In this paper, we present a theoretical investigation into the quantum-nondemolition (QND) properties of a system which utilizes intracavity harmonic generation. It is found that a nearly ideal QND measurement of the amplitude of a signal can be performed using second-harmonic generation, while subharmonic generation can be utilized to give a nearly ideal QND measurement of a phase. The results show a significant improvement over the QND properties of other systems studied previously and should be accessible to experiment.

Minimum-uncertainty states in variables which are quadratic in mode creation and annihilation operators can be used to increase the accuracy of interferometric measurements. For a general interferometer we show how to find the relevant uncertainty relation. We consider two specific examples, a Mach-Zehnder interferometer and a Mach-Zehnder interferometer in which the first beam splitter has been replaced by a four-wave mixer. For both devices su(2) squeezed minimum-uncertainty states can be used to achieve phase-measurement accuracies of 1/N, where N is the total photon number at the input. We also describe a method of producing approximate versions of these states.

The squeezing properties of the radiation field in a micromaser are analyzed. In particular, we consider the effect of finite Q on the steady-state squeezing inside the cavity. We also discuss the effect of pump fluctuations on squeezing and show that the regularization of pumping may lead to an increase in the steady-state squeezing.

Degenerate four-wave-mixing phase conjugation is analyzed in the case of two-photon transitions between levels having unequal dipole moments. The theoretical treatment is based upon a nonperturbative solution of the density matrix and does not require the rotating-wave approximation. We give an analytical expression for the polarization component oscillating at the field frequency for a medium excited by stationary waves. In the weak-probe-beam limit, the contributions in the direction of the probe and conjugated beams are evaluated. For instance, by using this two-photon process, instead of the usual single-photon one which is independent of the permanent dipole moments even in the strong-field limit, it is shown that lower intensities are sufficient to reach higher reflectivities. The influences of the permanent dipole moments, relaxation rates, and field frequency are also discussed.

A laser Ginzburg-Landau equation with fourth- and higher-order diffusion terms is derived from the Maxwell-Bloch equations describing a laser. It is shown that the higher-order diffusion terms in the laser Ginzburg-Landau equation are crucial for the transverse structure formation. Laser-hydrodynamical equations are derived, and the correspondence between laser light dynamics and the dynamics of a compressible, viscous, quantized fluid is demonstrated.

We investigate the dynamical evolution of nonclassical states of light undergoing a two-photon absorption process. We consider two distinct cases of initial states, a squeezed coherent state and an eigenstate of the two-photon annihilation operator (a superposition of macroscopically distinct coherent states). We analyze the fluctuations in the photon-number operator and in the quadrature components of the field. Whereas one-photon linear damping rapidly destroys quantum features such as squeezing, we demonstrate that substantial coherence is retained when such light interacts with a two-photon-absorbing reservoir. This surviving coherence is responsible for the preservation of squeezing in the steady state despite the effect of dissipation. We relate the origin of squeezing of initially unsqueezed light interacting with two-photon absorbers with the squeezing generated by simple superposition states of light.

A theory for a bound system of charges interacting with electromagnetic fields is developed, with special emphasis on the radiation-pressure effects imposed by the fields on the gross motion of the charges. The theory is particularly simple and transparent for the case of two opposite charges of finite mass M in the electric-dipole approximation. This enables a rigorous description to be given for the quantum electrodynamics of the problem with the various interaction terms affecting the gross motion being easily identified. Here, too, it becomes clear that, even in the dipole approximation, the conventional interaction term should be supplemented by the so-called Röntgen term. Besides ensuring the consistency of the theory for the overall (charges plus fields) system regarding conservation laws, the Röntgen term has dynamical consequences. This is established by explicit calculations of the expectation values of the mechanical momentum 〈MṘ〉 and the radiation pressure force 〈MR¨〉 on a two-level atom in the dipole approximation. Results of calculations are displayed for the case of a single light beam and for counterpropagating beams. The feasibility of experimentally observing the transverse force is explored. Generalizations of the well-known friction force arising in one-dimensional optical molasses are given and discussed.

