The notion of squeezing is generalized to the case of multimodes. The multimode generalized squeezing operator has similar algebraic properties to those of the single-mode case and reduces to the usual two-mode squeezing operator in the case of two modes. It is also shown that the generalized multimode squeezed state is a multimode minimum-uncertainty state if the squeezing parameters are real.

We present a theoretical investigation of complex dynamical behaviors of an injection-locked semiconductor laser. A period-doubling bifurcation route to chaos and bistability has been identified. The boundaries for period-doubling bifurcations and chaos are mapped out in the injection-level–frequency-detuning plane. It was shown that there exist two locally stable attractors of limit cycles. The centers of the attractors shift nonlinearly with injection level. The shift of the center of the electric-field phase is estimated by the harmonic balance method.

We use the Greenberger-Horne-Zeilinger [in Bell’s Theorem, Quantum Theory,and Conceptions of the Universe, edited by M. Kafatos (Kluwer Academic, Dordrecht, 1989)] approach to present three demonstrations of the failure of Einstein-Podolsky-Rosen (EPR) [Phys. Rev. 47, 777 (1935)] local realism for the case of two spinless particles in a two-particle interferometer. The original EPR assumptions of locality and reality do not suffice for this. First, we use the EPR assumptions of locality and reality to establish that in a two-particle interferometer, the path taken by each particle is an element of reality. Second, we supplement the EPR premises by the postulate that when the path taken by a particle is an element of reality, all paths not taken are empty. We emphasize that our approach is not applicable to a single-particle interferometer because there the path taken by the particle cannot be established as an element of reality. We point out that there are real conceptual differences between single-particle, two-particle, and multiparticle interferometry.

Localized wave-packet tunneling through a one-dimensional potential barrier is investigated with respect to the distribution of tunneling times. Specifically, we construct a quantity— termed the ‘‘tunneled flux’’—which has properties of a tunneling-time probability distribution. The tunneled flux is determined completely in terms of the time evolution of an initially localized wave packet. The tunneled flux and the corresponding tunneling-time distributions are investigated analytically via a semiclassical approximation. Additionally, numerical studies are performed with both the semiclassical and quantal versions of the tunneled flux. The semiclassical calculation qualitatively reproduces numerical tunneled fluxes, converging quantitatively as ħ→0. It also provides a wealth of insight into the nature of tunneling processes. Our principle conclusions are as follows: (i) There is an essential disparity between the time- and energy-domain pictures of wave-packet tunneling; (ii) the tunneling-time distribution can be modeled as an exponential of a skewed Lorentzian function, with a width governed primarily by the phase-space asymmetry of the initial wave packet; and (iii) tunneling is faster than simple classical motion from one side of the barrier to another, even assuming an instantaneous transit between the turning points. Effectively, the barrier acts as a filter for the high-momentum components of the initial wave packet.

We cast the nonadiabatic geometric phase in terms of the geometric distance function and the geometric length of the curve for arbitrary cyclic evolution of the quantum states. An interpretation is given to the geometric phase as the value of the integral of the contracted length of the curve along which the system traverses. It is found that for arbitrary cyclic evolution of the quantum states the geometric phase β(C) acquired by the system cannot be greater than the total length of the curve l(C). We have argued that the geometric phase arises because of the fundamental inequality between the length of the curve and the distance function. Finally, we have illustrated the calculation of the geometric phase based on the geometric distance function and the geometric length of the curve.

Studies of Rydberg-atom destruction in collisions with the polar targets ${\mathrm{CH}}_3$I and ${\mathrm{CH}}_2$${\mathrm{Br}}_2$ for values of principal quantum number n in the range 100–400 are reported. Analysis of the data using the essentially-free-electron model suggests that, for ultralow electron energies (∼80 μeV to 1 meV), the cross section σ(ɛ) for rotationally inelastic scattering of electrons by a polar target varies approximately as 1/ɛ, where ɛ is the electron energy. The Born approximation does not predict such behavior at very low collision energies, and possible reasons for this discrepancy are discussed.

A strong asymmetry has been observed in 1-keV ${\mathrm{H}}^{+}$+Na(3p)→H(2p)+${\mathrm{Na}}^{+}$ collisions after Na(3p) preparation with left- and right-handed circularly polarized laser light. The H(2p) state is characterized by Lyman-α photon detection. This angle-differential ion-atom-collision study shows that direct information on higher final-state multipoles can be extracted from a well-defined nonisotropic system.

We report the ratio of double-to-single photoionization of He at several photon energies from 2 to 12 keV. By time-of-flight methods, we find a ratio consistent with an asymptote at 1.5%±0.2%, essentially reached by hν≊4 keV. Fair agreement is obtained with older shake calculations of Byron and Joachain [Phys. Rev. 164, 1 (1967)], of Åberg [Phys. Rev. A 2, 1726 (1970)], and with recent many-body perturbation theory (MBPT) of Ishihara, Hino, and McGuire [Phys. Rev. A 44, 6980 (1991)]. The result lies below earlier MPBT calculations by Amusia et al. [J. Phys. B 8, 1248 (1975)] (2.3%), and well above semiempirical predictions of Samson [Phys. Rev. Lett. 65, 2861 (1990)], who expects no asymptote and predicts σ(${\mathrm{He}}^{2+}$)/σ(${\mathrm{He}}^{+}$)=0.3% at 12 keV.

