The numerical integration of the three-dimensional time-dependent Schrödinger equation that appears in the impact-parameter approximation is carried out using a Fourier collocation method that is based on a splitting technique for the time evolution operator. The final wave function is analyzed in momentum space using a histogram approach. Singly and doubly differential cross sections for ionization as a function of polar emission angle and energy are calculated near the maximum in the total cross section, i.e., at 50–60-keV impact energy. They compare well with recent experimental results for intermediate and high electron emission energies, while some discrepancies remain at low energies due to the neglect of residual Coulomb interactions in the ionized wave packet. In order to achieve convergence and to reproduce experimental results at electron energies below 20 eV one needs to extend the calculations to final internuclear separations beyond 30 a.u.

The ${\mathrm{Li}}^{2+}$ to ${\mathrm{Li}}^{+}$ production cross-section ratio of ground-state atomic Li by electron-impact ionization has been measured for electron energies ranging from 200 eV to 1500 eV. The measurements were done using a pulsed, ion imaging time-of-flight spectrometer with Li atoms confined in a magneto-optical trap. The ratios are more accurate than the single earlier result for the ${\mathrm{Li}}^{2+}$ to ${\mathrm{Li}}^{+}$ ratios, a composite of two absolute measurements, and are systematically lower. Both experiments show similar energy dependences that disagree with the trend predicted by a semiempirical formulation. These measurements provide a benchmark for theoretical studies of electron-impact double ionization.

We suggest a method to derive the nonlinear wave equations suitable for describing the propagation of light pulses as short as two optical cycles in transparent nonlinear optical media. The equations are suitable for efficient numerical simulation of the propagation of extremely short pulses without the need to resort to any type of envelope approximation, although they contain these as a limiting case. We demonstrate the power of this approach by modeling some recent experiments in which ultrabroadband radiation was generated upon propagating 150 fs duration 390 nm pulses in a deuterium-gas-filled hollow-core waveguide. The calculated spectrum agrees well with the measured spectrum.

We propose a scheme to implement the two-qubit Grover’s quantum search algorithm using cavity quantum electrodynamics. In this scheme long-lived electronic states in circular Rydberg atoms are used as quantum bits (qubits). The atoms interact with the electromagnetic field of a nonresonant cavity. The quantum gate dynamics is provided by a cavity-assisted collision. We present the detailed procedure and analyze the experimental feasibility.

We investigate the spatial distribution of electrons photodetached from a polar molecular anion in an external static electric field. The large electron-molecule scattering amplitude strongly enhances the rescattering effect. This opens an opportunity for experimental investigation of collisions of ultraslow electrons (in the energy range below 0.1 meV) by polar molecules.

Displaced Fock states of the electromagnetic field have been synthesized by overlapping the pulsed optical single-photon Fock state $|1〉$ with coherent states on a high-reflection beam splitter and completely characterized by means of quantum homodyne tomography. The reconstruction reveals nonclassical properties of displaced Fock states, such as negativity of the Wigner function and photon number oscillations. To our knowledge, this is the first time complete tomographic reconstruction has been performed on a highly nonclassical optical state.

We report the implementation of two- and three-spin quantum erasers using nuclear magnetic resonance (NMR). Quantum erasers provide a means of manipulating quantum entanglement, an important resource for quantum information processing. Here, we first use a two-spin system to illustrate the essential features of quantum erasers. The extension to a three-spin “disentanglement eraser” shows that entanglement in a subensemble can be recovered if a proper measurement of the ancillary system is carried out. Finally, we use the same pair of orthogonal decoherent operations used in quantum erasers to probe the two classes of entanglement in tripartite quantum systems: the Greenberger-Horne-Zeilinger state and the W state. A detailed presentation is given of the experimental decoherent control methods that emulate the loss of phase information in strong measurements, and the use of NMR decoupling techniques to implement partial trace operations.

A lattice-gas algorithm for the one-dimensional diffusion equation is realized using radio frequency pulses in a one-dimensional spin system. The model is a large array of quantum two-qubit nodes interconnected by the nearest-neighbor classical communication channels. We present a quantum protocol for implementation of the quantum collision operator and a method for initialization and reinitialization of quantum states. Numerical simulations of the quantum-classical dynamics are in good agreement with the analytic solution for the diffusion equation.

We study the scissor mode (orbital $M1\mathrm{}$ excitations) in small Na clusters, triaxial metal clusters ${\mathrm{Na}}_{12}$ and ${\mathrm{Na}}_{16}$ and the close-to-spherical ${\mathrm{Na}}_9^{+},$ all described in density-functional theory with detailed ionic background. The scissor modes built on spin-saturated ground and spin-polarized isomeric states are analyzed in virtue of both macroscopic collective and microscopic shell-model treatments. It is shown that the mutual destruction of Coulomb and the exchange-correlation parts of the residual interaction makes the collective shift small and the net effect can depend on details of the actual excited state. The cross-talk with dipole and spin-dipole modes is studied in detail. In particular, a strong cross-talk with spin-dipole negative-parity mode is found in the case of spin-polarized states. Triaxiality and ionic structure considerably complicate the scissor response, mainly at the expense of stronger fragmentation of the strength. Nevertheless, even in these complicated cases the scissor mode is mainly determined by the global deformation. The detailed ionic structure destroys the spherical symmetry and can cause finite $M1\mathrm{}$ response (transverse optical mode) even in clusters with zero global deformation. But its strength turns out to be much smaller than for the genuine scissor modes in deformed systems.

