Solving the one-dimensional time-dependent Schrödinger equation for simple model potentials, we investigate resonance-enhanced high-order harmonic generation, with emphasis on the physical mechanism of the enhancement. By truncating a long-range potential, we investigate the significance of the long-range tail, the Rydberg series, and the existence of highly excited states for the enhancements in question. We conclude that the channel closings typical of a short-range or zero-range potential are capable of generating essentially the same effects.

We estimate the amount of temperature-dependent squeezing and entanglement in the collective excitations of trapped Bose-Einstein condensates. We also demonstrate an alternative method of temperature measurement for temperatures much less than the critical temperature $(T\ll T_c).\mathrm{}$

We investigate the lowest state of a rotating, weakly interacting Bose-Einstein condensate trapped in a harmonic confining potential that is driven by an infinitesimally asymmetric perturbation. Although in an axially symmetric confining potential the gas has an axially symmetric single-particle density distribution, we show that in the presence of the small asymmetric perturbation its lowest state is the one given by the mean-field approximation, which is a broken-symmetric state. We also estimate the rate of relaxation of angular momentum when the gas is no longer driven by the asymmetric perturbation and identify two regimes of “slow” and “fast” relaxation. States of certain symmetry are found to be more robust.

Quantum trajectories describe the stochastic evolution of an open quantum system conditioned on continuous monitoring of its output, such as, by an ideal photodetector. Here we derive (non-Markovian) quantum trajectories for realistic photodetection, including the effects of efficiency, dead time, bandwidth, electronic noise, and dark counts. We apply our theory to a realistic cavity QED scenario and investigate the impact of such detector imperfections on the conditional evolution of the system state. A practical theory of quantum trajectories with realistic detection will be essential for experimental and technological applications of quantum feedback in many areas.

A single inverted two-level atom is used as a theoretical model for a quantum noise detector to investigate fundamental properties of excess noise in an unstable optical resonator. For a symmetric unstable spherical mirror cavity, we develop an analytic quantum description of the field in terms of a complete set of normalizable biorthogonal quasimodes and adjoint modes. Including the interaction with a single two-level atom leads to a description analogous to the Jaynes-Cummings model with modified coupling constants. One finds a strong position and geometry-dependent atomic decay probability proportional to the square root $\sqrt{K}$ of the excess noise factor K at the cavity center. Introducing an additional homogeneous gain one recovers the K-fold emission enhancement that has been predicted before for the linewidth of an unstable cavity laser. We find that excess noise may be viewed as a spatial redistribution of the field quantum noise inside the resonator. Taking a position average of the atomic decay rate over the cavity volume leads to a cancellation of the excess noise enhancement.

The nuclear forward scattering of synchrotron radiation (NFSSR) of two spatially separated but otherwise identical stainless-steel targets is strongly influenced by the application of approximately stepwise Doppler energy shifts to one of the targets in synchronization with the synchrotron radiation (SR) pulse. The decisive role of radiative coupling in NFSSR, i.e., the influence of the radiation field emitted by the upstream target on the oscillators of the downstream target, has been clarified in decoupling and coupling experiments. In decoupling experiments, radiative coupling is initially strong and becomes drastically reduced when the Doppler motion is started some time interval after the prompt excitation of the two-target system by the SR pulse. In addition, due to the Doppler energy shift a pronounced quantum beat (QB) develops. Depending on the starting time of the Doppler motion, the onset of the QB manifests itself as a rapid decrease or increase of the NFSSR intensity. In coupling experiments, the Doppler motion is started before the SR pulse arrives and a QB is observed right after the SR excitation. Some time interval thereafter the motion is stopped, so that the resonance energies of both targets coincide. As a consequence, the QB disappears and radiative coupling can develop. Depending on the time when the Doppler motion is stopped, this coupling can be responsible for the main contribution to the NFSSR intensity. The changes of the evolution due to decoupling or coupling also strongly depend on the details of the Doppler velocity profile. We show that precise velocity profiles on a nanosecond time scale can be derived by applying the method of quantum-beat analysis and independently by a fitting procedure based on the dynamical theory of scattering. All NFSSR evolutions investigated are interpreted and quantitatively described within the dynamical theory in terms of the interference of the wave fields reemitted by the scattering system.

The nonresonant interaction between the high-density excitons in a quantum well and a single-mode cavity field is investigated. An analytical expression for the physical spectrum of the excitons is obtained. The spectral properties of the excitons, which are initially prepared in the number states or the superposed states of the two different number states by the resonant femtosecond pulse pumping experiment, are studied. A numerical study of the physical spectrum is carried out and a discussion of the detuning effect is presented.

The width of the electromagnetically induced transparency resonance is studied in rubidium vapor. Nonlinear dependence of the width on drive intensity that is caused by Doppler broadening and optical pumping is found. Density-matrix analysis supports the observed dependence. The discrepancies between experimental data and theoretical prediction are discussed.

We present a method of distillation of Greenberger-Horne-Zeilinger states from arbitrary tripartite pure states by local operations and classical communication. We go further to discuss the various results we get and calculate the efficiency of the protocol.

We propose a practical arrangement that allows one to reach the Heisenberg limit in precision phase-shift measurements. This is achieved via phase-shift amplification. The arrangement we propose is based on experimental processes already carried out and does not require the use of any special quantum state.

