the energy levels of the excited states with Doppler-free precision.

Quantum fluctuations of optical solitons in fibers are discussed on the basis of the exact quantum states in the frame of the quantum nonlinear Schrödinger equation. Position and momentum fluctuations as well as quadrature phase fluctuations are calculated, and explicit results are expressed in terms of the intrinsic quantum parameters. Soliton wave-packet spreading and phase-diffusion phenomena are clearly demonstrated, and the corresponding squeezing effects are analyzed. In particular, soliton amplitude squeezing is investigated in an experimental setup of a Mach-Zehnder interferometer containing an optical fiber. Coherent states for quantum solitons are discussed with attention given to their dynamical preservability. Compared with previous linearized approaches, the present study may provide a more fundamental treatment to quantum fluctuations of solitons.

We present an experimental study of cw nearly degenerate four-wave-mixing emission in a Doppler broadened Rb atomic system. With the fixed detuning of an intense pump laser for the Rb ${\mathit{D}}_2$ line, the nearly degenerate four-wave-mixing emission exhibits three peaks as a probe laser is scanned across the same Rb ${\mathit{D}}_2$ line. The observed three-peaked spectrum can be interpreted as the resonances in the energy structure of the dressed atomic states.

We consider a quantum model for a degenerate optical parametric oscillator below threshold in a cavity with spherical mirrors. The spatial correlation function of a generic quadrature component of the signal field is calculated analytically in terms of an expansion over the Gauss-Laguerre modes. The critical behavior and the quantum image phenomenon, which emerge when threshold is approached, are described in detail. We calculate also the spatial correlation function of the intensity fluctuations, which can be more easily measured by direct detection and exhibits the same qualitative features.

We propose a general method for reconstructing directly the density matrix of a single light mode in optical homodyne tomography. In our scheme the density matrix 〈a‖ρ^‖${\mathit{a}}^{\prime }$〉 is obtained by averaging a set of pattern functions ${\mathit{F}}_{\mathit{aa}}^{\prime }$(${\mathit{x}}_{\mathrm{\theta }}$,θ) with respect to the homodyne data ${\mathit{x}}_{\mathrm{\theta }}$. The functions show the typical features of the quadrature distributions for the corresponding density-matrix elements. It is also possible to compensate the effect of detection losses which requires, however, extra effort in both experimental and numerical precision. We calculate the pattern functions for the coherent-state and Fock representations and study their properties. We believe that our method is the most efficient way for reconstructing the density matrix from homodyne measurements.

We present a study of the higher-order subthreshold transverse modes in a single-frequency laser resonator. A theory is developed to describe the gain behavior when an input beam is reinjected into the laser, and tuned around the resonances that correspond to each of the subthreshold transverse modes of the cavity. Some of the predictions of the theory are confirmed by experiments with an argon-ion laser and these experiments also illustrate the use of spatial filtering for optimization of the gain, as well as an associated increase in intensity-fluctuation noise. Further experiments have been carried out to demonstrate the perturbation of the modes caused by the insertion of intracavity obstructions; the data are compared with the results from theoretical modeling of the perturbed resonator. In addition, the experimental techniques are used to track the behavior of a transverse mode as it approaches, and finally exceeds, the lasing threshold.

The free-induction decay (FID) after saturation by a laser radiation pulse of finite duration is studied for systems with spectral diffusion. The exact solution of the FID signal shape has been obtained in the framework of the telegraph noise model. This solution takes into account the finite duration of the saturating field and is valid at arbitrary values of the spectral exchange rate and the amplitude of the coherent field. The exact expression of the FID signal is derived in the weak-external-field limit. It is shown that the FID signal suddenly disappears after a time equal to the pulse width. This confirms the validity of the theorem on coherent transients, which has been proved for the Bloch equations [A. Schenzle, N. C. Wong, and R. G. Brewer, Phys. Rev. A 22, 635 (1980)].

