Super-radiant Rayleigh scattering from a Bose-Einstein condensate, driven by a single far-detuned laser beam, is strongly limited by dephasing due to inhomogeneous broadening. We demonstrate that the dephasing can be reversed applying a Bragg pulse of area $\pi$ after the first superradiant scattering, stimulating a superradiant echo and inducing a further transfer of atoms from the initial condensate to a condensate with relative momentum $2ħk$ in the direction of the laser.

We study the measurement-induced suppression or acceleration of the radiative decay of an atom embedded in an anisotropic photonic crystal. Due to the presence of a band gap in the electromagnetic density of states, repeated projections onto the excited state of the atom can lead to a suppression or acceleration effect already at rather low repetition rates. It is shown that in contrast to the isotropic band-gap materials, both suppression and acceleration effects are possible, depending on the detuning of the atomic transition from the band edge and the frequency of measurements.

The relativistic configuration interaction valence-bond method has been used to calculate permanent and transition electric dipole moments of the KRb heteronuclear molecule as a function of internuclear separation. The permanent dipole moment of the ground-state $X{}^1{\Sigma }^{+}$ potential is found to be 0.30(2) ${\mathrm{ea}}_0$ at the equilibrium internuclear separation with excess negative charge on the potassium atom. For the $a{}^3{\Sigma }^{+}$ potential the dipole moment is an order of smaller magnitude (1 ${\mathrm{ea}}_0=8.47835\mathrm{}{10}^{-30}\mathrm{Cm}).\mathrm{}$ In addition, we calculate transition dipole moments between the two ground-state and excited-state potentials that dissociate to the $\mathrm{K}\mathrm{}\left(4s\right)+\mathrm{Rb}\mathrm{}\left(5p\right)\mathrm{}$ limits. Using this data we propose a way to produce singlet $X{}^1{\Sigma }^{+}$ KRb molecules by a two-photon Raman process starting from an ultracold mixture of doubly spin-polarized ground state K and Rb atoms. This Raman process is only allowed due to relativistic spin-orbit couplings and the absence of gerade-ungerade selection rules in heteronuclear dimers.

When the laser frequency is tuned to be equal to the molecular electronic excitation, high-order harmonics are generated due to the electronic dipole transitions between the corresponding two potential-energy surfaces (PES). A natural, often taken, choice is the PES of the field-free molecular system. In this special choice the ionization phenomenon is not considered. Only the effect of the dissociation is considered. The method we developed enables one to remain within the framework of the 2-PES approximation and yet to include also the ionization effect in the calculations of molecular high-order harmonic generation spectra. In this approach the coupling between the electronic and nuclear motions is taken into consideration by using coupled complex adiabatic PES. As an illustrative numerical example, we calculated the high harmonic generation (HHG) spectra of ${\mathrm{H}}_2^{+}$ in a 730-nm laser with the intensity of $8.77\times {10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^2.$ The inclusion of the ionization in our approach not only enables the electrons to tunnel through the effective static potential barrier, but also apply an asymmetric force which accelerates the electron before ionization takes place. Therefore, indirectly the inclusion of the ionization by the laser field may lead eventually to an enhanced HHG spectra in comparison with the calculated one when the “natural” choice of the field-free 2PES is taken.

We present analytical relations for the quantum evolution of the number difference of bosons between the two sites of a double-well potential, by using perturbative results for small tunneling amplitudes in the two-mode approximation. Results are obtained for two different initial conditions: completely localized states and coherent spin states. In the former case both the short and the long time behavior is investigated and the characteristic Bohr frequencies of the energy spectrum are determined. In the latter case we calculate the short time-scale evolution of the number difference. The analytical expressions compare favorably with direct numerical solutions of the quantum problem. Finally, we discuss the corresponding Gross-Pitaevskii (i.e., mean-field) dynamics and we point out the differences between the full quantum evolution and the mean-field evolution.

We consider a rotating Bose-Einstein condensate in a harmonic trap and investigate numerically the behavior of the wave function which solves the Gross-Pitaevskii equation. Following recent experiments [P. Rosenbuch, V. Bretin, and J. Dalibard, Phys. Rev. Lett. 89, 200403 (2002)], we study in detail the line of a single quantized vortex, which has a U or S shape. We find that a single vortex can lie only in the $x-z$ or $y-z$ plane. S-type vortices exist for all values of the angular velocity $\Omega$ while U vortices exist for $\Omega$ sufficiently large. We compute the energy of the various configurations with several vortices and study the three-dimensional structure of vortices.

We report the generation of optical squeezed vacuum states by means of polarization self-rotation in rubidium vapor following a proposal by Matsko et al. [Phys. Rev. A 66, 043815 (2002)]. The experimental setup, involving in essence just a diode laser and a heated rubidium gas cell, is simple and easily scalable. A squeezing of $(0.85\mathrm{}\pm 0.05)$ dB was achieved.

The behavior of both the survival $S\left(t\right)\mathrm{}$ and the nonescape $P\left(t\right)\mathrm{}$ probabilities at long times for the one-dimensional free-particle system is shown to be closely connected to that of the initial wave packet at small momentum. We prove that both $S\left(t\right)\mathrm{}$ and $P\left(t\right)\mathrm{}$ asymptotically exhibit the same power-law decrease at long times, when the initial wave packet in momentum representation behaves as $k^m$ with $m=0\mathrm{}$ or $1$ at small momentum. On the other hand, if the integer m becomes greater than $1,$ $S\left(t\right)\mathrm{}$ and $P\left(t\right)\mathrm{}$ decrease in different power laws at long times.