We maintain that the reservations against the use of stimulated Raman adiabatic photoassociation in a nondegenerate gas, expressed by Mackie and Javanainen in a recent manuscript [Phys. Rev. A 60, 3174 (1999)], do not account for the coherence induced by the photoassociating laser. The authors treat the problem of two-color stimulated photoassociation using a set of decoupled three-mode problems, each containing a box eigenstate and the two bound states. We argue that such decomposition is not justified, since in the limit of a very large box all three-mode problems are, in fact, coherently coupled. The three-mode condition can only be met with a set of wave packets whose bandwidth matches the spectral width of the photoassociating pulse. Using this basis set, we show that by the authors’ own formalism; stimulated Raman photoassociation is a viable process in the thermal ensemble.

In the preceding Comment [Phys. Rev. A 65, 027401 (2002)], the authors maintain that we have used incorrect initial conditions in our calculations on photoassociation of a thermal, nondegenerate gas [Phys. Rev. A 58, R789 (1998)], and that our conclusion that stimulated Raman adiabatic passage (STIRAP) is absent in two-color photoassociation is therefore invalid. We refute the technical argument of the Comment. Nonetheless, corresponding to the fact that in quantum mechanics the decomposition of a mixed state into pure states is not unique, the question of the presence or absence of STIRAP turns out to be a matter of semantics.

We demonstrate the possibility of energy-selective removal of cold atoms from a tight optical trap by means of parametric excitation of the trap vibrational modes. Taking advantage of the anharmonicity of the trap potential, we either selectively remove the most energetic trapped atoms or excite those at the bottom of the trap by tuning the parametric modulation frequency. This process, which had been previously identified as a possible source of heating, also appears to be a robust way for forcing evaporative cooling in anharmonic traps.

We trap and cool a gas composed of ${}^{40}\mathrm{K}$ and ${}^{87}\mathrm{Rb},$ using a two-species magneto-optical trap (MOT). This trap represents the first step towards cooling the Bose-Fermi mixture to quantum degeneracy. Laser light for the MOT is derived from laser diodes and amplified with a single high-power semiconductor amplifier chip. The four-color laser system is described, and the single-species and two-species MOTs are characterized. Atom numbers of $1\times {10}^7$ ${}^{40}\mathrm{K}$ and $2\times {10}^9$ ${}^{87}\mathrm{Rb}$ are trapped in the two-species MOT. Observation of trap loss due to collisions between species is presented and future prospects for the experiment are discussed.

We present a probabilistic quantum processor for qudits on a single qudit of dimension N. The processor itself is represented by a fixed array of gates. The input of the processor consists of two registers. In the program register the set of instructions (program) is encoded. This program is applied to the data register. The processor can perform any operation on a single qudit of dimension N with a certain probability. For a general unitary operation, the probability is ${1/N}^2,$ but for more restricted sets of operators the probability can be higher. In fact, this probability can be independent of the dimension of the qudit Hilbert space of the qudit under some conditions.

The computed core spectra of Be and Mg dimers exhibit interesting nonatomic features. The spectra are analyzed and related to the underlying core-ionized states and their potential energy curves. Band shapes of the core-level spectra of the dimers are shown to be sensitive to changes of the interatomic distances. An analysis of the spectra computed at various distances reveals interesting details of the evolution of the spectra from a superposition of the respective atomic ones obtained at the dissociation limit to the spectra of the dimers. The vibrationally resolved spectra of the transitions from the ground to the main core-ionized states are computed as well.

We have calculated the photodissociation cross section of NaH through the $B{}^1\Pi$ state. We have solved the time-dependent Schrödinger equation by using the Chebyshev-polynomial scheme and Fourier grid Hamiltonian method. We have used four different sets of potential-energy curves for $X{}^1{\Sigma }^{+}$ and $B{}^1\Pi$ states [E. S. Sachs, J. Hinze, and N. H. Sabelli, J. Chem. Phys. 62, 3367 (1975); R. E. Olson and B. Liu, J. Chem. Phys. 73, 2817 (1980); W. C. Stwalley, W. T. Zemke, and S. C. Yang, J. Phys. Chem. Ref. Data 20, 156 (1991)]. We have found that the values of maximum photodissociation cross section and the corresponding photon energy depend on the choice of the potential-energy curves. But all these results differ significantly from an earlier calculation [K. Kirby and A. Dalgarno, Astrophys. J. 224, 444 (1978)]. We have also found that the photodissociation occurs mainly due to the Franck-Condon transitions.

Fission of doubly charged metal clusters is studied using the open-shell two-center jellium Hartree-Fock approximation. Two-overlapping-spheroids shape parametrization allows one to obtain independent variation of deformations in the parent and daughter clusters. Results of calculations of the electronic structure and fission barriers for the symmetric channel associated with the process ${\mathrm{Na}}_{18}^{2+}\to 2{\mathrm{Na}}_9^{+}$ are presented. The importance of cluster-deformation effects in the fission process is reported. The model developed forms the basis for the systematic many-body theory of metal-cluster-fission phenomena.

Narrow s-wave features with subthermal widths are predicted for the ${}^1{\Pi }_g$ photoassociation spectra of cold alkaline-earth-metal atoms. The phenomenon is explained by numerical and analytical calculations. These show that only a small subthermal range of collision energies near threshold contributes to the s-wave features that are excited when the atoms are very far apart. The resonances survive thermal averaging, and may be detectable for Ca cooled near the Doppler cooling temperature of the $4{}^{1}P\leftarrow 4{}^{1}S$ laser-cooling transition.