Total electron scattering cross sections of ${\mathrm{PH}}_3$ and ${\mathrm{SiH}}_4$ molecules have been obtained for 90–3500-eV electrons by measuring the attenuation of the electron beam through a gas cell. The present cross sections are compared to existing experimental cross sections as well as to theoretical predictions. No previous experimental electron scattering cross sections of ${\mathrm{PH}}_3$ are reported in the literature.

We study helium single ionization with emission of low-energy $({\varepsilon }_k<10\mathrm{}\mathrm{eV})$ electrons in collisions with $2-\mathrm{M}\mathrm{e}\mathrm{V}/\mathrm{u}$ ${\mathrm{C}}^{6+}$ ions. We explore, both experimentally and theoretically, longitudinal and transverse momentum distributions of the final reaction products. We present in-depth discussion of mechanisms resulting in the forward-backward asymmetry in the longitudinal spectra of emitted electrons and recoil ions. By comparing our experimental data and calculations we display clear signatures of the interaction between the projectile and the target core (the $n-n$ interaction) for the transverse distributions of the recoil ion and projectile. For the collision system in question the $n-n$ interaction is shown to represent an important mechanism of the momentum exchange.

The effect of magnetic dichroism in the partial and total ion yield of chromium, i.e., the absorption of polarized chromium vapor was observed in the gas phase. The measurements were performed at the $2p$ edge and at photon energies above the $2p$ edge. The structure of the dichroism at the $2p$ edge can be understood by including the coupling of the $2p$ hole with the $3d$ and $4s$ shells. Our experimental results for the dichroism at the $2p$ edge are similar to results of solid-state experiments. Implications for the sum rules used as a standard tool to calculate the spin and orbital momentum are discussed.

An analytical expression for the transition amplitude is obtained by means of the continuum distorted-wave approximation of Cheshire, in order to study the double capture and excitation process. The Dodd-Greider formalism is used to provide a way of connecting the diagrams in the subseries by introducing an intermediate channel. This expression, so derived, is a rigorous first-order term of a perturbation series.

For Rabi frequencies comparable to, or even larger than, the transition frequency of a two-level system, the regime of extreme nonlinear optics is reached. Here, we give an overview of the radiated light intensity as a function of carrier frequency of light, transition frequency, Rabi frequency, spectrometer frequency, as well as of the shape and duration of the exciting optical pulses. The graphical representations reveal an amazing complexity and beauty of the nonlinear optical response. Analytical results within the “square-wave approximation” qualitatively reproduce many of the intricate features of the exact numerical calculations.

We show that a model for the collective atomic recoil laser, previously introduced to include collisions with an external buffer gas, can be reduced to a single dynamical equation for the probe amplitude. This is the result of a clever adiabatic elimination of the atomic variables and of the assumption of a negligible effect of the probe field onto the atomic motion. This reduced model provides a fairly accurate description of the phase diagram of the original set of equations and allows for the investigation of more realistic regimes, where the direct simulation of the full model would be otherwise unfeasible. As a result, we find that the onset of a coherent field can be either described by a second- or first-order transition, the former scenario being observable only below a given temperature. Moreover, the first-order transition is accompanied by an intrinsic optical bistability region.

Neutron-multiwave-interference phenomena based on Ramsey’s resonance method of “separated oscillating fields” are studied. A neutron passes through N successive resonant coils $(ħ{\omega }_0=2\mathrm{}{\mu }_n{B}_0),$ which flip the neutron spin with a probability $\rho$ smaller than 1. These coils are separated by path lengths L over which a homogeneous field $B_1$ is present. Because the spin-flip probability $\rho$ is smaller than 1, the number of waves for a neutron is doubled after each flipper, so as to produce $2^N$ neutron waves at the end of the setup. The phase difference between any pair of waves is a multiple of a “phase quantum” determined by the line integral of the field difference $B_1-B_0$ over the length L. Highly regular patterns of the quantum-mechanical probability R in ${(B}_1,\rho )$ space appear due to pair interference between individual waves. Possible applications of this phenomenon, such as a direct measurement of n-particle correlation function, are pointed out.

A theory of an atom beam splitter is developed involving the interaction of standing-wave field pulses with $\Lambda$-type atoms. The beam splitter consists of two interaction zones, both treated in the Raman-Nath approximation. There is an initial, off-resonant field (or fields) that prepares the initial atomic state, followed by interaction with a pair of standing-wave field pulses. Using this configuration one is able to create a large-angle beam splitter with a significant suppression of unwanted momentum components. The roles of relaxation and a frequency chirp are investigated, as are initial conditions for which both ground states of the $\Lambda$ scheme are prepared by the first field. It is shown that the scheme is rather robust against fluctuations of system parameters.

