A scheme is presented for the generation of macroscopic quantum-interference states for the motion of a trapped ion. In the scheme the ion is driven by a standing-wave laser beam whose carrier frequency is tuned to the ion transition. The distinct feature of the scheme is that the time required to generate the desired state is limited only by the available laser intensity, not by inherent mechanisms such as off-resonant excitations. Therefore, our scheme may allow the generation of the desired state at a high speed, which is important in view of decoherence. In the two-dimension case, entangled coherent states can be obtained. The scheme can also be used to generate entangled states for multiple modes of $N$ trapped ions with a single strong standing-wave laser pulse.

A general scheme for an adiabatic geometric phase gate is proposed which is maximally robust against parameter fluctuations. While in systems with SU(2) symmetry geometric phases are accompanied by dynamical phases and are thus not robust, we show that in the more general case of an $\text{SU}\left(2\right)\otimes \text{SU}\left(2\right)$ symmetry it is possible to obtain a nonvanishing geometric phase without dynamical contributions. The scheme is illustrated for a phase gate using two systems with dipole-dipole interactions in external laser fields which form an effective four-level system.

We examine the entanglement of general mixed states of a two-qubit Heisenberg $XYZ$ chain in the presence of a magnetic field, and its detection by means of different criteria. Both the exact separability conditions and the weaker conditions implied by the disorder and the von Neumann entropic criteria are analyzed. The ensuing limit temperatures for entanglement in thermal states of different $XYZ$ models are then examined and compared with the limit temperature of the symmetry-breaking solution in a mean-field-type approximation. The latter, though generally lower, can also be higher than the exact limit temperature for entanglement in certain cases, indicating that symmetry breaking does not necessarily entail entanglement. The reentry of entanglement for increasing temperatures is also discussed.

In this work, we generalize the quantum-secret-sharing scheme of Hillery, Bužek, and Berthiaume [Phys. Rev. A 59, 1829 (1999)] into arbitrary multiparties. Explicit expressions for the shared secret bit is given. It is shown that in the Hillery-Bužek-Berthiaume quantum-secret-sharing scheme the secret information is shared in the parity of binary strings formed by the measured outcomes of the participants. In addition, we have increased the efficiency of the quantum-secret-sharing scheme by generalizing two techniques from quantum key distribution. The favored-measuring-basis quantum-secret-sharing scheme is developed from the Lo-Chau-Ardehali technique [H. K. Lo, H. F. Chau, and M. Ardehali, e-print quant-ph∕0011056] where all the participants choose their measuring-basis asymmetrically, and the measuring-basis-encrypted quantum-secret-sharing scheme is developed from the Hwang-Koh-Han technique [W. Y. Hwang, I. G. Koh, and Y. D. Han, Phys. Lett. A 244, 489 (1998)] where all participants choose their measuring basis according to a control key. