Phase properties of the micromaser field are studied for different initial states of the cavity field and the atom. It is shown that a multipeak phase structure develops when the cavity field is initially in a coherent state and the atoms enter the cavity in their excited state, which eventually leads to the randomization of the phase. If initially the field is thermal and the atoms entering the cavity are polarized, the steady state with a considerably well defined phase is reached, which asymptotically can be one of the cotangent states or some mixed state depending on the atom-cavity interaction time. An interesting effect of switching between the two- and three-peak phase structure caused by each subsequent atom passing the cavity has been found. © 1996 The American Physical Society.

The exact solutions of both the classical and quantum versions of two models describing stimulated Raman scattering in the spatially homogeneous approximation are obtained. The quantum problem is a dynamical problem and the exact solution for the out-states of this problem is presented. We also include medium dissipation and inhomogeneous broadening of the resonance transition and present the solution of that quantum problem. The expectation value of the intensity of the generated Stokes pulse is obtained and discussed. The recursion relation for the expectation value of a general nodal operator is given and it is solved, with the solution being given as either an infinite sum or a contour integral. © 1996 The American Physical Society.

We identify the universal mechanism that is responsible for superluminal (faster-than-light) traversal times as well as the narrowing of wave packets transmitted through various nondissipative media. This mechanism is shown to be predominantly destructive interference between successive wave-packet components traversing all accessible causally retarded paths. It strongly depends on wave-packet coherence and width, and can cause superluminal traversal not only in evanescent-wave ‘‘tunneling’’ but also in allowed propagation. © 1996 The American Physical Society.

We describe one of the possible ways of development of a spatial instability of counterpropagating strong light beams in resonant media. The interplay between self-focusing and saturation effects results in a formation of counterpropagating stable filaments. The nondegenerate four-photon scattering in these waveguiding structures leads to a far-field multiconical emission with the cones’ frequencies shifted to both red and blue sides of the resonant transition. © 1996 The American Physical Society.

The nonlinear differential equation for the electron density in an arbitrary closed shell for a bare Coulomb field as derived by Cooper [Phys. Rev. A 50, 1040 (1994)] is reduced to a Sturm-Liouville problem. Using supersymmetry inspired ladder-operator relations for the continuum Coulomb problem, the appropriate electron density is expressed exactly in the same way as the bound-state case. © 1996 The American Physical Society.

Using both the continuum and tight-binding models, we have investigated analytically the effects of a statistical phase factor on the persistent charge and magnetic moment currents of ‘‘free’’ hard-core anyons in one-dimensional mesoscopic rings. In particular, we find that the ground state of the system could, in most cases, self-sustain the Aharonov-Bohm and Aharonov-Casher effects simultaneously. © 1996 The American Physical Society.

The Stark shift of the (4s${)}^2$${\mathrm{}}^1$${\mathit{S}}_0$→(4s4p)${\mathrm{}}^3$${\mathit{P}}_1$ transition in calcium was measured using a laser to excite an atomic beam as it traversed through a uniform electric field and a field-free region. The laser frequency was scanned across the transition, while fluorescence produced by the radiative decay of the excited state was detected. The frequency scan was calibrated using an acousto-optic modulator to frequency shift part of the laser beam. The Stark shift rate was found to be -12.314±0.041 kHz/(kV/cm${)}^2$, where the minus sign indicates the transition frequency decreases when the electric field is applied. © 1996 The American Physical Society.

The effect of electromagnetically induced transparency on laser-induced collisional autoionization is investigated. The process, occurring only during atomic collisions and transient in nature, is characterized by the formation of a quasimolecule that can be ionized through the excitation to a pair of interfering dressed states decaying into the same continuum of ionization. Ionization suppression of the order of 70% is predicted for dressing laser intensities of the order of a few tens of MW/${\mathrm{cm}}^2$. © 1996 The American Physical Society.

The time-independent mean-field method for the calculation of matrix elements of nonrelativistic propagators is based on a variational principle whose nonlinearity induces a multiplicity of variational solutions. Several of them can break any symmetry shared by the Hamiltonian and initial and final states. We describe a soluble model where, in particular, time reversal and parity breakings occur. Such breakings account for important properties of propagation amplitudes. © 1996 The American Physical Society.