We calculate the spectrum of low-lying collective excitations in a gaseous cloud formed by a Bose-Einstein condensate and a spin-polarized Fermi gas over a range of the boson-fermion coupling strength extending from strongly repulsive to strongly attractive. Increasing boson-fermion repulsions drives the system towards spatial separation of its components (“demixing”), whereas boson-fermion attractions drives it towards implosion (“collapse”). The dynamics of the system is treated in the experimentally relevant collisionless regime by means of a random-phase approximation, and the behavior of a mesoscopic cloud under isotropic harmonic confinement is contrasted with that of a macroscopic mixture at given average particle densities. In the latter case the locations of both the demixing and the collapse phase transitions are sharply defined by the same stability condition, which is determined by the softening of an eigenmode of either fermionic or bosonic origin. In contrast, the transitions to either demixing or collapse in a mesoscopic cloud at fixed confinement and particle numbers are spread out over a range of boson-fermion coupling strength, and some initial decrease of the frequencies of a set of collective modes is followed by hardening, as evidenced by blue shifts of most eigenmodes. The spectral hardening can serve as a signal of the impending transition and is most evident when the number of bosons in the cloud is relatively large. We propose physical interpretations for these dynamical behaviors with the help of suitably defined partial compressibilities for the gaseous cloud under confinement.

A many-body density-matrix theory is derived by including quasiparticle renormalization of kinetic energy and dipole coupling to an external electromagnetic field, as well as the screening and quantum-interference effects. This theory is applied to a three-level resonant asymmetric double-quantum-well system in which the ground subband is coupled to the upper tunneling-split doublet by a strong external electromagnetic field. By using this theory, the quasiparticle energy-level separations and off-diagonal radiative-decay coupling rates, absorption coefficient, refractive-index function, and scaled subband electron density are calculated as functions of incident photon energy. The effects of quasiparticle renormalization on the quantum interference between a pair of optically induced polarizations are analyzed. The quantum interference is shown to be robust against the Coulomb-interaction effect in the mean-field approximation. The roles played by the dephasing rate and electron density are explained.

We analyze theoretically and experimentally the polarization dynamics of vertical-cavity surface-emitting lasers exposed to isotropic optical feedback. The theoretical investigations are based on a model that takes into account different inversion populations for charge carriers with opposite spin. In the deterministic case, we observe toruslike synchronized low-frequency fluctuations of one or both polarization modes in the vicinity of the solitary laser threshold, depending on the parameters and initial conditions. Both in experiment and in simulations including spontaneous emission noise, asymmetric low-frequency fluctuations are observed above and below the solitary laser threshold as well as a transition to coherence collapse further above threshold. The amount of excitation of polarization degrees of freedom and thus the occurrence of simultaneous low-frequency fluctuations of both polarization modes is shown to depend on the magnitude of the dichroism. Apart from the correlation properties on the external-cavity time scale, we find a good qualitative agreement between experiments and simulations.

We investigate two models for performing topological quantum gates with the Aharonov-Bohm (AB) and Aharonov-Casher (AC) effects. Topological one- and two-qubit Abelian phases can be enacted with the AB effect using charge qubits, whereas the AC effect can be used to perform all single-qubit gates (Abelian and non-Abelian) for spin qubits. Possible experimental setups suitable for a solid-state implementation are briefly discussed.

A general model for an intersection of two electronic states dependent on one nuclear coordinate is considered. A perturbational series expansion resulting from the inclusion of the nonadiabatic coupling starting from the Born-Oppenheimer approximation in zeroth order is set up. It is proven that the series has a finite radius of convergence and that it can be summed up exactly.

Atoms in high-$(n,\mathcal{l})$ states formed in cold Rydberg plasmas decay to the ground state in a succession of radiative transitions populating intermediate excited states. A classical treatment presents radiative cascade in a physically transparent way and reveals the “trajectory” in $n\mathcal{l}$ space obeyed by the cascade, scaling rules, and other aspects hidden within the quantal approach. Quantal-classical correspondence in radiative decay is directly demonstrated. Classical transition probabilities are also presented and are in excellent agreement with quantal transition probabilities, even for moderate quantum numbers.