We report on the observation of ${\mathrm{Ar}}_2^{*}$ emission at 126 nm by use of a hollow fiber to guide the high-intensity laser propagation in high-pressure Ar. A small-signal gain coefficient of $0.05{\mathrm{cm}}^{-1}$ was measured at 126 nm by this method. The increase of the ${\mathrm{Ar}}_2^{*}$ emission intensity was measured to be $\exp \mathrm{}\left(2.5\right),$ with an increase of the fiber length up to 50 cm. Kinetic analysis revealed that the ${\mathrm{Ar}}_2^{*}$ production processes were initiated by the electrons produced by the high-intensity laser through an optical-field-induced ionization (OFI) process. A rapid conductive cooling of the OFI electrons is favorable to initiate the ${\mathrm{Ar}}_2^{*}$ formation kinetics more efficiently. This high-density cold plasma production method should be applicable for other vacuum ultraviolet rare-gas excimer lasers.

Two-dimensional noncentral potentials are derived in the exact classical limit of quantum mechanics where the quantum correction to the classical Hamilton-Jacobi equation is canceled. It is then shown explicitly that there exist Ermakov-type invariants I for a large class of such potentials. Some of the invariants are found to be positive definite, $I>\mathrm{}0,$ and for the other, we have found $I=0.\mathrm{}$

Partial-anion and- cation yields from ${\mathrm{H}}_2\mathrm{O}$ are presented for photon energies near the oxygen K edge. The ${\mathrm{O}}^{-}$ yield exhibits a feature above threshold attributed to doubly excited states, in contrast to the ${\mathrm{H}}^{-}$ and cation yields, which are nearly featureless above threshold. Additionally, the lack of the ${\mathrm{OH}}^{-}$ fragment indicates radiative decay and provides a negligible amount of anion formation.

Different forms of bursting oscillations with frequencies from a few MHz to hundreds of MHz separated by intervals of no activity have been observed experimentally for an optical parametric oscillator (OPO) system subject to thermal effects. These oscillations have been simulated numerically using previously derived equations for two interacting transverse modes. In this paper, we investigate one particular case in detail and show that a simple phase-plane analysis explains the bursting cycle. Furthermore, by taking advantage of the values of the parameters, we determine an approximation for the solution of the OPO equations, leading to estimates of the oscillation frequency and of the threshold of the bimode regime.

We show that the set of s-parametrized quasiprobability distribution functions corresponding to an electromagnetic field in a cavity subject to dissipation can be directly measured. Such distributions contain whole information of the quantum state, therefore making it possible to recover information after losses have occurred.

The thermal entanglement in the Heisenberg $\mathrm{XYZ}$ chain is investigated in the presence of an external magnetic field B. In the two-qubit system, the critical magnetic field $B_c$ is increased by introducing the interaction of the z component of two neighboring spins $J_z.$ This interaction not only improves the critical temperature $T_c,$ but also enhances the entanglement for a particular fixed B. We also analyze the pairwise entanglement between nearest neighbors in three qubits. The pairwise entanglement, for a fixed T, can become strong by controlling B and $J_z.$

We propose a scheme to generate a four-particle Greenberger-Horne-Zeilinger (GHZ) state of distant atoms that are trapped separately in leaky cavities. This scheme uses cavity decay to inject photons into a setup of optical devices that consist of a symmetric series of beam splitters and photon detectors. Photon detection on the output modes of the beam splitters projects the atom-cavity-system state onto the GHZ state. It is briefly pointed out that this scheme can be extended to generate GHZ states of $4m$ atoms.

A direct classical analog of the quantum dynamics of intrinsic decoherence in Hamiltonian systems, characterized by the time dependence of the linear entropy of the reduced density operator, is introduced. The similarities and differences between the classical and quantum decoherence dynamics of an initial quantum state are exposed using both analytical and computational results. In particular, the classicality of early-time intrinsic decoherence dynamics is explored analytically using a second-order perturbative treatment, and an interesting connection between decoherence rates and the stability nature of classical trajectories is revealed in a simple approximate classical theory of intrinsic decoherence dynamics. The results offer deeper insights into decoherence, dynamics of quantum entanglement, and quantum chaos.

We study the mechanism and complexity of an efficient quantum repeater, employing double-photon guns, for long-distance optical quantum communication. The guns create polarization-entangled photon pairs on demand. One such source might be a semiconducter quantum dot, which has the distinct advantage over parametric down-conversion that the probability of creating a photon pair is close to 1, while the probability of creating multiple pairs vanishes. The swapping and purifying components are implemented by polarizing beam splitters and probabilistic optical controlled-NOT gates. We also show that the bottleneck in the efficiency of this repeater is due to detector losses.

We show that deterministic quantum computing with a single bit can determine whether the classical limit of a quantum system is chaotic or integrable using $O\left(N\right)\mathrm{}$ physical resources, where N is the dimension of the Hilbert space of the system under study. This is a square-root improvement over all known classical procedures. Our study relies strictly on the random matrix conjecture. We also present numerical results for the nonlinear kicked top.

We present a theoretical study of the properties of an electron wave packet created by a linearly chirped pulse in the strong-field regime. Radial focusing of the wave packet at macroscopic distances which occurs in the case of a weak laser pulse is still possible but at different time and position. Two different situations are reported. (1) The case of a weakly chirped pulse, for which power broadening of the energy distribution occurs. The electron wave packet focuses to less than the inverse of the pulse spectrum bandwidth. (2) The case of highly chirped pulses, for which a new regime is highlighted where power narrowing of the energy distribution occurs. Saturation effects are shown in this case to improve the quality of focusing by reducing the temporal distortions of the wave packet at the focus point.