High-order harmonic generation from molecules (${\mathrm{O}}_2$, ${\mathrm{N}}_2$, ${\mathrm{H}}_2$, and $\mathrm{C}{\mathrm{O}}_2$) and atoms (Xe, Ar, and Kr) has been studied in the few optical cycle domain. Two laser peak intensities; $2\times {10}^{14}$ and $6\times {10}^{14}\mathrm{W}{\mathrm{cm}}^{-2}$, were compared. At the lower intensity spectra were approximately the same for molecules and atoms with the same ionization potential, at higher laser intensity the cutoff of ${\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ extends far beyond the cutoff of Xe and Kr, respectively, in contrast with ${\mathrm{N}}_2$ and ${\mathrm{H}}_2$ which exhibit cutoffs very close to that of Ar. This behavior is well explained by adopting an atomlike approximation for the molecule response in the high-field regime and employing the Lewenstein’s model, properly modified in order to account for the nonlinear dipole moment of a randomly oriented molecule ensemble.

A formalism for multiport devices is developed in which the creation and annihilation operators at the output are related with those at the input through a finite Fourier transform. By considering two specific inputs of number states and coherent states, we show how these devices can be used to produce multimode Schrödinger cat states and can be combined to create multipath interferometers. We introduce uncertainty relations for the distributions of particles in the input modes and the correlations between the output modes. These allow us to determine how the phase resolution of the output state scales with the number of ports and particles. This scaling is of fundamental interest for schemes that seek to use multipath interferometers for enhanced measurement precision.

A generalized nonlocal energy-balance equation for excited carriers and phonons is established for studying the laser cooling of a lattice of a wide-band-gap semiconductor such as AlN using a laser through a three-photon nonlinear excitation process. The power-exchange densities of the system are calculated and compared for different strengths of the excitation field. When the power-exchange density is positive, it implies laser cooling of the lattice. The effects of initial lattice temperature and field-frequency detuning on the laser-cooling phenomenon under the three-photon nonlinear excitation process are described. The power-exchange densities are compared for both laser cooling and heating using linear and nonlinear excitations. We find that the linear excitation seems more favorable than the nonlinear excitation for laser cooling. However, the resonant three-photon nonlinear absorption will allow us the use of a common laser for laser cooling of the lattice in AlN, rather than a more expensive ultraviolet laser.

We propose a design for a quantum-information processor where qubits are encoded into hyperfine states of ions held in a linear array of individually tailored linear microtraps and sitting in a spatially varying magnetic field. The magnetic field gradient introduces spatially dependent qubit transition frequencies and a type of spin-spin interaction between qubits. Single- and multiqubit manipulation is achieved via resonant microwave pulses as in liquid-NMR quantum computation while the qubit readout and reset is achieved through trapped-ion fluorescence shelving techniques. By adjusting the microtrap configurations we can tailor, in hardware, the qubit resonance frequencies and coupling strengths. We show that the system possesses a sideband transition structure which does not scale with the size of the processor, allowing scalable frequency discrimination between qubits. By using large magnetic field gradients, one can reset individual qubits in the ion chain via frequency selective optical pulses to implement quantum-error correction, thus avoiding the need for many tightly focused laser beams.

A systematic method for generating bound entangled states in any bipartite system, with ranks ranging from five to full rank, is presented. These states are constructed by mixing separable states with unextendable product basis–generated bound entangled states that are positive under partial transposition (PPT). A subset of this class of PPT bound entangled states, having less than full rank, is shown to satisfy the range criterion [P. Horodecki, Phys. Lett. A 232, 333 (1997)].

The nonlinear collective electron dynamics of a cluster irradiated by a strong near-infrared linearly polarized short laser pulse are studied by classical molecular-dynamics simulations for a small model cluster with $\sim {10}^3$ particles. The model brings forth almost all of the features of a cluster exposed to a strong laser pulse, such as inner and outer ionization, expansion of the ion core, etc. When the frequency of the incident radiation is near three- or five-photon resonance with the (time-dependent) frequency of the dipole Mie-plasmon excitation in the laser-ionized and expanding cluster, both the total electron acceleration and the local electric field acting on the ions inside the cluster exhibit a resonant enhancement at the odd harmonics of the fundamental frequency. The time evolution during the laser pulse of these odd harmonics is discussed for different parameters of the laser-cluster interaction. The presence of even low-order harmonics (in particular, of the second) in the local electric field at ion positions off the cluster center is also demonstrated. This indicates nonlinear laser excitation of the quadrupole surface plasmon.

