We demonstrate the experimental feasibility of quantum-state endoscopy [P. J. Bardroff, E. Mayr, and W. P. Schleich, Phys. Rev. A 51, 4963 (1995)] to measure the complete quantum state of a single mode of the electromagnetic field in a cavity. We perform numerical simulations of an experiment in progress. © 1996 The American Physical Society.

We analyze transient properties of light amplification without population inversion (LWI) in a closed three-level @sV -type system and study the system evolution from transient LWI into steady-state LWI. From the time evolution of the atomic coherence and population distribution with and without an incoherent pump field, we elucidate the light amplification mechanism and derive the conditions under which the @sV system exhibits LWI in any state basis. We derive analytical solution for the steady-state LWI and discuss other mechanisms of light amplification such as stimulated Raman scattering and population inversion in the dressed states. © 1996 The American Physical Society.

Experimentally achievable laser-field intensities of the order of ${10}^{16}$ W/${\mathrm{cm}}^2$ and higher may accelerate free and bound electrons to velocities of the order of the velocity of light. In this regime magnetic-field forces become comparable to electric-field forces. We propose and investigate the application of a strong static magnetic field in the propagation direction of the laser beam as an additional means to tailor the harmonic spectrum in this direction. Even harmonic generation is shown to be a signature of this additional interaction. © 1996 The American Physical Society.

We study spontaneous pattern formation in a system consisting of an alkali-metal vapor cell placed in front of a feedback mirror and driven by a vector optical field. Through a combination of linear and nonlinear analysis we are able to predict the formation of squares, rolls, rhomboids, hexagons or ‘‘honeycombs’’ for appropriate parameter values. Our predictions are confirmed by numerical integration of the model equations. © 1996 The American Physical Society.

A simple model of a class-B laser subject to a delayed feedback is investigated. The response of the laser exhibits strongly pulsating intensity oscillations which we analyze using asymptotic methods. We concentrate on a single branch of periodic solutions and progressively increase the amplitude of the feedback. We show that the response of the laser (amplitude of the oscillations and period) follows simple scaling laws which are typical to class-B lasers subject to optoelectronic feedback. Specifically, the intensity of the laser field quickly saturates at a constant amplitude and collapses as the feedback surpasses a critical amplitude. © 1996 The American Physical Society.

Sonoluminescence is explained in terms of quantum radiation by moving interfaces between media of different polarizability. In a stationary dielectric the zero-point fluctuations of the electromagnetic field excite virtual two-photon states, which become real under perturbation due to motion of the dielectric. The sonoluminescent bubble is modeled as an optically empty cavity in a homogeneous dielectric. The problem of the photon emission by a cavity of time-dependent radius is handled in a Hamiltonian formalism, which is dealt with perturbatively up to first order in the velocity of the bubble surface over the speed of light. A parameter dependence of the zeroth-order Hamiltonian in addition to the first-order perturbation calls for a novel perturbative method combining standard perturbation theory with an adiabatic approximation. In this way the transition amplitude from the vacuum into a two-photon state is obtained, and expressions for the single-photon spectrum and the total energy radiated during one flash are given both in full and in the short-wavelength approximation when the bubble is larger than the wavelengths of the emitted light. A model profile is assumed for the time dependence of the bubble during the collapse, and in this model the radiated energy and the spectrum are calculated numerically and in the short-wavelength limit also analytically. It is shown analytically that the spectral density has the same frequency dependence as blackbody radiation; this is purely an effect of correlated quantum fluctuations at zero temperature. The present theory clarifies a number of hitherto unsolved problems and suggests explanations for several more. Possible experiments that discriminate this from other theories of sonoluminescence are proposed. © 1996 The American Physical Society.

