We show that nonadiabatic, resonant amplitude- and phase-modulated pulses can be frequency converted with greater efficiency than adiabatic resonant pulses in a coherently prepared $\Lambda$ system. Indeed, conversion efficiencies close to unity, similar to those achieved using highly detuned pulses, can been obtained by using highly nonadiabatic resonant pulses. Moreover, by solving the Maxwell-Bloch equations using Fourier transforms, we derive analytical expressions for the probe and the generated four-wave mixing (FWM) pulses as a function of time and propagation distance. From these expressions, which are valid for either adiabatic or nonadiabatic pulses, we derive the result that starting from a nonadiabatic probe pulse, an asymptotically matched probe-FWM pulse pair with the same shape as the initial probe pulse is obtained. In addition, we show that, starting with a nonadiabatic matched pulse pair or a pair of matched pulse trains, we obtain propagation of these pulses without either deformation or losses.

We show that the intracavity Kerr nonlinear coupler is a potential source of bright continuous variable entangled light beams which are tunable and spatially separated. We use a linearized fluctuation analysis to calculate the necessary correlations in regimes where it is valid. This means that we are treating regimes where the system exhibits Gaussian statistics so that well-known criteria are both necessary and sufficient to demonstrate entanglement. This system may be realized with integrated optics and thus provides a potentially rugged and stable source of bright entangled beams.

Close to converged energies and expectation values for PsH are computed using a ground state wave function consisting of 1800 explicitly correlated gaussians. The best estimate of the ${\mathrm{Ps}}^{\infty }\mathrm{H}$ energy was $-0.789196740\text{hartree}$ which is the lowest variational energy to date. The $2\gamma$ annihilation rate for ${\mathrm{Ps}}^{\infty }\mathrm{H}$ was $2.47178\times {10}^9{\mathrm{s}}^{-1}$.

We present a spin-bus concept of quantum computing where an electron spin $S=1∕2$ acts as a bus qubit connected to a finite number $N$ of nuclear spins $I=1∕2$ serving as client qubits. Spin-bus clusters are considered as local processing units and may be interconnected with other spin-bus clusters via electron-electron coupling in a scaled up version. Here we lay the ground for the basic functional unit with long qubit registers, provide the theory and experimental verification of correlated qubit states, and demonstrate the Deutsch algorithm. Experiments were performed on a qubyte plus one nuclear spin in a solid state system.

Electron impact ionization cross sections (EIICS) of 30 $L$-shell targets, with open- and closed-shell configurations in the isoelectronic sequences ranging from Li to Ne, are evaluated using the generalized parameters of our recent modification of BELL formula (MBELL) [Haque et al., Phys. Rev. A 73, 012708 (2006)]. Three sets of parameters, one each for the $1s$, $2s$, and $2p$ orbits, provide an excellent account of the experimental EIICS data of atomic targets, neutral and ionic, up to the atomic number $Z=92$ and incident energies up to about $250\mathrm{MeV}$. In comparison with the quantum mechanical predictions, it is found that the present MBELL cross sections are in better agreement with the experimental results.

We discuss and model an experiment to study quasicondensate growth on an atom chip. In particular, we consider the addition of a deep dimple to the weak harmonic trap confining an ultracold one-dimensional atomic Bose gas, below or close to the characteristic temperature for quasicondensate formation. The subsequent dynamics depends critically on both the initial conditions and the form of the perturbing potential. In general, the dynamics features a combination of shock-wave propagation in the quasicondensate and quasicondensate growth from the surrounding thermal cloud.

In this paper we are concerned with the problem of stability for quantum feedback networks. We demonstrate in the context of quantum optics how stability of quantum feedback networks can be guaranteed using only simple gain inequalities for network components and algebraic relationships determined by the network. Quantum feedback networks are shown to be stable if the loop gain is less than one—this is an extension of the famous small gain theorem of classical control theory. We illustrate the simplicity and power of the small gain approach with applications to important problems of robust stability and robust stabilization.

In this paper we report an observation of the two-photon absorption in a four-level system in hot ${}^{87}\mathrm{Rb}$ vapor based on the proposal of Harris and Yamamoto [Phys. Rev. Lett. 81, 3611 (1998)]. We show that this effect is reduced in hot atoms due to the non-Doppler-free nature of this scheme. Then we report a phenomenon that could be used in the same application of Harris and Yamamoto. The main result is a great enhancement of electromagnetically induced transparency (EIT) effect in hot ${}^{87}\mathrm{Rb}$ vapor caused by optical pumping. We find that when the single photon detuning is near zero the EIT signal is dramatically enhanced by an optical pumping field. More interestingly when the single photon detuning is larger the signal can be changed from a sharp Raman peak to a sharp EIT dip. The full width at half maximum of the peak and dip are narrow and subnatural.

