The dc-field-induced tunneling rates for He were computed semiclassically and quantum mechanically, the latter case being the result of the application of our previously published nonperturbative theory which accounts for electronic structure, electron correlation, and field-induced effects. As the field strength increases beyond 0.15 a.u., the deviation between the two calculations is enhanced, a phenomenon which is intrinsic to the WKB approximation and is observed explicitly, via the known expressions, in the hydrogen atom.

Stimulated Raman scattering (SRS) is investigated in a liquid parahydrogen droplet. We show that the liquid hydrogen droplet readily leads to a high-cavity Q value exceeding ${10}^9$ for whispering gallery modes, and that the SRS in the liquid-hydrogen droplet proceeds not only for vibrational transition but also for rotational transition as in the gas-phase ${\mathrm{H}}_2$ system, leading to multiorder SRS sidebands covering the whole visible spectral range.

Given a bipartite quantum system represented by a Hilbert space ${\mathcal{H}}_1\otimes {\mathcal{H}}_2,$ we give an elementary argument to show that if either $\dim {\mathcal{H}}_1=\infty$ or $\dim {\mathcal{H}}_2=\infty ,$ then the set of nonseparable density operators on ${\mathcal{H}}_1\otimes {\mathcal{H}}_2$ is trace-norm dense in the set of all density operators (and the separable density operators nowhere dense). This result complements recent detailed investigations of separability, which show that when $\dim {\mathcal{H}}_i<\infty$ for $i=1,2,$ there is a separable neighborhood (perhaps very small for large dimensions) of the maximally mixed state.

Single and multiple transitions in $p^{\pm }+\mathrm{Ne}$ collisions are discussed within an effective single-particle picture relying on an accurate description of the target ground state in terms of the optimized potential method. We show that in the case of proton scattering total cross sections for capture and ionization can be extracted over a broad range of impact energies from the single-particle solutions obtained with the basis generator method. In particular, we analyze the role of dynamical screening effects in multiple ionization and propose a simple model for their inclusion in the case of antiproton scattering.

A high-order perturbation theory is presented for efficient and accurate computation of multiphoton and above-threshold ionization cross sections of atoms and molecules in weak to medium strength laser fields. The procedure is based on a Raleigh-Schrödinger perturbative expansion of the time-independent non-Hermitian Floquet Hamiltonian. The reduced Green function and generalized pseudospectral discretization techniques are extended to facilitate the calculation of complex quasienergy resonance states without the need of diagonalizing the full Floquet Hamiltonian. Explicit expressions are presented for the determination of intensity-dependent total and partial rates and electron angular distributions. The theory is applied to a case study of multiphoton detachment of ${\mathrm{H}}^{\mathrm{-}}$ for a range of laser frequencies (corresponding to the absorption of a minimum of two photons) and laser intensities from ${10}^7$ to ${10}^{12}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ It is found that a 16th-order perturbative Floquet procedure provides an excellent description of the two-photon-dominant detachment processes for laser intensity up to $2\times {10}^{11}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ The predicted electron angular distributions are in good agreement with recent experimental data.

We study the magneto-optical trap (MOT) as an optical lattice with a setup providing full phase control for all light fields. Although completely different light fields are possible for various phases, we have found experimental evidence that stored atoms are generally localized in micropotentials of the six-beam lattice. The influence of the phase variation is surprisingly small, suggesting that the robust behavior of the MOT is a consequence of this fact. We find furthermore good agreement of our experimental data with a simple theoretical model which reduces the complicated MOT to a description of steady-state atoms localized at points of the deepest adiabatic light-shift potential.

The conventional Galton board illustrates diffusion in a classical-mechanics context: it is composed of balls performing a random walk on a downward sloping plane with a grid of pins. We introduce a wave-mechanical variety of the Galton board to study the influence of interference on the diffusion. This variety consists of a wave, in our experiments a light wave, propagating through a grid of Landau-Zener crossings. At each crossing neighboring frequency levels are coupled, which leads to spectral diffusion of the initial level populations. The most remarkable feature of the spectral diffusion is that below a certain single-crossing transition probability (around 0.7–0.8) the initial spectral distribution almost perfectly reappears periodically when the wave penetrates further and further into the grid of crossings. We compare our experimental results with numerical simulations and with an analytical description of the system based on a paper by Harmin [Phys. Rev. A 56, 232 (1997)].

