Circular dichroism of a chiral molecule, S(+)2-butanol, is investigated in intense laser light. Although theory suggests the possibility of enhanced differences in absorption (leading to ionization) of circularly polarized light of a particular handedness by a random ensemble of chiral molecules, we report results of experiments that fail to observe such effects. While the ionization probability does not significantly change with the ellipticity of the incident light, fragmentation decreases with enhanced ellipticity. The kinetic-energy release accompanying ${\mathrm{CH}}^{+}$ formation decreases as the laser light becomes more intense, a possible manifestation of bond softening.

Photoabsorption spectra have been measured as a continuous function of energy and applied electric field for the $m=0,$ 1, and 2 Rydberg series in sodium. The spectra are Fourier transformed in scaled coordinates, generating scaled recurrence maps. The continuous nature of the scaled maps allows us to investigate patterns in the recurrence strength with good resolution.

We show that the decoherence of a superposition of two coherent states of the electromagnetic field in a cavity may be interpreted as a phase-diffusion process.

The relativistic configuration-interaction method based on numerical Dirac-Fock wave functions (CIDF) is employed to determine $6s6p{}^3{P}_{0,1,2},$ ${}^1{P}_1–6s7s{}^3{S}_1{,}^1{S}_0$ transition energies and probabilities in mercury. The influence of electron correlation on both transition energies and probabilities is investigated. An alternative approach to the generation of spin orbitals used to construct configuration interaction wave functions is proposed. This is a configuration interaction method with relativistic wave functions generated using an ab initio model potential [CIDF(MP)]. The CIDF(MP) method is tested for resonance spin-forbidden and spin-allowed ${6s}^2{}^1{S}_0–6s6p$ transitions in mercury and later successfully used for $6s6p–6s7s$ transition array.

When excited hydrogen atoms are driven by collinear static and microwave electric fields, we discover a series of strong-field resonances that can ionize most substates with a given principal quantum number $n_0.$ We describe a quantal theory using an adiabatic basis that explains the origin of the resonances and why they disappear at certain ratios of the two fields. Our three-dimensional classical Monte Carlo calculations reproduce this behavior. We demonstrate n selectivity sufficient for continued development of this process as a practical detector.

A numerical model to calculate the high-order harmonics spectrum of a macroscopic gas target irradiated by a few-optical-cycle laser pulse is presented. The single-atom response, calculated within the nonadiabatic strong-field approximation, is the source term of a three-dimensional propagation code. The simulation results show remarkably good agreement with experiments performed in neon using laser pulses with durations of 30 and 7 fs. Both simulations and experiments show discrete and well-resolved harmonics even for the shortest driving pulses.

We propose a general method for diagonalizing a wide class of nonlinear Hamiltonians describing different quantum optical models. This method makes use of a nonlinear deformation of the usual su(2) algebra and when some physical parameter, dictated by the particular model under consideration, becomes small, it gives a diagonal effective Hamiltonian that describes correctly the dynamics for arbitrary states and long times. We apply as well the technique to three-level systems interacting with quantum fields, showing that it is possible to engineer resonant interactions through nonresonant processes.

Experimental measurements of scaling with cluster and laser parameters of the energies of ions produced in the explosion of atomic clusters have been obtained, with a view to experimental optimization of the cluster explosion temperature. The noble-gas clusters were irradiated with high-intensity, 200-fs laser pulses. Ion energy scalings with cluster size (ranging from ${10}^2–{10}^5$ atoms per cluster), laser intensity ${(10\mathrm{}}^{14}–{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{-}2}),$ and laser wavelength (780 and 390 nm) are presented for both Xe and Kr clusters. Numerical calculations of the interaction, treating the cluster as a spherical nanoplasma, are also presented.