In molecules with ionic contributions to the binding, the contribution of nuclear displacements (due to the external field) to the static polarizability can be decisive. Using the finite field method, we optimized the structure with and without a finite external electric field by a total energy minimization and we calculated the polarizability from the induced dipole moment. In ${\mathrm{C}}_{60}{\mathrm{F}}_n$, fluorination mostly increases the polarizability. Only for $n=2$ and 18, where the molecule without an external field has a very large dipole moment, does fluorination decrease it. For large $n$ ($n=20$, 36, and 48), the polarizability per added F atom due to nuclear displacements is increased by a factor of about 2. The validity of the additivity model has been discussed.

We propose and experimentally demonstrate a mechanism to achieve coherent control of the polarization rotation of an optical field in a multilevel electromagnetically induced transparency (EIT) system in rubidium atoms. By choosing a properly polarized coupling field and transition energy levels, the symmetry of the atomic medium to the propagation of two orthogonal polarization components of a weak linearly polarized probe field can be broken, which leads to a coherently controlled rotation of the probe field polarization. This mechanism of coherently controlled optical polarization rotation makes use of asymmetry in EIT subsystems for the two circular polarization components of the probe beam with a contribution from different transition strengths (due to different Clebsh-Gordan coefficients) in this multilevel atomic system.

We perform stochastic simulations of the quantum Zeno and anti-Zeno effects for a two-level system and for a decaying one. Instead of a simple projection postulate approach, a more realistic model of a detector interacting with the environment is used. The influence of the environment is taken into account using the quantum trajectory method. Simulation of the measurement for a single system exhibits probabilistic behavior showing the collapse of the wave packet. When a large ensemble is analyzed using the quantum trajectory method, the results are the same as those produced using the density matrix method. The results of numerical calculations are compared with the analytical expressions for the decay rate of the measured system, and good agreement is found. Since the analytical expressions depend on the duration of the measurement only, the agreement with the numerical calculations shows that other parameters of the model are not important.

We demonstrate an optical-fiber-based source of polarization-entangled photon pairs with improved quality and efficiency, which has been integrated with off-the-shelf telecom components and is, therefore, well suited for quantum communication applications in the 1550-nm telecom band. Polarization entanglement is produced by simultaneously pumping a loop of standard dispersion-shifted fiber with two orthogonally polarized pump pulses, one propagating in the clockwise and the other in the counterclockwise direction. We characterize this source by investigating two-photon interference between the generated signal-idler photon pairs under various conditions. The experimental parameters are carefully optimized to maximize the generated photon-pair correlation and to minimize contamination of the entangled photon pairs from extraneously scattered background photons that are produced by the pump pulses for two reasons: (i) spontaneous Raman scattering causes uncorrelated photons to be emitted in the signal and idler bands and (ii) broadening of the pump-pulse spectrum due to self-phase modulation causes pump photons to leak into the signal and idler bands. We obtain two-photon interference with visibility $>90\%$ without subtracting counts caused by the background photons (only dark counts of the detectors are subtracted), when the mean photon number in the signal (idler) channel is about 0.02/pulse, while no interference is observed in direct detection of either the signal or idler photons.

We propose an asymmetric quantum cloning scheme. Based on the proposal and experiment by Andersen et al. [Phys. Rev. Lett. 94, 240503 (2005)], we generalize it to two asymmetric cases: quantum cloning with asymmetry between output clones and between quadrature variables. These optical implementations also employ linear elements and homodyne detection only. Finally, we also compare the utility of symmetric and asymmetric cloning in an analysis of a squeezed-state quantum key distribution protocol and find that the asymmetric one is more advantageous.