The statistics of the atomic population distributions in nonlinear matter-wave processes, such as four-wave mixing of matter waves (or output coupled wave packets produced by Bragg scattering) from Bose-Einstein condensates (BECs), are determined. Fluctuations of the populations of atoms in the four-wave mixing wave packet can be due to (a) fluctuations of the laser fields that produce the separate momentum wave packets of the BEC, (b) quantum fluctuations arising from finite temperature effects, and (c) the quantum-mechanical nature of the mean-field BEC wave function. We focus on the latter source of fluctuations. The distribution of the number of atoms in the four-wave mixing wave packet is binomial and reduces to a Gaussian distribution for strong conversion. We calculate the skewness and kurtosis of the distribution. The differences in the nature of the fluctuations in nonlinear phenomena for atoms (matter waves) and photons are discussed.

We have succeeded in precisely measuring spin-precession angles of tunneling and nontunneling neutrons through double-rectangular, triple-rectangular, and multirectangular potential barriers represented by a $\left[\mathrm{Permalloy}45\mathrm{}\right(\mathrm{PA})-\mathrm{g}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{u}\mathrm{m}(\mathrm{Ge})\mathrm{}{]}^n-\mathrm{PA}$ Fabry-Perot magnetic thin-film resonator for $n=1,$ 2, and 10, respectively. The spin-precession angle due to the Fabry-Perot resonator shows the oscillation curve as a function of the incident angles, and the curve is well reproduced by the theoretical phase difference of ↑ and ↓ spin neutron wave functions based on the one-dimensional Schrödinger equation. We demonstrate that the amplitude of the curve is proportional to the number of germanium layers (wells) although the transmission probability remains constant.

In this paper, decoherence in a system consisting of two Bose-Einstein condensates is investigated analytically. It is indicated that decoherence can be controlled by manipulating the interaction between the system and environment. The influence of the decoherence on quantum coherent atomic tunneling (AT) between two condensates with arbitrary initial states is studied in detail. Analytical expressions of the population difference (PD) and the AT current between two condensates are found. It is shown that the decoherence leads to the decay of the PD and the suppression of the AT current.

Femtosecond Raman spectroscopy has been developed to investigate ultrafast photoinduced structural changes of materials. Vibrational modes in the photogenerated transient species are measured by stimulated Raman scattering using a Raman pump pulse with narrow bandwidth and a femtosecond supercontinuum probe pulse. The Raman signal can be measured without slowing the temporal response and broadening the spectrum, because the temporal and spectral resolutions of the present method can be improved independently without the restriction of the transform limit. The transient Raman spectra of the trans- cis photoisomerization process in the DCM (4-dicyanomethylene-2-methyl-6-$p$-dimethylaminostyryl-4H-Pyran) dye solution were observed with the resolutions of 250 fs and $25{\mathrm{cm}}^{-1}.$

We demonstrate, in a very general fashion, the considerable slowing down of decoherence and relaxation by fast frequency modulation of the system-heat–bath coupling. The slowing occurs as the decoherence rates are now determined by the spectral components of bath correlations which are shifted due to fast modulation. We present several examples including the slowing down of the heating of a trapped ion, where the system-bath interaction is not necessarily Markovian.

Electron-impact ionization of the collinear $Z=1/4\mathrm{}$ model atom is investigated in order to examine a nonclassical behavior of the ionization cross sections. Slightly above the ionization threshold, ab initio calculations reveal a distinctive dip due to short-range dynamics. The dip is a strongly energy-dependent feature in the usually smooth and structureless ionization cross section and is foreign to treatments based on classical, as well as semiclassical mechanics. The $Z=1/4\mathrm{}$ model thus serves as a counterexample to the standard Wannier treatment of near-threshold ionization. The hyperspherical hidden-crossing theory is applied to identify the origin of the dip.