In this paper we report results on second-harmonic generation (SHG) from aqueous suspensions of the purple membrane of Halobacterium halobium. A detailed theory describing this SHG signal is supported by results for the angular distribution of the harmonic emission and validated by experimental measurements with circularly polarized pump radiation—a condition that normally precludes SHG. Hence we identify the precise mechanism for second-harmonic emission in this macroscopically isotropic but microscopically ordered system, resolving confusion over whether the signal should be considered coherent SHG or incoherent hyper-Rayleigh scattering. © 1996 The American Physical Society.

We present a theoretical formalism that describes the nonlinear phase shift acquired by a laser beam in passing through an anisotropic material oriented at an arbitrary angle with respect to the beam. We use this theory to analyze the results of z-scan measurements made on layered composite materials. Samples are constructed from alternating, subwavelength-thick layers of titanium dioxide and the conjugated polymer poly (p-phenylenebenzobisthiazole). Effective-medium theory predicts an enhancement of 35% in the third-order susceptibility when the electric field is polarized normal to the layers. Good agreement between theory and experiment is observed. © 1996 The American Physical Society.

We present results of an experimental investigation into alteration of the spontaneous emission spectrum of GaAs from within one-dimensional photonic band gap (PBG) structures. The PBG samples are multilayer AlAs/${\mathrm{Al}}_{0.2}$${\mathrm{Ga}}_{0.8}$As/GaAs p-i-n light-emitting diodes, with layers arranged as a distributed Bragg reflector. The emission spectra normal to the layers are measured, and we use a simple method to model the power spectrum of spontaneous emission from within the structures. We find that the emitted power is enhanced by a factor of 3.5 at the frequencies near the photonic band edge. © 1996 The American Physical Society.

We report two-photon correlation experiments using spontaneous parametric down-conversion under a severe manipulation of the input pump field. Considering the case of passing the laser beam through a focusing lens before the down-conversion crystal, theoretical calculations and a series of imaging experiments demonstrate two-photon geometric optics effects. In particular, the imaging in coincidence counts of an aperture placed in one of the down-conversion beams is found to be the analog of a simple spherical mirror system, which displays a ‘‘vacuum dispersion’’ effect in that the object and image distances are wavelength weighted. © 1996 The American Physical Society.

We revisit the problem of spontaneous emission by a two-level system in a cavity, concentrating in particular on those aspects associated with the finite propagation time of light between the atom and the cavity boundaries. We find that spontaneous emission always occurs first at the free space rate and there is then an abrupt change in these dynamics when light reflected by the cavity mirrors returns to the atom, leading to interference effects that result either in an enhancement or a reduction of this rate or in a periodic exchange of excitation between the atom and the cavity field. From this point of view, reversible spontaneous emission in terms of single-mode ‘‘vacuum Rabi oscillations’’ can be interpreted as resulting from the interference between the multimode partial waves reflected at the cavity mirrors and the instantaneously radiated light. © 1996 The American Physical Society.

The behavior of a nonresonant Brillouin laser is studied both theoretically and experimentally. Equations for the laser are revisited in order to derive the steady states. The stability of the stationary solutions is studied numerically in the case of a short length cavity. This study reveals unstable regimes and a modal bistability around the antiresonant frequency. Experimental results confirm these predictions. © 1996 The American Physical Society.

In this paper we present a summary of the experimental observations of the polarization behavior of a Zeeman and Faraday laser and discuss difficulties with the usual interpretation in terms of two circularly polarized modes. To illustrate a single-mode view we solve analytically the problem of a laser with a weak dichroism and a weak Faraday rotation. The theory explains essentially all of the experimental observations. Equally important, we show that circularly polarized modes by themselves do not satisfy the accepted laser equations and consequently that a two-mode interpretation is not tenable. Our treatment is couched in the language of nonlinear dynamics. The transition from a fixed-point solution to a periodic solution that occurs at a critical field is identified as a saddle-point instability. In three separate appendixes, we consider: a polarization propagation picture that gives us physical insight into the behavior of a dichroic Zeeman or Faraday laser, possible extensions of our calculations to cavities with large anisotropies, and we discuss the merits of ‘‘bare cavity’’ laser models. © 1996 The American Physical Society.

