The undepleted-pump photorefractive four-wave mixing equations for the case of orthogonally polarized pumps are solved taking into account the linear absorption of the medium. A phenomenon of the enhancement of the phase conjugate reflectivity due to linear absorption is found to occur for a certain range of the phase conjugator parameters. This phenomenon is shown to occur as a consequence of the existence of the self-oscillatory modes in the presence of linear absorption. © 1996 The American Physical Society.

We show experimentally that under suitable conditions two globally coupled laser modes showing chaotic dynamics can spontaneously synchronize together. Under the synchronization the frequencies of the two modes lock together and the chaotic dynamics of each mode show similarities to those of the single-mode laser. © 1996 The American Physical Society.

The mechanism of coherent destruction of tunneling found by Grossmann et al. [Phys. Rev. Lett. 67, 516 (1991)] is studied from the viewpoint of quantum optics by considering the photon statistics of a single mode cavity field which is strongly coupled to a two-level tunneling system (TS). As a function of the interaction time between TS and cavity the photon statistics displays the tunneling dynamics. In the semiclassical limit of high photon occupation number n, coherent destruction of tunneling is exhibited in a slowing down of an amplitude modulation for certain parameter ratios of the field. The phenomenon is explained as arising from interference between displaced number states in phase space which survives the large n limit due to identical ${\mathit{n}}^{\mathrm{-}1/2}$ scaling between orbit width and displacement. © 1996 The American Physical Society.

We study the generation of atomic-squeezed states in an optical cavity. The cavity encloses a pair of two-level atoms and is coupled to a broadband squeezed vacuum. Using the bad-cavity limit, cavity field is adiabatically eliminated to obtain the equations of motion for collective atomic operators. These equations are then employed to obtain the atomic density-matrix elements and to study the generation of atomic-squeezed states in the steady state. To characterize atomic-squeezed states we use the squeezing parameter defined recently by Wineland et al. [Phys. Rev. A 46, 6797 (1992)] for spectroscopic purposes. The aforementioned parameter is also compared with the squeezing parameter obtained from the uncertainty principle satisfied by the collective atomic operators. We show that in a cavity arrangement, unlike free space, atomic-squeezed states can be generated only for a restricted range of values of the parameters λ and N which characterize the cavity and the squeezed field driving the cavity, respectively. © 1996 The American Physical Society.

We investigate the effect of non-Poissonian pump fluctuations on the micromaser dynamics. Non-Poissonian micromaser pumping has been described by two, seemingly unrelated, theoretical approaches. The first employs a discrete pumping process where the pump atoms are allowed to arrive, with certain probability p, only at regularly spaced instants of time, whereas the second refers to continuously distributed arrival times of the pump atoms. Based on a generalization of the latter, we present a unified approach that can handle both situations on an equal footing. We find that for any kind of non-Poissonian pumping the resulting dynamics of the micromaser field is a non-Markovian one. For a micromaser with discrete non-Poissonian pumping, we show the equivalence of ensemble averaging and time averaging, providing a rare example where the validity of the ergodic hypothesis can be explicitly demonstrated. Moreover, we investigate the time-delayed field-field correlation function and a generalized k-photon spectrum of the cavity field, which for k=1 corresponds to the usual power spectrum. For the case that the micromaser is operated under the k-photon trapping condition, we derive exact analytical expressions for the k-photon spectrum and the corresponding correlation functions that result from the exact solution of a non-Markovian evolution problem. Provided that p>1/2, the spectrum is found to be split into several equidistant peaks for certain values of the interaction parameters. © 1996 The American Physical Society.

The influence of spontaneous emission on the dynamical localization in atomic momentum transfer observed in recent experiments is investigated. Our numerical and analytical results indicate that contrary to previous expectation quantum diffusion due to decoherence by spontaneous emission is non-negligible in the experimental regime and may actually be visible in the measured data. Modifications of the experiments to strengthen this point are proposed. © 1996 The American Physical Society.

The transverse correlation between the pairs of photons produced in optical spontaneous parametric down-conversion is analyzed. The interesting features of the correlation arise from the form of the two-photon state generated in the process. The physics of the rings of radiation emerging from the crystal is described. We discuss the theory behind the recent experiments on two-photon geometric and physical optics. © 1996 The American Physical Society.

In isolated-core excited Rydberg systems the destruction of quantum coherence by spontaneous emission of photons is investigated. For this purpose a theoretical description is developed which is based on a decomposition of the atomic density operator into N-photon contributions. Using methods from multichannel quantum defect theory, the relevant atomic transition amplitudes are represented in the form of semiclassical path representations which are associated with repeated returns of the Rydberg electron to the ionic core. Apart from numerical advantages, this approach also yields a clear physical picture of the intricate interplay between the incoherent photon emission process, the coherent laser-modified electron correlation effects, and the semiclassical aspects of the dynamics of the Rydberg electron. Various examples of this interplay are analyzed within the framework of the developed theoretical tools. © 1996 The American Physical Society.

We report calculations for stimulated Raman scattering (SRS) with pump photons from a soft-x-ray laser. The Raman transitions from the 1${\mathrm{}}^1$${\mathit{S}}_0$ ground state to the 2${\mathrm{}}^1$${\mathit{S}}_0$ and 3${\mathrm{}}^1$${\mathit{S}}_0$ metastable states of helium at 20.6 and 22.9 eV, respectively, are considered. We calculate ${\mathrm{\chi }}^{\mathrm{}\left(3\right)\mathrm{}}$ for pump photons with an energy close to the autoionizing resonances (2s2p${)}^1$P at 60.15 eV and (sp,23+${)}^1$P at 63.66 eV. SRS gain coefficients are derived for x-ray laser lines in close resonance with the autoionizing levels, viz., the 3p(3/2,3/2${)}_2$→3s(3/2,1/2${)}_1$ transition in neonlike selenium (λ=20.64 nm) and the analogous transition in neonlike bromine (λ=19.47 nm). It is shown that the Raman gain coefficient with experimentally realistic parameters is significant and that a high gain length product can be achieved even with pump intensities below ${10}^{11}$ W/${\mathrm{cm}}^2$. © 1996 The American Physical Society.