An efficient method is investigated for the generation of circularly polarized high-order harmonics by a bichromatic laser field whose two components with frequencies $\omega$ and $2\omega$ are circularly polarized in the same plane, but rotate in opposite directions. The generation of intense harmonics by such a driving-field configuration was already confirmed by a previous experiment. With the help of both a semiclassical three-step model as well as a saddle-point analysis, the mechanism of harmonic generation in this case is elucidated and the plateau structure of the harmonic response and their cutoffs are established. The sensitivity of the harmonic yield and the polarization of the harmonics to imperfect circular polarization of the driving fields are investigated. Optimization of both the cutoff frequency and the harmonic efficiency with respect to the intensity ratio of the two components of the driving field is discussed. The electron trajectories responsible for the emission of particular harmonics are identified. Unlike the case of a linearly polarized driving field, they have a nonzero start velocity. By comparison with the driving-field configuration where the two components rotate in the same direction, the mechanism of the intense harmonic emission is further clarified. Depending on the (unknown) saturation intensity for the bichromatic field with counter-rotating polarizations, this scheme might be of practical interest not only because of the circular polarization of the produced harmonics, but also because of their production efficiency.

We study analytically the quantum corrections to the Gross-Pitaevskii equation for two weakly linked Bose-Einstein condensates. The goals are (1) to investigate dynamical regimes at the borderline between the classical and quantum behavior of the bosonic field, and (2) to search macroscopic quantum coherence phenomena not observable with other superfluid/superconducting systems. Quantum fluctuations renormalize the classical Josephson oscillation frequencies. Large amplitude phase oscillations are modulated, exhibiting collapses and revivals. We describe an interwell oscillation mode with a vanishing (ensemble averaged) mean value of the observables, but with oscillating mean square fluctuations. We finally discuss the limit in which the classical Gross-Pitaevskii (Josephson) dynamics is recovered.

Dirac bras and kets are explicitly defined for phase-space representations of quantum mechanical observables, and simple transformation rules for changing representations are constructed. Derivations in the Wigner-Weyl representation of an expression for the kernel of the quantum Liouville operator, and of a relation between its stationary and nonstationary eigenstates, demonstrate the power of the resultant algebra.

We experimentally investigate the resonance fluorescence spectrum of single ${}^{171}\mathrm{Yb}$ and ${}^{172}\mathrm{Yb}$ ions that are laser cooled to the Lamb-Dicke regime in a radiofrequency trap. While the fluorescence scattering of ${}^{172}\mathrm{Yb}$ is continuous, the ${}^{171}\mathrm{Yb}$ fluorescence is interrupted by quantum jumps because a nonvanishing rate of spontaneous transitions leads to electron shelving in the metastable hyperfine sublevel ${}^2{D}_{3/2}\mathrm{}(F=2\mathrm{}).$ The average duration of the resulting dark periods can be varied by changing the intensity of a repumping laser field. Optical heterodyne detection is employed to analyze the fluorescence spectrum near the Rayleigh elastic scattering peak. It is found that the stochastic modulation of the fluorescence emission by quantum jumps gives rise to a Lorentzian component in the fluorescence spectrum, and that the linewidth of this component varies according to the average duration of the dark fluorescence periods. The experimental observations are in quantitative agreement with theoretical predictions.

We introduce a formalism for the calculation of the time of arrival t at a space point for particles traveling through interacting media. We develop a general formulation that employs quantum canonical transformations from the free to the interacting cases to compute t in the context of the positive-operator-valued measures. We then compute the probability distribution in the times of arrival at a point for particles that have undergone reflection, transmission or tunneling off finite potential barriers. For narrow Gaussian initial wave packets we obtain multimodal time distribution of the reflected packets and a combination of the Hartman effect with unexpected retardation in tunneling. We also employ explicitly our formalism to deal with arrivals in the interaction region for the step and linear potentials.

We theoretically study a double-quantum-dot hydrogen molecule in the GaAs conduction band as the basic elementary gate for a quantum computer, with the electron spins in the dots serving as qubits. Such a two-dot system provides the necessary two-qubit entanglement required for quantum computation. We determine the excitation spectrum of two horizontally coupled quantum dots with two confined electrons, and study its dependence on an external magnetic field. In particular, we focus on the splitting of the lowest singlet and triplet states, the double occupation probability of the lowest states, and the relative energy scales of these states. We point out that at zero magnetic field it is difficult to have both a vanishing double occupation probability for a small error rate and a sizable exchange coupling for fast gating. On the other hand, finite magnetic fields may provide finite exchange coupling for quantum computer operations with small errors. We critically discuss the applicability of the envelope-function approach in the current scheme, and also the merits of various quantum-chemical approaches in dealing with few-electron problems in quantum dots, such as the Hartree-Fock self-consistent-field method, the molecular-orbital method, the Heisenberg model, and the Hubbard model. We also discuss a number of relevant issues in quantum dot quantum computing in the context of our calculations, such as the required design tolerance, spin decoherence, adiabatic transitions, magnetic-field control, and error correction.