The most basic scenario of quantum control involves the organized manipulation of pure dynamical states of the system by means of unitary transformations. Recently, Vilela Mendes and Man’ko have shown that the conditions for controllability on the state space become less restrictive if unitary control operations may be supplemented by projective measurement. The present work builds on this idea, introducing the additional element of indirect measurement to achieve a kind of remote control. The target system that is to be remotely controlled is first entangled with another identical system, called the control system. The control system is then subjected to unitary transformations plus projective measurement. As anticipated by Schrödinger, such control via entanglement is necessarily probabilistic in nature. On the other hand, under appropriate conditions the remote-control scenario offers the special advantages of robustness against decoherence and a greater repertoire of unitary transformations. Simulations carried out for a two-level system demonstrate that, with optimization of control parameters, a substantial gain in the population of reachable states can be realized.

We probe the dynamic evolution of a Stark wave packet in cesium using weak half-cycle pulses (HCP’s). The state-selective field-ionization spectra taken as a function of HCP delay reveal wave-packet dynamics such as Kepler beats, Stark revivals, and fractional revivals. A quantum-mechanical simulation explains the results as multimode interference induced by the HCP.

We present a generalization of the electron localization function (ELF) that can be used to analyze time-dependent processes. The time-dependent ELF allows the time-resolved observation of the formation, the modulation, and the breaking of chemical bonds, and can thus provide a visual understanding of complex reactions involving the dynamics of excited electrons. We illustrate the usefulness of the time-dependent ELF by two examples: the $\pi \text{−}\pi *$ transition induced by a laser field, and the destruction of bonds and formation of lone pairs in a scattering process.

We describe an all-optical method of making a ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate (BEC). Atoms are first cooled in an optical lattice and loaded into a crossed dipole trap with $1.06\mu \mathrm{m}$ light. Then the density is increased by dynamically reducing the trap size, while the atoms are evaporatively cooled by reducing the light intensity. Every $3.3\mathrm{s}$, we obtain a nearly pure BEC with $3.5\times {10}^5$ atoms in their lowest internal energy state.

Relativistic four-component electronic structure theory using both wave-function (Dirac-Coulomb-Hartree-Fock and second-order many-body perturbation-theory) and density-functional based methods (local density, hybrid, and generalized gradient approximations) is applied to discuss the current status on the accuracy of parity-violation calculations for molecules. As a test case we choose the $\mathrm{C}\text{‐}\mathrm{F}$ stretching mode of CHFClBr, which is currently being investigated by molecular-beam spectroscopy. We show that electron correlation effects are important and cannot be neglected anymore for the parity nonconservation contribution to the total electronic energy. However, electron correlation contributions to parity violation in vibrational transitions of the $\mathrm{C}\text{‐}\mathrm{F}$ stretching mode are less important. The density functionals tested give somewhat different results, but the Becke three-parameter Lee-Yang-Parr functional agrees quite well with the second-order many-body perturbation-theory values. The calculations suggest that electron correlation effects have to be considered for future investigations in parity-violation effects in electronic transitions. The performance of density-functional based methods for this property needs further statistics.

We present the concept of threshold collaborative unitary transformation or threshold quantum cryptography, which is a kind of quantum version of threshold cryptography. Threshold quantum cryptography states that classical shared secrets are distributed to several parties and a subset of them, whose number is greater than a threshold, collaborates to compute a quantum cryptographic function, while keeping each share secretly inside each party. The shared secrets are reusable if no cheating is detected. As a concrete example of this concept, we show a distributed protocol (with threshold) of conjugate coding.

We have measured the double-to-single photoionization ratio of beryllium $\left(1s^{2}2s^2\right)$ between 28 and $40\mathrm{eV}$ and determined the relative single- and double-photoionization cross sections. In this energy region only simultaneous but not sequential emission of both $2s$ electrons can take place. We also compare our data with recent theoretical calculations and find good agreement with our data. The previously found scaling law for the double-to-single photoionization ratio is confirmed with high accuracy.