One of the dominant systematic effects that shift resonance lines in high-precision measurements of two-photon transitions is the dynamic (ac) Stark shift. For suitable laser frequencies, the ac Stark shift acquires an imaginary part which corresponds to the rate of resonant one-photon ionization of electrons into a continuum state. At the current level of spectroscopic accuracy, the underlying time-dependent quantum dynamics governing the atomic two-photon excitation process must be well understood, and related considerations are the subject of the present paper. In order to illustrate the basic mechanisms in the transient regime, we investigate an analytically solvable model scenario for the population dynamics in the density matrix formalism and describe in detail how to generalize the corresponding equations of motion for individual experimental use. We also calculate the dynamic Stark shift for two-photon $S\text{−}S$ and $S\text{−}D$ transitions in bound two-body Coulomb systems and the corresponding two-photon transition matrix elements. In particular, we investigate transitions for which the $1S$ ground state or alternatively the metastable $2S$ state acts as the lower-energy state, and for which states with $n⩽20$ represent the upper states. Relativistic and radiative corrections to the excitation dynamics, and the corresponding limitations to the accuracy of the measurements, are briefly discussed. Our considerations suggest the general feasibility of a detection mechanism, offering high quantum efficiency, based on two-step three-photon resonant ionization spectroscopy, for large classes of experimentally relevant two-photon transitions in two-body Coulomb systems.

We have measured absolute cross sections for electrons $(9–700\mathrm{eV})$ incident on water molecules. By fitting synthetic vibrational bands to the experimental emission spectra, we measured the emission cross sections for the $\mathrm{OH}$ $A{}^2\Sigma ^{+}–X{}^2\Pi$ $(0,0)$, $(0,1)$, $(1,0)$, $(1,1)$, $(2,1)$, $(2,2)$, $(3,2)$, and $(3,3)$ bands. The branching fractions from our cross sections are consistent with those we measured in the emission of a rf discharge and with our calculated transition rates. We have also calculated apparent level cross sections for the $A{}^2\Sigma ^{+}{v}^{\prime }=0$, 1, 2, and 3 vibrational levels where the effects of predissociation are obvious.

We have shown that spin-orbit coupling plays an important role in determining the structures of platinum nanoclusters. We have used spin-orbit coupling self-consistently with density functional theory to study the geometry of platinum clusters and their binding energy. It has been found that the relative energies of the isomers of a cluster of a given size can be changed due to the inclusion of spin-orbit coupling. Planar structures were found for ${\mathrm{Pt}}_n$ clusters with $n⩽5$.

We present a theory of dark soliton dynamics in trapped quasi-one-dimensional Bose-Einstein condensates, which is based on the local-density approximation. The approach is applicable for arbitrary polynomial nonlinearities of the mean-field equation governing the system as well as to arbitrary polynomial traps. In particular, we derive a general formula for the frequency of the soliton oscillations in confining potentials. A special attention is dedicated to the study of the soliton dynamics in adiabatically varying traps. It is shown that the dependence of the amplitude of oscillations vs the trap frequency (strength) is given by the scaling law $X_0\propto {\omega }^{-\gamma }$ where the exponent $\gamma$ depends on the type of the two-body interactions, on the exponent of the polynomial confining potential, on the density of the condensate, and on the initial soliton velocity. Analytical results obtained within the framework of the local-density approximation are compared with the direct numerical simulations of the dynamics, showing a remarkable match. Various limiting cases are addressed. In particular for the slow solitons we computed a general formula for the effective mass and for the frequency of oscillations.

We consider the formation of bright solitons in a mixture of Bose and Fermi degenerate gases confined in a three-dimensional elongated harmonic trap. The Bose and Fermi atoms are assumed to effectively attract each other whereas bosonic atoms repel each other. Strong enough attraction between bosonic and fermionic components can change the character of the interaction within the bosonic cloud from repulsive to attractive making thus possible the generation of bright solitons in the mixture. On the other hand, such structures might be in danger due to the collapse phenomenon existing in attractive gases. We show, however, that under some conditions (defined by the strength of the Bose-Fermi components attraction) the structures which neither spread nor collapse can be generated. For elongated enough traps the formation of solitons is possible even at the “natural” value of the mutual Bose-Fermi (${}^{87}\mathrm{Rb}\text{−}{}^{40}\mathrm{K}$ in our case) scattering length.

By using the ghost imaging technique, we experimentally demonstrate the reconstruction of the diffraction pattern of a pure phase object by using the classical correlation of incoherent thermal light split on a beam splitter. The results once again underline that entanglement is not a necessary feature of ghost imaging. The light we use is spatially highly incoherent with respect to the object ($\approx 2\mu \mathrm{m}$ speckle size) and is produced by a pseudo-thermal source relying on the principle of near-field scattering. We show that in these conditions no information on the phase object can be retrieved by only measuring the light that passed through it, neither in a direct measurement nor in a Hanbury Brown-Twiss (HBT) scheme. In general, we show a remarkable complementarity between ghost imaging and the HBT scheme when dealing with a phase object.