We measured the polarized binary collision-induced light scattering spectrum from room-temperature gaseous helium over the greatly extended spectral domain $\nu =5\mathrm{}–1200{\mathrm{cm}}^{\mathrm{-}1},$ and then deduced the isotropic spectrum that we report in the interval $\nu =100\mathrm{}–1200{\mathrm{cm}}^{\mathrm{-}1}.$ Our experimental results were calibrated on an absolute scale. These data were compared to those from quantum-mechanical computations, where use of advanced induced-polarizability incremental traces was made. This comparison enabled us to check several trace models and to show that for certain ab initio ones the computed spectra agree well with our measurements. As was borne out from our computations the main contributions to the spectral intensities come from the exchange component of the incremental trace and its negative mixing with the rest of the trace components.

Photoionization of the ground ${1s}^2{2s}^2{}^{1}S$ state of atomic beryllium is studied in the energy region between the ${\mathrm{Be}}^{+}\mathrm{}\left(2s\right)\mathrm{}$ and ${\mathrm{Be}}^{+}\mathrm{}\left(3d\right)\mathrm{}$ thresholds using a noniterative variational R-matrix method combined with multichannel quantum-defect theory at the R-matrix surface. The autoionizing Rydberg series of resonances converging to the ${\mathrm{Be}}^{+}$ $(2p,$ $3s,$ $3p,$ and $3d)$ thresholds are identified and effective quantum numbers assigned. Also calculated are the photoelectron angular distribution asymmetry parameters β for the processes leaving the ion in the ${\mathrm{Be}}^{+}$ ${(1s}^{2}np,$ $n=2,3)$ states. The present photoionization cross sections extend earlier work and are in good agreement with previous calculations and experiments. There is excellent agreement between length and velocity gauges in our calculation.

A very simple analytical model is presented to compute the time evolution of an atomic electron interacting with an ultrastrong high-frequency laser field. The model takes into account the evolution of the electronic wave function in terms of the action and trajectory of a classical electron. It shows the possibility of atomic excitation by a space-dependent shift of the wave function. It is an extension of the well-known strong-field approximation and the dipole Volkov states to the case of space-dependent fields, in the very-high-frequency regime. The great advantage of the model lies in its applicability in situations where the dipole approximation does not hold and, therefore, ab initio numerical computation of the time-dependent Schrödinger equation would require extremely long run times.

We have applied the state-specific expansion approach (SSEA) to the solution of the time-dependent Schrödinger equation describing the interaction of He with laser pulses of $ħ\omega =5\mathrm{}\mathrm{eV},$ which have a shape with a sharp rise and peak intensities in the range $5\times {10}^{14}–3.46\times {10}^{15}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ The calculations include the effects of the state-specific self-consistent field, the electron correlation, and the $n=2\mathrm{}$ intrashell doubly excited states. The results show that, provided the pulses have fast rise times, neutral He can generate harmonics of higher order than previously thought before the ${\mathrm{He}}^{+}$ contribution takes over. The computed spectra agree with the observations of Sarukura et al. [Phys. Rev. A 43, 1669 (1991)]. In particular, the SSEA results reveal “anomalously” high peaks for the 13th and 21st harmonics, in agreement with experiment.

We discuss how the nature of the relative phase between two entangled condensates differs depending on whether the entanglement is established by measurements or by coupling. In the former case, the limit of the resulting phase resolution scales inversely with the square root of the number of atoms. This limit can be surpassed, however, when the condensates are entangled by coupling and can reach the fundamental Heisenberg limit where the phase resolution scales inversely with the number of atoms involved.