Recent studies of the decoherence induced by the quantum nature of the laser field driving a two-state atom [J. Gea-Banacloche, Phys. Rev. A 65, 022308 (2002); S. J. van Enk and H. J. Kimble, Quantum Inf. Comput. 2, 1 (2002)] have been questioned by Itano [W. M. Itano, Phys. Rev. A 68, 046301 (2003)] and the proposal made that all decoherence is due to spontaneous emission. We analyze the problem within the formalism of cascaded open quantum systems. Our conclusions agree with the Itano proposal. We show that the decoherence, nevertheless, may be divided into two parts—that due to forwards scattering and to scattering out of the laser mode. Previous authors attribute the former to the quantum nature of the laser field.

We report on the demonstration of broadband squeezed laser beams that show a frequency-dependent orientation of the squeezing ellipse. Carrier frequency as well as quadrature angle were stably locked to a reference laser beam at $1064\mathrm{nm}$. This frequency-dependent squeezing was characterized in terms of noise power spectra and contour plots of Wigner functions. The latter were measured by quantum state tomography. Our tomograph allowed a stable lock to a local oscillator beam for arbitrary quadrature angles with $\pm 1°$ precision. Frequency-dependent orientations of the squeezing ellipse are necessary for squeezed states of light to provide a broadband sensitivity improvement in third-generation gravitational-wave interferometers. We consider the application of our system to long-baseline interferometers such as a future squeezed-light upgraded GEO 600 detector.

We consider intensity-field correlation functions for a two-level atom in a degenerate optical parametric oscillator (OPO), which would result from a conditioned homodyne measurement. Analytic results are obtained in the limit of weak driving fields using quantum trajectory methods for both the transmitted and fluorescent fields. This system is unique in that after detection of a photon, it is known that one excitation is in the system, in either the atom or cavity mode. We find large violations of inequalities satisfied by classical fields, for both transmitted and fluorescent fields. This is in contrast to the usual cavity QED system of an atom in a driven cavity where we do not find nonclassical behavior in the intensity-field correlation function in fluorescence. This is understood in terms of a relationship between the intensity-field correlation function and the second-order intensity correlation function, as well as the different amount of field-atom entanglement in the two systems. We show that for weak-field cavity QED one must have photon bunching to have nonclassical behavior in the intensity-field correlation function. We compare our results to those of an ordinary OPO. Finally, we also consider cross correlations, where we examine the transmitted (fluorescent) field conditioned by detection of a fluorescent (transmitted) photon.

Vibrational optical activity spectroscopies utilizing either circularly polarized ir or circularly polarized visible beams were theoretically investigated by considering the infrared and visible sum-frequency-generation (IV-SFG) schemes. In addition to the purely electric dipole-allowed chiral component of the IV-SFG susceptibility, the polarizability–electric quadrupole hyperpolarizability term also contributes to the vibrationally resonant IV-SFG susceptibility. The circular-intensity-difference signal is shown to be determined by the interferences between the all-electric dipole-allowed chiral component and the polarizability–electric-dipole or electric-dipole–electric-quadrupole Raman optical activity tensor components. The circularly polarized SFG methods are shown to be potentially useful coherent spectroscopic tools for determining absolute configurations of chiral molecules in condensed phases.

Metastability of a particle trapped in a well with a time-periodically oscillating barrier is studied in the Floquet formalism. It is shown that the oscillating barrier causes the system to decay faster in general. However, avoided crossings of metastable states can occur with the less stable states crossing over to the more stable ones. If in the static well there exists a bound state, then it is possible to stabilize a metastable state by adiabatically increasing the oscillating frequency of the barrier so that the unstable state will eventually cross over to the stable bound state. It is also found that increasing the amplitude of the oscillating field may change a direct crossing of states into an avoided one.

We present a generalized tomographic quantum key distribution protocol in which the two parties share a Bell diagonal mixed state of two qubits. We show that if an eavesdropper performs a coherent measurement on many quantum ancilla states simultaneously, classical methods of secure key distillation are less effective than quantum entanglement distillation protocols. We also show that certain classes of Bell diagonal states are resistant to any attempt at incoherent eavesdropping.

We show that the two-slit experiment in which a single quantum particle interferes with itself can be interpreted as a quantum fingerprinting protocol: the interference pattern exhibited by the particle contains information about the environment it encountered in the slits which would require much more communication to learn classically than is required quantum mechanically. An extension to the case where the particle has many internal degrees of freedom is suggested, and its interpretation is discussed. The interpretation of these results is discussed in detail, and a possible experimental realization is proposed.