We propose a scheme for amplification of faint optical images with coherent light which preserves the signal-to-noise ratio in the image. The scheme consists of a ring-cavity optical parametric amplifier as an active element and two imaging lenses. This system can amplify images either ‘‘point by point,’’ when a small region is amplified at a time, or by narrow rings, depending on its operation; in both cases it works as an image scanner. We have evaluated the inherent noise of this amplification scheme which has its physical origin in the phenomenon of spontaneous parametric radiation. This noise gives a lower ultimate limit for the intensity of the image that can be noiselessly amplified by our scheme. We have estimated spatial resolution of the scheme and formulated criteria for its noiseless operation.

We present an analytical solution of the Maxwell-Schrödinger set of equations describing propagation of a two-component short electromagnetic pulse through a medium consisting of atoms with a @sV-type level scheme with an initially inverted population. This solution demonstrates solitonlike waves traveling at a velocity exceeding the vacuum speed of light c (or even a negative velocity) until they reach the forward (or, respectively, trailing) pulse front. The front propagation velocity recognized as information transmission velocity is explicitly found to be equal to c.

A theoretical treatment is given of the dynamical and steady-state behavior of an anisotropic laser. The model is based on a homogeneously broadened, pumped J=1/2⇆J=1/2 atomic transition, and is solved to all orders of the electric field, in contrast to previous studies of anisotropic lasers. A survey of the system behavior is presented, and complementary analytic descriptions developed to describe the small- and large-field evolutions are given. The small-field polarization behavior is shown to be oscillatory in general, and dominated by the properties of the cavity. The large-field description allows the polarization stability of the final output states to be analyzed, and shows that stable steady states in this regime must be linearly polarized along either one or the other of the two anisotropy axes. The possible range of cavity lengths which allows bistable output between these two polarizations is characterized analytically, and a regime identified where the polarization undergoes sustained oscillation.

We consider a state of N correlated particles, each with spin j, in which the quantum expectation value of an operator A^ is evaluated. This is found to exceed a bound on A^ obtained from considerations of classical realism. It is indicated from numerical results that this contradiction between quantum mechanics and classical realism persists in the ‘‘strong’’ macroscopic limit, i.e., when the values of both N and j are arbitrarily increased. In this argument, the notion of classical realism embodies the twin assumptions of locality (no action at a distance) and noncontextuality (a measurement simply reveals the preexisting value of an observed quantity).

The role of anapole moments (a→) arising from the P-violating and T-invariant theory of electron-nucleon interactions in the atom is examined. The standard model of the electroweak theory is used to compute numbers for ‖a→‖ for alkali-metal atoms and muonic hydrogen. The physical implications of the results obtained are discussed.

In this paper we examine the reported comparisons between theoretical and experimental results for the ratio between the Lyman-α line fine-structure components of the hydrogenlike ions Ti XXII, Cr XXIV, and Ni XXVIII, observed in tokamak plasmas. We also present a table of calculated Lyman-α doublet intensity ratios for these ions for a broad range of plasma temperatures and densities.

The Coulomb potential produced by an ultrarelativistic particle (such as a heavy ion) in uniform motion is shown in the appropriate gauge to factorize into a longitudinal δ(z-t) dependence times a simple two-dimensional potential solution in the transverse direction. This form makes manifest the source of the energy independence of the interaction.

Electron excitation from the fundamental state of hydrogen atoms by impact of bare ions is studied at intermediate and high collision velocities. Total cross sections for final np states by impact of protons, alpha particles, and ${\mathrm{He}}^{+}$ ions are calculated using the symmetric eikonal approximation and compared with experimental data. This comparison supports the existence of distortion effects recently predicted by Bugacov and co-workers [Phys. Rev. A 47, 1052 (1993)]. The validity of scaling laws is analyzed.

It is shown that the high-frequency approximation describes the lack of stabilization for the zero-range models in dimensions higher than 1. The same conclusion has been drawn from the exact solution of this model in the three-dimensional space. Hence, there are no contradictions between the predictions of the exactly soluble models and the high-frequency approximation. On the other hand both the exact numerical analysis and this approximation leads to the stabilization for the one-dimensional δ potential. This means that the high-frequency approximation does correctly predict both the presence and the absence of stabilization effects.