We demonstrate the implementation of an entanglement swapping operation on a four-qubit liquid-state nuclear-magnetic-resonance (NMR) quantum-information processor. We use this experiment as a benchmark to illustrate the progress made in the field of quantum control using strongly modulating pulses and a correction scheme for removing distortions introduced by the nonlinearities in the transmitter and probe circuits. The advances include compensating for incoherent errors caused by the spatial variation of the system Hamiltonian in the NMR sample. The goal of these control sequences is to cause the collapse of the Kraus operator-sum representation of the superoperator into one unitary operator so that the ensemble appears to evolve as one coherent whole.

We provide the expression of the exchange-correlation potential in the adiabatic connection fluctuation-dissipation (ACFD) framework, for arbitrary time-dependent (TD) kernels. We investigate the asymptotic behavior of the ACFD potential in three relevant approximations: the random-phase approximation, the exact-exchange kernel in two-electron systems, and the adiabatic local-density approximation. We show that these potentials have the expected $-{1/r+Q/r}^3-\alpha {/(2r}^4)$ tail (in closed-shell systems with spherical symmetry), where Q and $\alpha$ depend on the TD kernel and reflect the physics included in each approximation. We also discuss approximate ACFD potentials that are much simpler to compute than the exact ones while being likely of reasonable accuracy.

Total cross sections (TCSs) for electron and positron scattering from hexafluorobezene $\left({\mathrm{C}}_6{\mathrm{F}}_6\right)$ molecules have been measured by the linear transmission time-of-flight method and are presented. In addition, new systematic measurements have been also carried out for benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$-molecule targets. The impact energies are 0.4–1000 eV and 0.2–1000 eV for electron and positron scattering, respectively. For the case of ${\mathrm{C}}_6{\mathrm{F}}_6,$ two resonance peaks have been observed in ${\mathrm{C}}_6{\mathrm{F}}_6$ electron TCSs: one at 0.8 eV attributed to the resonant capture of the incident electron into the ${\pi }^{*}$ orbital, with the formation of a transient ${\mathrm{C}}_6{\mathrm{F}}_6^{-}$ anion, and the broad one in the range above 7 eV showing weak spikes at 14 and 30 eV, of which the 14-eV one has been attributed to another negative-ion resonance formation with decomposition either through the reemission of the electron or through dissociative electron attachment leading to ${\mathrm{F}}^{-},$ ${\mathrm{C}}_5{\mathrm{F}}_3^{-},$ and ${\mathrm{C}}_6{\mathrm{F}}_5^{-}$ fragmented ions. In contrast, the positron TCSs show no conspicuous strong structure but 2 to 3 weak shoulders overlaid on a broad (6–100)-eV peak arising from a series of electronic excitations, positronium formations, and ionizations. A close comparison with those for ${\mathrm{C}}_6{\mathrm{H}}_6$ sheds much light on better understanding of the underlying dynamics.

The theoretical developments presented in a preceding companion paper by Wassaf et al. [Phys. Rev. A $67,$ 053405 (2003)], for simulating photoelectron spectra, are used to address several issues regarding the harmonic generation process. Both above-threshold Ionization (ATI) and high-order harmonic generation are observed when atoms are submitted to a laser field with intensity around ${I=10\mathrm{}}^{14}{\mathrm{W}\mathrm{}\mathrm{cm}}^{-2}.$ Here, we demonstrate that the resonances, together with multiple recollisions processes, which have been shown to be at the origin of enhancements of the magnitudes of ATI peaks in the high-energy range, can also play a determining role on the magnitudes of harmonic lines within the plateau. These findings have been obtained via a set of quantum and classical simulations for two classes of one-dimensional model potentials, i.e., either long range (Coulomb-like) or short range (with an exponentially decreasing tail). They are confirmed by following the time evolution of the emission rate of selected harmonics with the help of a (waveletlike) Gabor time-frequency analysis.

In this paper we present a double-slit experiment where two indistinguishable photons produced by type-I parametric down-conversion are each sent to a well-defined slit. Data about the diffraction and interference patterns for coincidences are presented and discussed. An analysis of these data allows a test of standard quantum mechanics against the de Broglie–Bohm theory.

The Kullback-Leibler divergence offers an information-theoretic basis for measuring the difference between two given distributions. Its quantum analog, however, fails to play a corresponding role for comparing two density matrices, if the reference states are pure states. Here it is shown that nonadditive quantum information theory inspired by nonextensive statistical mechanics is free from such a difficulty and the associated quantity, termed the quantum q-divergence, can in fact be a good information-theoretic measure of the degree of state purification. The correspondence relation between the ordinary divergence and the q-divergence is violated for the pure reference states, in general.