In this work we undertake an analytical and numerical investigation of the motion of a neutral two-level atom in a plane standing electromagnetic wave using a semiclassical approximation. In our study we neglect the energy loss from radiation. We show that, for experimentally achievable parameter values, the dynamics of the atom can be chaotic.

Strong, short laser pulses have been used to ionize diatomic molecules and rare-gas atoms. Using mixed species targets, intensity-dependent ionization yield ratios have been measured directly for pairs of molecules and atoms with similar ionization potentials. Specifically, ionization rate ratios for homonuclear $({\mathrm{N}}_2:\mathrm{Ar},{\mathrm{F}}_2:\mathrm{Ar},{\mathrm{D}}_2:\mathrm{Ar},{\mathrm{O}}_2:\mathrm{Xe},{\mathrm{S}}_2:\mathrm{Xe})$ and heteronuclear (CO:Kr, NO:Xe, SO:Xe) molecules have been obtained. Our experimental results are compared to the predictions of several approximate theoretical models. In general, these models fail to accurately describe the detailed differences in the intense laser ionization rates of atomic and diatomic targets.

Closed-orbit theory was initially developed as a qualitative and quantitative tool to interpret the dynamics of excited hydrogen in static external fields: the modulations in the photoabsorption spectrum were explained in terms of classical orbits closed at the nucleus. We consider the closed-orbit theory formalism appropriate for molecules in fields. The theoretical extensions are described, and semiclassical calculations based on this formalism are undertaken and compared to quantum R-matrix calculations for model molecules in a static magnetic field. We find that the spectral modulations can be analyzed simply in terms of the scattering of the excited electron on the molecular core. In addition to elastic scattering, modulations produced by inelastic scattering are essential to account for the photoabsorption spectrum. Through this process, an electron along a closed orbit in the classically chaotic regime exchanges energy with the core and comes out along an orbit in the near integrable regime. The relative importance of elastic and inelastic scattering depends on the molecular quantum defects.

We consider classical and quantum dynamics on potentials that are asymptotically unbounded from below. By explicit construction we find that quantum bound states can exist in certain bottomless potentials. The classical dynamics in these potentials has the property that only a set of zero measure of classical trajectories can escape to infinity. All other trajectories get trapped as they get further out into the asymptotic region.

We analyze the interaction of N laser fields with a $\mathrm{}(N+1\mathrm{})\mathrm{}$-level quantum system. A general analytic expression for the steady-state linear susceptibility for a probe-laser field is obtained and we show that the system can exhibit multiple electromagnetically induced transparency, with at most $N-1$ transparency windows occurring in the system. The group velocity of the probe-laser pulse can also be controlled.

We present in this paper a study of the radiation emitted after the explosion of clusters of propane and carbon dioxide. The highly charged ions produced in the explosion can resonantly transfer an electron from the buffer gas, helium, into highly excited states. Radiation of wavelength as short as 11.6 nm was measured from ${\mathrm{O}}^{5+},$ and shorter than 4 nm from ${\mathrm{C}}^{4+}.$

The macroscopic coherent tunneling through the barriers of a periodic potential is used as an atom-optical filter to separate the center of mass of the condensate and the thermal components of a ${}^{87}\mathrm{Rb}$ mixed cloud. We condense in the combined potential of a laser standing wave superimposed on the axis of a cigar-shaped magnetic trap and induce condensate dipole oscillation in the presence of a static thermal component. The oscillation is damped due to the interaction with the thermal fraction and we investigate the role played by the periodic potential in the damping process.

We present some applications of high-efficiency quantum interrogation (“interaction-free measurement”) for the creation of entangled states of separate atoms and of separate photons. The quantum interrogation of a quantum object in a superposition of object-in and object-out leaves the object and probe in an entangled state. The probe can then be further entangled with other objects in subsequent quantum interrogations. By then projecting out those cases in which the probe is left in a particular final state, the quantum objects can themselves be left in various entangled states. In this way, we show how to generate two-, three-, and higher-qubit entanglement between atoms and between photons. The effect of finite efficiency for the quantum interrogation is delineated for the various schemes.

The question of convergence in two-center close-coupling expansions is addressed via the study of positron-hydrogen scattering. It is found that the major cross sections do converge if sufficient number of states from a complete basis, centered separately on the hydrogen atom and positronium, are used to expand the total scattering wave function. The underlying equations are computationally highly ill conditioned, and a simple, numerically efficient technique is given that alleviates the problem. Generally, we find good agreement with available experiment and some previous theory. However, calculations that only used eigenstates for the positronium center yielded cross sections for positronium formation in the $2s$ and $2p$ states that are higher than the convergent ones obtained in this work.

A two-center continuum approximation with correct boundary conditions in the entrance and exit channels is introduced to study the $\mathrm{}(e,2e)\mathrm{}$ reaction for ${\mathrm{H}}_2$ targets. The case of impact of fast projectiles for highly asymmetric coplanar geometries is investigated. The influence of the description of the ejected electron on triple differential cross sections is analyzed. Particular attention is paid to the relevance of the screening of the nuclei of the target produced by the nonejected electron. The model, which implies relatively small computational effort, gives a good representation of absolute experimental data.