The Mandel Q factor, antibunching effect, and entropy are calculated for the Jaynes-Cummings model with the atom being initially in a coherent superposition of the upper and lower states. The value of Mandel’s Q factor in the collapse region is found and compared with the behavior of the field entropy.

We present a theoretical analysis of the stability characteristics of each of the four eigenpolarization configurations of two light waves counterpropagating through a nonlinear medium. Our treatment generalizes previous treatments in that we allow the possibility of the development of transverse instabilities in either the intensities or the polarizations of the waves. Indeed, we find that under many conditions the instability develops in directions other than the symmetry axis of the interaction and that the instability can affect the intensity or the polarization of the waves. We also find through numerical computations that the polarizations of the waves can become unstable in the nonlinear regime at intensities above the threshold for the transverse amplitude instability but below the threshold predicted by linear stability analysis for the corresponding polarization instability. We conclude that in many cases the inclusion of the tensor properties of the nonlinear interaction is important in describing the dynamical behavior of counterpropagating waves.

We examine the properties of gap solitaires in media exhibiting two-photon absorption alone, and also in media with linear gain as well. Using an effective-particle approach, we derive expressions for the time evolution of the solitaire’s frequency detuning and velocity. We predict that gap solitaires are slowed down for a wide range of parameters, which is partly confirmed by comparison with results from numerical simulations. However, these also show that solitaires are destroyed before significant slowing can occur due to the uncontrolled growth of side wings.

An exact analysis has been carried out for general analytical expressions for the second threshold of a single-mode homogeneously broadened laser and for the initial pulsation frequency at the second threshold, for arbitary physical values of the relaxation rates, and at an arbitrary detuning between the cavity frequency and the atomic resonance frequency. These expressions also give correspondingly exact forms for asymptotic cases that have been previously studied with some approximations. Earlier approximate results are partly confirmed and partly improved by these more general expressions. The physical status of various expressions and approximations is reconsidered and specified more clearly, including an analysis of what reasonably can be attained in lasers or masers. A general analytical proof is given of the fact that, for a larger detuning of the laser cavity from resonance, a higher value of the laser excitation is required to destabilize the steady-state solution (the second threshold). We also present results for the minimum value of the second threshold at fixed detuning as a function of the other parameters of the system and for the dependence of the ratio of the second threshold to the first threshold as a function of detuning. Minima of the second threshold and of the threshold ratio occur only if the population relaxation rate is equal to zero. The minima of the threshold ratio are shown to be bounded from above as well as from below (as functions of the relaxation rates, so long as the second threshold exists). The upper bound on the minima ratio is equal to 17. The variation of the second threshold in the semi-infinite parameter space of the decay rates is shown at various detunings in plots with a finite domain by normalizing the material relaxation rates to the cavity decay rate.

The dressed-atom approach to the fluctuation problem in lasers is described. Such an approach allows one to analyze the fluctuations in various kinds of laser models. The intensity fluctuations in a regularly pumped laser are considered in the framework of the dressed-atom approach. The Fokker-Plank equation for the field density matrix is derived from first principles without heuristic assumptions. An alternative interpretation of the noise suppression in the regularly pumped laser is given. It is based upon the nonstationary picture for atomic variables. In such a regime the statistics of the output laser field may be controlled by the input statistics of the atomic initial conditions. It is the regular pump that provides the best input statistics. In the latter case the atoms are essentially nonstationary and coherently oscillate at the Rabi frequency. They are treated in a framework of the time-dependent picture in spite of the short atomic lifetime as compared with the cavity mode lifetime. The nonstationary version of the dressed-atom approach is employed to treat the regular pump case. It is demonstrated that a coherent coupling of the Rabi oscillations in the quadratic dressed-atom terms gives rise to a survival of a linear inverse proportional dependence of the atomic correlation functions upon field intensity. This survival in turn accounts for the anomalously large dressed-atom contribution to the fluctuations. Our results are in quantitative agreement with those of other approaches.

