The evolution of squeezed and displaced number states in the free and dissipative third-order nonlinear oscillator is investigated from the point of view of nonclassical phenomena as the number squeezing in both the strong and the weak sense, the principal squeezing of vacuum fluctuations, and the generation of superposition states. A destructive effect of losses on quantum coherence is demonstrated.

Two theorems on the group velocity are presented in this paper. First a simple proof is given that for any dispersive dielectric, there must be a frequency at which the group velocity of an electromagnetic pulse becomes abnormal, i.e., greater than the vacuum speed of light, infinite, or negative. Second, at the frequency at which the attenuation (or gain) is a maximum, the group velocity must be abnormal (or normal). This second theorem is more widely applicable, e.g., to propagation in waveguides or through multilayer dielectrics. To illustrate these theorems we discuss dispersion in a medium with two resonance lines, one absorption and the other gain. We find that the group velocity is abnormal within the absorption line and in a transparent region outside the gain line.

The steady-state lasing and the photon statistics have been analyzed for a closed four-level-laser system pumped only by a single coherent field. The system exhibits a transition from light amplification by population inversion to light amplification by coherence. Under suitable operating conditions, the laser-intensity fluctuations may be reduced more than 50% below the shot-noise limit.

Milburn has recently proposed [Phys. Rev. A 44, 5401 (1991)] a variation of the Schrödinger equation which models decoherence as the system evolves through intrinsic mechanisms beyond conventional quantum mechanics rather than dissipative interaction with an environment. In this paper we give an exact solution of this equation and apply it to the Jaynes-Cummings model of atom-field interaction with nontrivial dynamics. We show that the intrinsic decoherence is responsible for deterioration of quantum coherence effects in this model, such as revivals of the atomic inversion. We discuss the applicability of the Milburn equation.

High-order harmonic radiation from a thin solid layer illuminated by a subpicosecond laser pulse is investigated. For laser intensities ${\mathit{I}}_{\mathit{L}}$ and wavelengths ${\lambda }_{\mathit{L}}$ with ${\mathit{I}}_{\mathit{L}}$${\lambda }_{\mathit{L}}^2$>${10}^{14}$ W μ${\mathrm{m}}^2$/${\mathrm{cm}}^2$, free electrons oscillate with amplitudes δ extending over many lattice periods ${\mathit{l}}_{\mathit{c}}$ of the solid and, due to interaction with the lattice, radiate a spectrum of harmonics with a sharp cutoff at ${\mathit{n}}_{\max }$≃2πδ/${\mathit{l}}_{\mathit{c}}$. Laser pulses (of order 100 fs) shorter than the lattice disintegration time are required. The harmonics emission crucially depends on electron correlations. Competing spontaneous atomic radiation is discussed.

The absorption spectrum for weak radiation of a two-state atom near the surface of a phase-conjugating four-wave mixer has been evaluated. Also included is the option that the atom is illuminated by a near-resonant laser field. In the secular limit with respect to the laser-dressed atomic states the absorption spectrum has two lines, just as for a free atom. The strengths and the widths of these lines depend on the phase-conjugate reflectivity, as well as on the optical parameters. It is shown that increasing the reflectivity drives the atom towards saturation through three-photon processes. Also the driving laser tends to saturate the transition, but it is shown that both mechanisms have a different effect on the absorption profile, and can hence be distinguished experimentally.

We have studied the diffractive coupling of Stokes, anti-Stokes, and pump fields in stimulated Raman scattering. We find that, depending on the strength of the anti-Stokes field relative to that of the Stokes, both soliton dynamics and beam-pointing fluctuations are affected for finite Fresnel numbers. The generation of the anti-Stokes waves can inhibit a gain-focusing mechanism that we predicted for the Stokes field, when a soliton propagates in the medium. Although no qualitative differences arise in the beam-pointing fluctuations of the Stokes field, we predict that the anti-Stokes beam may be much more stable than the Stokes counterpart. Our analysis shows that for a Fresnel number of 5, the difference can be as large as 70%, while for Fresnel numbers of order unity, it is nearly 10%.

We describe a hydrogen maser operating at very low temperatures in which most of the hydrogen atoms are condensed on a superfluid helium surface in long-lived states. This proposed maser can be used to obtain information on the properties of the hydrogen–liquid-helium-surface system. In addition, it promises to be an interesting system from the point of view of nonlinear dynamics. It is found that the surface recombination to molecular hydrogen, which might be considered as undesirable, is actually necessary to achieve the masing conditions. We develop the maser equations and consider a number of realistic conditions for operation.

We calculate intracavity photon-number and extracavity photocount distributions for a pump-noise-suppressed laser, using both the Lamb-Scully laser model in the number-state representation and the Haken-Risken laser model in the coherent representation. When pump fluctuations and spontaneous decay are suppressed, a pump-noise-suppressed laser has intracavity photon-number fluctuations squeezed by a factor up to 1/2. The extracavity photocount fluctuations are at the Poisson level for counting times much shorter than the cavity lifetime, whereas the variance for photocounts can be reduced to the mean of the intracavity photon number, which is markedly sub-Poissonian, under ideal conditions when the counting time is much longer than the cavity lifetime.