In the holonomic approach to quantum computation, information is encoded in a degenerate eigenspace of a parametric family of Hamiltonians and manipulated by the associated holonomic gates. These are realized in terms of the non-Abelian Berry connection and are obtained by driving the control parameters along adiabatic loops. We show how it is possible for a specific model to explicitly determine the loops generating any desired logical gate, thus producing a universal set of unitary transformations. In a multipartite system unitary transformations can be implemented efficiently by sequences of local holonomic gates. Moreover, a conceptual scheme for obtaining the required Hamiltonian family, based on frequently repeated pulses, is discussed, together with a possible process whereby the initial state can be prepared and the final one can be measured.

We present a detection scheme that, using imperfect detectors, and imperfect quantum copying machines (which entangle the copies), allows one to extract more information from an incoming signal than with the imperfect detectors alone.

A local realistic theory of single neutral kaons and of correlated neutral kaon pairs was formulated [F. Selleri, Phys. Rev. A 56, 3493 (1997)] by deducing the consequences of three assumptions: (i) Einstein-Podolsky-Rosen reality criterion; (ii) locality; (iii) lack of retroactive causality. Variables were introduced for every kaon determining the stable $\mathrm{CP}$ value and the changing value of strangeness S. That theory is here generalized by taking into full account the history-dependent nature of kaons. If kaon pairs are produced in $\phi$ meson decays, the local realistic probability of observing $K^0{K}^0$ pairs at certain different proper times turns out to differ considerably from the quantum-mechanical predictions.

The semiclassical approximation for a quantum amplitude is given by the sum of contributions from intersections of the appropriate manifolds in classical phase space. The intersection overlaps are just the Van Vleck determinants multiplied by a phase given by a classical action. Here we consider two nonstandard instances of this semiclassical prescription which would appear to be on shaky ground, yet the corresponding physical situations are not unusual. The first case involves momentum-space WKB theory for scattering potentials; the second is a propagator for the whisker map that arises in generic two-dimensional systems. In the former case two manifolds become asymptotically tangent, and the semiclassical formula needs to be uniformized in order to give a meaningful wave function. We give a uniformization procedure. In the latter case, there are an infinite number of intersections in phase space within a zone with the area of Planck’s constant (the limit of resolution for quantum mechanics), yet the semiclassical sum over all contributions is shown to be correct.

The unitary evolution of linear optics can be used to model quantum computational networks. In this paper, a quantum simulation of a classically chaotic map (the baker’s map) is developed using linear optics. Two different models are presented. The first model employs only 50-50 beam splitters and phase shifters to simulate universal 2-qubit gates of a quantum computer. The second model uses the discrete Fourier transform generated by symmetric $N\times N$ fiber couplers. If single photons are used as inputs for these linear optics models, the result is a physical realization of the quantum baker’s map.

We propose a generalization of the method proposed by Prudente and Acioli [Chem. Phys. Lett. 302, 249 (1999)] to compute rovibrational spectra of molecular systems. We present the expressions of the generalized rotating wave function for n-body systems. We discuss the advantages of the current approach over traditional variational methods. We also obtain an important relation for the Euler angles in terms of the Cartesian components of Jacobi vectors, expressions that are needed to compute the matrix elements utilizing the correlation function quantum Monte Carlo method.

High-resolution measurements of the photoinduced L and M x-ray spectra of metallic thorium and M x-ray spectrum of metallic uranium were performed with transmission-type and reflection-type bent-crystal spectrometers. Linewidths of $31L$ and $10M$ x-ray emission lines of thorium and of $12M$ x-ray emission lines of uranium were extracted. Using these results and those of Hoszowska et al. [Phys. Rev. A 50, 123 (1994)] for the L x rays of uranium, by means of a least-squares method we have determined the level widths of the subshells $L_1$ to $N_5$ of thorium and uranium.