Fullerene ions, ${\mathrm{C}}_{60}^{\mathit{q}+}$, with q up to 9, have been observed in a study of their production by slow (v<0.5 au) impact of the projectiles ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}_{}^{4,5,8,12,16,17+}{}_{}{}^{}$, ${}_{}{}^{136}{}_{}{}^{}\mathrm{Xe}_{}^{27+}{}_{}{}^{}$, ${}_{}{}^{86}{}_{}{}^{}\mathrm{Kr}_{}^{28+}{}_{}{}^{}$, ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}_{}^{20,38,44,46+}{}_{}{}^{}$, and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}_{}^{46+}{}_{}{}^{}$ on a neutral fullerene beam. The distribution of ion yields for each projectile is representable by a binomial form; variation of the biniomial fit parameters with projectile charge suggests the maximum positive charge for the fullerene ion. Correlations between the time of flight of first and second ions are shown to provide details of the fragmentation of fullerenes in close collisions. © 1996 The American Physical Society.

Applying an appropriate unitary transformation, a nonperturbative result for the T matrix for potential scattering in a strong elliptically polarized laser field is derived. The approximation obtained is exact in the high-intensity laser field limit. For lower laser field intensity, this result is an off-shell low-frequency approximation. The second result is a modified on-shell low-frequency approximation. For an elliptically polarized laser field, the momenta shifts are complex, but energies are still on shell. In the linear polarization case, these shifts are real and the range of applicability is substantially wider. © 1996 The American Physical Society.

We present a momentum-space solution for the time-dependent Schrödinger equation of a realistic hydrogen atom in a strong laser pulse. The method can integrate the nonperturbative system to thousands of optical cycles, previously thought not yet feasible. A hydrogen atom under a 285-nm (4.357-eV), 97.5-TW/${\mathrm{cm}}^2$ short pulse is found to produce two prominent photoelectron peaks above threshold, and significant intermediate bound-state resonance with the 4p state. With a 248-nm (5-eV) probe laser pulse that is ten times longer, the resonant structure is explored. © 1996 The American Physical Society.

Recently [Phys. Rev. A 52, R21 (1995)] a process for coherent hard-x-ray generation in crystalline solids was suggested by us. In the current work the theoretical treatment of this process is formulated in the language of nonlinear optics. An approximate solution of the coupled Schrödinger and Maxwell equations is presented that governs the creation and the evolution of the x-ray field. The theory generalizes our previous study by providing a description of the pulse evolution and of the temporal shape of the x-ray pulse. © 1996 The American Physical Society.

The basic requirement that, in quantum theory, the time evolution of any state is determined by the action of a unitary operator, is shown to be the underlying cause for certain ‘‘exact’’ results that have recently been reported about the time dependence of transition rates in quantum theory. Departures from exponential decay, including the ‘‘quantum Zeno effect,’’ as well as a theorem by Khalfin about the ratio of reciprocal transition rates, are shown to follow directly from such considerations. At sufficiently short times, unitarity requires that reciprocity must hold, independent of whether T invariance is valid. If T invariance does not hold, unitarity restricts the form of possible time dependence of reciprocity ratios. © 1996 The American Physical Society.