The intrinsic property of laser systems with many internal degrees of freedom to generate a squeezed light is studied. The dressed-atom approach to fluctuations, described in the preceding paper [Khazanov, Koganov, and Shuker, Phys. Rev. A 48, 1661 (1993)], is employed. A minimization principle which governs noise quenching in laser systems is described comprehensively. This principle allows one to avoid complicated quantum-mechanical calculations to assess the squeezing capacity of the system. It is shown that noise in a laser system can be decomposed into noise states. These states interact coherently. For instance, each nonactive level in a multilevel lasing scheme may represent separate noise states under certain conditions while both lasing levels make only one noise state. The squeezing capacity of the system is determined by a quantity called the noise dimension. The theory is extended to laser schemes with many photons (two-photon generation, more than one lasing transition, etc.). The validity of the minimization principle is established for this type of system. Some consequences from the theory, which are relevant to the experiment, are discussed.

A common physical origin is demonstrated for the recently discovered differential-flow-induced chemical instability (DIFICI) and the multimode instability in a laser with an intracavity saturable absorber (LSA). The DIFICI occurs in the chemical systems involving an activator (autocatalytic) reacting species and an inhibitor reacting species, which are locally disengaged by their relative bulk motion—the differential flow. It has been shown that the LSA may be treated as an activator-inhibitor-like system and the motion of light plays the role of the differential flow. This analogy allows one to gain a deeper understanding of the physical basis of the laser instability and explain some its spectral properties.

We introduce the concept of quantum random walk, and show that due to quantum interference effects the average path length can be much larger than the maximum allowed path in the corresponding classical random walk. A quantum-optics application is described.

Hyperfine coupling constants for small radicals have been calculated using the density-functional theory with a gradient-corrected (GC) local-spin-density approximation (LSDA). The agreement between the calculated and observed results is fairly good. The GC-LSDA is essential to calculate the isotropic hyperfine coupling constants for π radicals. For σ radicals, the simple LSDA also leads to relatively good results.

We report on beam-foil time-of-flight measurements of forbidden transitions in H-like and He-like Nb and Ag. The lifetimes of the 2 ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$ state of He-like Nb, the 2${\mathit{s}}_{1/2}$ state of H-like Ag, and the 2 ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_2$ state of He-like Ag have been measured with precisions ranging from 1.7% to 10%. These are the highest-Z ions for which these lifetimes have been measured.

Differential cross sections and asymmetries at 90° and 30° are calculated for electron-hydrogen elastic scattering over the energies of the lowest ${}_{}{}^1{}_{}{}^{}$S and ${}_{}{}^3{}_{}{}^{}$P resonances using a nine-state coupled-channel calculation with and without continuum effects, which are represented by an equivalent local polarization potential. The polarization potential improves agreement with experiment in general for the spin-averaged cross sections. It is suggested that continuum effects would be critically tested by asymmetry measurements at 30° over the ${}_{}{}^1{}_{}{}^{}$S resonance.

The linear polarization of L${\mathrm{\alpha }}_1$ and L${\mathrm{\alpha }}_2$ x rays of gold induced by 60-keV photons has been determined with a polarimeter using coherent scattering as the analyzing process. The L${\mathrm{\alpha }}_1$ and L${\mathrm{\alpha }}_2$ lines are found to show opposite polarizations. The present results support the results of our earlier experiments on the angular distribution of fluorescent x rays of gold that the spatial anisotropy of the L${\mathrm{\alpha }}_1$ line is opposite to that of the L${\mathrm{\alpha }}_2$ line [Phys. Rev. A 44, 4379 (1991)].

We calculate the force of radiation on a two-level atom near a small dielectric such as a nanometer-scale fiber probe. Our results show that a spontaneously emitted photon may have large momentum by the position-momentum uncertainty principle.