We propose a scheme for obtaining sub-Doppler resolution for one transition of an inhomogeneously broadened, three-level atomic system, by using an intense control field at the other transition. Analytical and numerical calculations are presented to delineate the mechanism responsible for this sub-Doppler resolution, and quantify the extent to which Doppler broadening can be reduced. © 1996 The American Physical Society.

As a realistic system for observing the distinctive features of the interaction of squeezed light with atoms, we consider a single, coherently driven, two-level atom in a resonant optical cavity coupled to a broadband squeezed vacuum field. In the bad cavity limit we derive equations of motion for the atomic operators that are identical to those for the free-space situation, except for having modified parameters. We focus our attention on the resonance fluorescence from this system and show that, in the above limit, many of the unusual spectral features previously reported for the free atom situation persist. We also confirm the link between these anomalous spectra and the collapse of an atom into a pure state. The cavity parameter values needed to verify some of these interesting effects appear to be within the range of present day technology. © 1996 The American Physical Society.

We present an analytic construction of the three-bit quantum conditional swap (Fredkin) gate that uses only five quantum gates, each acting on only two qubits. Our implementation is based on previous work on the three-bit quantum conditional-NOT (Toffoli) gate. Numerical evidence suggests that this is a minimal implementation. © 1996 The American Physical Society.

A technique based on a dispersive atom–cavity-field interaction is proposed for preparing two or more atoms in macroscopically separated entangled states. After suitably prepared atoms interact with the cavity field a subsequent measurement on this field projects the atoms onto the entangled states. Two-particle entangled states are discussed as well as a three-particle state of the type proposed by Greenberger, Horne, and Zeilinger. © 1996 The American Physical Society.

Measurements of positronium (Ps) -formation cross sections (upper and lower limits) for positrons (1–17 eV) scattered by Rb atoms are reported. These measurements, along with recent measurements and calculations of total cross sections, provide evidence that coupling effects between Ps formation and other scattering channels may be very important for energies below 10 eV for Rb. © 1996 The American Physical Society.

We have investigated the recently reported discrepancy between theory and a new experiment [Palathingal et al., Phys. Rev. A 51, 2122 (1995)] for single-quantum annihilation of positrons with bound electrons. Fully relativistic calculations of total cross sections for the production and the annihilation of positrons involving an electron bound in the K or L shells of atoms have been performed in screened atomic potentials, for a number of elements ranging from germanium (Z=32) to thorium (Z=90) and for positron kinetic energies up to 4 MeV above threshold. Very good agreement with earlier calculations, that had been performed over a more restricted range of elements and energies, is obtained, also confirming that the Z and shell dependence previously seen continues at higher energies. The results presented here extend theory to energies where recent experiments have been performed, at forward angle, which were assumed to characterize the Z and shell dependence of the corresponding total cross sections. Our new theoretical predictions are not consistent with the strong dependence of the K- and L-shell bound-pair-annihilation cross sections on the nuclear charge of the target reported in those experiments. An explanation of these discrepancies is proposed, namely, that it is not correct to assume the same ratios for total cross sections as for forward distributions. This is demonstrated using existing data for the related photoeffect process, and it is seen that such forward data are indeed consistent with the pair-annihilation experiments.

We study 1s→2s(2p) excitation of hydrogen as a function of the nuclear charge of the projectile, ${\mathit{Z}}_{\mathit{P}}$, for a fixed impact velocity, v=1.3 a.u., for which electron excitation, capture, and ionization are competitive processes. For this purpose, we have performed large ab initio close-coupling calculations using a molecular basis in the framework of the impact-parameter method. Excitation probabilities do not saturate when ${\mathit{Z}}_{\mathit{P}}$ increases indefinitely, in agreement with recent work of Janev and with theoretical evidence on the behavior of transition probabilities at high impact energies. © 1996 The American Physical Society.