We present a simulation for the acceleration of multiply charged ions approaching metal surfaces by the interaction with its image charge, based on the classical over-barrier model. The image attraction leads to a gain in velocity perpendicular to the surface. The amount of energy gain is directly related to the distance of formation of hollow atoms. The role of a transient hollow negative ion is discussed. Comparison with recent data by Winter [Europhys. Lett. 18, 207 (1992)] shows good agreement.

We study the classical and quantum fluctuations of a general damped forced oscillator close to a bifurcation instability. Near the instability point, the fluctuations are strongly phase correlated and are squeezed. In the limit of low damping, it is shown that the system has universal features when scaled with the damping. The same scaling law applies to the classical and to the quantum regimes. We furthermore show that the coupling to the environment is crucial in the generation of squeezed fluctuations.

We suggest using a wave-collapse mechanism for the compression of optical pulses. This idea is illustrated by the example of the physical model that is described by the Ginzburg-Landau equation for a subcritical bifurcation. We find, for a large class of initial conditions in this model, the exact sufficient criterion of the collapse.

We analyze a recent proposal for a new phase operator along with the experiment [J. W. Noh, A. Fougères, and L. Mandel, Phys. Rev. Lett. 67, 1426 (1991)] using phase-space functions. We show that the measured dispersion in phase results from a phase distribution given by the appropriate Wigner function integrated over radius. We relate this experiment to a different phase operator [H. Paul, Fortschr. Phys. 22, 657 (1974)] and the integrated Q function.

We show how the Kramers-Henneberger (KH) frame can be used to interpret the generation of high-order harmonics in very strong high-frequency laser fields in which the ionization of the atom is suppressed. We show that although a single KH eigenstate produces little harmonic generation in a constant-amplitude laser field, a wave packet of bound KH states with a low ionization rate can result in a long comb of high-order harmonics. We verify that there is a close link between the production of strong even harmonics and the use of ultrafast laser-field turn-on times, and that the even-harmonic strength is reduced for larger and, therefore, perhaps more realistic rise times.

We analyze a three-level atomic system subjected to a strong-coupling irradiation and discuss the gain and absorption of a probe field on a different transition; the approach is a general one and can easily be extended to more complex systems. A physically important concept of level flow that allows us to restore the physical symmetry on which the principle of inversion rests is introduced. Thus the response to the probe field is shown to be the contribution of two processes that obey, separately, a criterion of flow inversion.

Significant offset and narrowing of the small-signal-gain line, i.e., spectrum focusing, in a three-level molecular system at the regime of the saturation and Stark (Autler-Townes) splitting of the pumping transition in the field of a strong collimated pump beam have been obtained and explained.

Resonant and nonresonant multiphoton ionization of xenon is studied using short, circularly polarized light pulses (100 fs, 597 nm, 22 TW/${\mathrm{cm}}^2$). A pump-probe measurement shows that, although bound states are substantially populated, they do not enhance the ionization signal. The bound states do not ionize because their high angular momentum repels the wave functions from the nucleus. Ionization does occur through intermediate states in the continuum, in spite of a large energy mismatch, because these states have more energy and therefore suffer less from the centrifugal barrier.

We have observed trapping of population in Rydberg states of atoms and ions exposed to intense laser radiation. The trapping occurs in nonstationary ‘‘wave-packet’’ superposition states, created, via ac Stark-shifted multiphoton resonances during the laser pulse. The formation of these highly excited bound states requires the nonresonant absorption of up to 20 or more photons; some of the ionic Rydberg states lie nearly 32 eV above the ionic ground state. All of the trapped states having binding energies much less than the photon energy but remain stable against ionization even when the multiphoton ionization probability of the atom is saturated.

Absolute generalized oscillator strengths (GOS’s) of S 2p excitations of ${\mathrm{SF}}_6$ have been determined as a function of momentum transfer K, using angle-resolved electron-energy-loss spectroscopy at 2.5-keV impact energy. The GOS for the dipole-forbidden S 2p→6${\mathit{t}}_{1\mathit{u}}$ transition was found to be significantly enhanced at large K, in contrast to the generally decreasing trend observed for the dipole-allowed discrete and shape-resonance states of the S 2p shell. A simple empirical analysis based upon the Lassettre series expansion showed that this state has a dominant quadrupole contribution.

Measurements of absolute total cross sections for electron-impact excitation of ${\mathrm{Ar}}^{7+}$(3s→3p) using a merged-beams electron-energy-loss technique show that near threshold the inelastically scattered electrons are ejected primarily in the backward direction. This unusual angular scattering has not been previously observed for atoms or ions, but may be typical for multiply charged ions. The total cross sections, measured over an energy range to 2.2 eV above threshold, agree with seven-state R-matrix close-coupling calculations. Both close-coupling and distorted-wave calculations also confirm the backscattering observed in these measurements.