For some applications the overall phase of a quantum state is crucial. For the so-called displaced squeezed number state (DSN), which is a generalization of the well-known squeezed coherent state, we obtain the position space representation with the correct overall phase, from the dynamics in a harmonic potential. The importance of the overall phase is demonstrated in the context of characteristic or moment generating functions. For two special cases the characteristic function is shown to be computable from the inner product of two different DSNs. © 1996 The American Physical Society.

Broad-area lasers exhibit complex spatiotemporal behavior due to multilateral mode operation, emitting filamented beams with low spatial coherence. Using weak external optical feedback we demonstrate that some degree of lateral mode selection and pattern stabilization of these lasers can be obtained under appropriate feedback conditions. © 1996 The American Physical Society.

The effect of changing the coupling laser transition, and hence the wavelength, on the coherently induced transparency seen by a probe beam in a Doppler-broadened cascade electromagnetically induced transparency (EIT) configuration is studied experimentally and theoretically. The transparency of the vapor is shown to be an effect not just of EIT, but of the varying Autler-Townes splitting for each velocity group within the vapor. Consequently, the best overall transparency for Rabi splittings less than the Doppler width is not at the position of matched coupling and probe wavelengths, but for coupling wavelengths less than that of the probe. © 1996 The American Physical Society.

The propagation of intense laser pulses through a four-level atomic system in a double-Λ scheme is examined. Under conditions of adiabatic perturbation of the atomic quantum state, paired pulses with arbitrary shapes establish a correlation between their amplitudes and reach a quasiform-stable regime of propagation. The amplitude correlation is a feature of pulse matching, while the propagation presents the same properties as the so-called adiabatons, predicted and observed in single-Λ systems. We show that in the double-Λ scheme the phenomena of pulse matching and adiabaton-type propagation are associated with two distinct propagation normal modes. © 1996 The American Physical Society.

We show that by combining nonselective and conditional measurements on a sequence of excited two-level atoms traversing a resonant cavity, one can construct a practical strategy for compressing an arbitrary photon-number distribution (thermal distributions included) into a desired Fock state with high probability of success and low number of measurements. © 1996 The American Physical Society.

Some generalizations are effected here of the work of Heilmann and Lieb [Phys. Rev. A 52, 3628 (1995)], who summed the squares of all the normalized bound-state wave functions for the hydrogen atom. One of their main results is shown to be equivalent to a spatial generalization of Kato’s theorem. Their asymptotic evaluation of the above sum for large r is used to obtain a property of the bound-state Slater sum in the high-temperature limit. The corresponding momentum space density is also briefly discussed. © 1996 The American Physical Society.

It is shown that in supersymmetric quantum mechanics the superpartner Hamiltonian ${\mathit{H}}_{+}$ is related to the Hamiltonian ${\mathit{H}}_{\mathrm{-}}$ by a semiunitary transformation. The properties of the semiunitary transformation are discussed. © 1996 The American Physical Society.

The method of complex rotation is used to establish the existence of a regime of strongly overlapping resonances (the Ericson regime) in stretched helium, a collinear model of the helium atom. The existence of an Ericson regime in helium is conjectured and a method for proving its existence experimentally is suggested. © 1996 The American Physical Society.

The autoionization distribution of the double Rydberg (DR) states ${\mathit{N}}^{\prime }$${\mathit{L}}^{\prime }$nl(${\mathit{N}}^{\prime }$<n;${\mathit{L}}^{\prime }$,l<4) in calcium has been investigated by using five-laser resonance excitation and sequential ionization with a pulsed and a strong constant electric field. We have found that the DR states autoionize with the ejected electron having its kinetic energy apparently related to that of the electron in the ${\mathrm{Ca}}^{+}$ Rydberg orbit ${\mathit{N}}^{\prime }$${\mathit{L}}^{\prime }$. We have discussed semiclassically the dynamics in the DR state and described two types of autoionizing processes, i.e., ‘‘penetration autoionization’’ and ‘‘polarization autoionization’’ in DR states. The former is responsible for our experimental results. © 1996 The American Physical Society.

It is shown that for single-center coupled-channel calculations, it is not possible to remove some unwanted part ${\mathit{W}}^{\prime }$ of the interaction by a gauge transformation, even if the coupled equations do not exhibit an explicit dependence on ${\mathit{W}}^{\prime }$. This dependence necessarily exists and enters implicitly via the convergence property of the basis expansion. In two-center expansions, an explicit dependence on the gauge function associated with ${\mathit{W}}^{\prime }$ always remains because of the nonorthogonality of the basis states belonging to different centers in the reaction zone. The observation discussed here and supported by physical examples is relevant to the assessment of the asymptotic cross-section behavior for electron-positron pair production in relativistic atomic collisions. © 1996 The American Physical Society.

In this work the electron-impact ionization of one- and two-electron atomic systems with arbitrary nuclear charge is investigated. Distortion of the electrons’ motion in the initial and final channels is taken into account. The effect of the charge state of the residual ion on the spin asymmetry in the total cross section is analyzed for varying incident energies. The influence of increasing electron-ion interactions on the excess-energy sharing between the two escaping electrons is elucidated. © 1996 The American Physical Society.