We present a comparative analysis of trilinear parametric processes for photon-number detection. Attention is focused on those processes that correspond to constituent devices in optical networks, such as second- and subharmonic sum- and difference-frequency generation and phase-sensitive and -insensitive amplification and attenuation. Gains, output Fano factors, and noise figures are given for input in both coherent and number eigenstates. The fully quantum processes (i.e., with no classical pump) are evaluated numerically. It is shown that the depletion of the quantum pump is never complete and this accounts for additional noise at the output. Second-harmonic and sum-frequency generation turn out to be more efficient and less noisy than the respective down-conversion processes. Ideal behaviors are also reported for comparison and the results are discussed with a brief parallel analysis of the various devices.

An analytic expression for the quasiprobability distributions of Cahill and Glauber [Phys. Rev. 177, 1882 (1969)] of the Jaynes-Cummings model is derived in the rotating-wave approximation for a vanishing thermal photon number. First the equations of motion for the density operator are transformed to a system of partial differential equations for the quasiprobability distributions. By a suitable expansion of these distributions into a Fourier series and into Laguerre functions a system of ordinary tridiagonally coupled differential equations for the expansion coefficients is obtained. By an appropriate choice of a scaling parameter and by a proper elimination procedure it is shown that the coefficients are only coupled to coefficients with the same index and to coefficients with the next upper index. Because of this coupling the Laplace transform can be given analytically. By using a partial fraction decomposition the eigenvalues and eigenvectors can be obtained in closed forms, thus leading to a simple analytic solution for the long-time behavior of the quasiprobability distributions. Finally it is shown that the method can also be applied if additional atomic damping is present.

The statistical properties of fluorescent light from a single two-level atom driven by nonclassical light beams that exhibit antibunching and squeezing are discussed in the weak-field limit. The fields produced in intracavity second-harmonic generation and those produced by superposing the light from a degenerate parametric oscillator with a coherent field are considered to be the models of nonclassical light beams. These fields can be described in terms of real Gaussian random processes with nonzero mean coherent components. Analytical expressions for the fluorescent light intensity, the second-order intensity correlation function, and the variance of the photon number in the steady state are obtained. The effects of driving-field correlations, detuning, and other parameters characterizing the atom-field interaction on the statistics of scattered light are investigated. It is found that for a certain range of parameters the second-order intensity correlation function may vanish at nonzero times. Under certain other circumstances the antibunching of the fluorescent photons is enhanced. These and other interesting features that appear in the scattered light are discussed.

The negative ions of the isotopes ${}_{}{}^{226}{}_{}{}^{}\mathrm{Ra}$, ${}_{}{}^{231}{}_{}{}^{}\mathrm{Pa}$, and ${}_{}{}^{244}{}_{}{}^{}\mathrm{Pu}$ have been observed by means of accelerator mass spectrometry and their properties compared with the negative ions of Th and U. The electron affinities of all these elements have been estimated to be similar and greater than 50 meV.

The local Wigner exchange-correlation functional and its scaled analog are tested by self-consistent calculations on ${\mathrm{H}}_2$ and ${\mathrm{N}}_2$ determining energy curves and atomic polarizabilities. The kinetic-energy contribution, which results from coordinate scaling of the exchange-correlation potential energy, is shown to be important in the density-functional theory.

Starting from the previously derived scattering matrix [P. Šťovíček, Phys. Lett. A 161, 13 (1991)] the differential cross section is calculated for electrons scattering on two infinitely thin parallel solenoids. The magnetic fluxes are arbitrary. The wave vector is assumed to lie in the asymptotic domain kρ≫1 with ρ being the distance of the solenoids.

We describe a rubidium circular-state-preparation technique suited to high principal quantum numbers. It combines an excellent selectivity and a good efficiency. Preliminary spectroscopic tests on the n=50→n=51 circular to circular transition at 51 GHz are reported.

We study the quantum dynamics of a Morse oscillator under an intense field with three different frequencies: (1) one much higher than the dissociation energy from an excited state, (2) one resonant with the dissociation energy from an excited state, and (3) an intermediate one. The calculations were performed by using the momentum-space Fourier-grid Hamiltonian method. The high-frequency stabilization and harmonic-generation spectra are shown. The laser frequencies discussed are currently experimentally available.

A recent article by Shih, Sergienko, and Rubin [Phys. Rev. A 47, 1288 (1993)] reports a two-photon interference effect in which fringes with a visibility of 59% were observed; for an ideal system, quantum mechanics predicts perfect visibility interference. In explanation of this discrepancy, the authors conclude that the reduced visibility is due to ‘‘nonperfect phase matching of the down-conversion,’’ the parent process of the correlated photon pairs. They imply that the nonperfect phase matching stems from the finite size of their down-conversion crystal. We argue that the crystal length was not a limiting factor in their measurement, citing three similar experiments which achieved higher visibilities [Kwiat, Steinberg, and Chiao, Phys. Rev. A 47, R2472 (1993); Brendel, Mohler, and Martienssen, Phys. Rev. Lett. 66, 1142 (1991); Europhys. Lett. 20, 575 (1992)].

The assertion of Kwait, Steinberg, and Chiao [preceding Comment, Phys. Rev. A 48, 3999 (1993)] that alignment problems and the size of the beam-defining irises explain the decrease in the visibility in two-photon interference experiments is examined. We argue that these effects can explain the reason that the visibility does not attain its maximum theoretical value, but that it fails to explain the decrease in the visibility with the path differences in the interferometers. We conclude that one explanation of this effect is the breakdown of perfect frequency phase matching in parametric down-conversion.