How does the classical notion of ‘‘phase’’ apply to a quantum harmonic oscillator H=1/2(q${\mathrm{^}}^2$+p${\mathrm{^}}^2$), [q^,p^]=iħ, which cannot have sharp position and momentum? A quantum state ρ^ can be assigned a definite classical phase only if it is a large-amplitude localized state. Our only demand, therefore, on a (Hermitian) phase operator φ^ is that the phase distribution P(Φ)= Tr{δ(φ^-Φ)ρ^} attribute the correct sharp phase to any such ‘‘classical phase’’ state. This requires that the Weyl symbol [φ^${]}_{\mathit{w}}$(q,p) of φ^ tend to θ mod2π as r→∞, where θ=${\tan }^{\mathrm{-}1}$(p/q) and r=(${\mathit{q}}^2$+${\mathit{p}}^2$${)}^{1/2}$. There are infinitely many such phase operators. Each is expressible as φ^=[${\tan }^{\mathrm{-}1}$(p^/q^)${]}_{\mathrm{\Omega }}$, where Ω specifies an ordering rule for q^ and p^. The commutator -i[H^,φ^]=1-2π[δ(${\tan }^{\mathrm{-}1}$p^/q^)${]}_{\mathrm{\Omega }}$ corresponds to the Poisson bracket {H,${\mathrm{\phi }}_{\mathrm{cl}}$${\mathrm{\}}}_{\mathrm{}\mathrm{PB}}$=1-2πδ(θ) for the single-valued classical phase ${\mathrm{\phi }}_{\mathrm{cl}}$=θ mod2π. Phase states Γ^(Φ) are defined by the condition that their Weyl symbols [Γ^(Φ)${]}_{\mathit{w}}$(r,θ)→δ(θ-Φ) as r→∞. If moreover ${\mathrm{\int }}_0^{2\mathrm{\pi }}$dΦΓ^(Φ)=1^, then Γ^(Φ) is a phase probability operator measure (POM). In particular, δ(φ^-Φ) is a phase POM.

Normalizable approximate phase states Γ${\mathrm{^}}^{\mathrm{\varepsilon }}$(Φ) are defined by [Γ${\mathrm{^}}^{\mathrm{\varepsilon }}$(Φ)${]}_{\mathit{w}}$(r,θ) =2π${\mathrm{\varepsilon }}^2$${\mathit{e}}^{\mathrm{-}\mathrm{\varepsilon }\mathit{r}}$[Γ^(Φ)${]}_{\mathit{w}}$(r,θ), ɛ≪1. Phase states are not, in general, ‘‘pure orthogonal’’ in the sense Γ^(Φ)Γ^(Φ′)=δ(Φ-Φ′)Γ^(Φ), unless they are of the form δ(φ^-Φ). However, any phase state Γ${\mathrm{^}}_1$(Φ) is trace orthogonal to any phase POM Γ${\mathrm{^}}_2$(Φ), in the sense that Tr{Γ${\mathrm{^}}_2$(Φ′)Γ${\mathrm{^}}_1^{\mathrm{\varepsilon }}$(Φ)}→δ(Φ-Φ′) as ɛ→0. This implies that measurement of the POM Γ${\mathrm{^}}_2$(Φ′) on the state Γ${\mathrm{^}}_1^{\mathrm{\varepsilon }}$(Φ) yields the outcome Φ with probability 1 as ɛ→0. Phase measurements of the first kind are possible in principle; they would allow one to prepare (approximate) phase states and monitor their phase evolution in a quantum (phase) nondemolishing manner. Cases of special interest are the Susskind-Glogower and the Cahill-Glauber ordered phase states and operators. The energy-phase (or number-phase) uncertainty relation is ΔHΔφ≥0, the lower limit ΔHΔφ=0 being realized by pure number states; however, for states whose Wigner functions are localized away from the origin and from the extremities of the (single-valued) phase window (0,2π), the uncertainty relation is effectively ΔHΔφ≥1/2. © 1996 The American Physical Society.

We study the time evolution of the distribution function for an ensemble of classical particles in a harmonic trap when the oscillation frequencies are varied with time. We compare our results with the near circular density distributions observed for the thermal part of the expanded cloud in a recent experiment demonstrating the Bose-Einstein condensation of an alkali-metal vapor. The shape and extent of this noncondensed component characterize the temperature of the original cloud.

As the peak electric field of the microwave pulse is increased, steps in the classical microwave ionization probability of the highly excited hydrogen atom are produced by phase-space metamorphosis. They arise from new layers of Kolmogorov-Arnold-Moser (KAM) islands being exposed as KAM surfaces are destroyed. Both quantum numerical calculations and laboratory experiments exhibit the ionization steps, showing that such metamorphoses influence pulsed semiclassical systems.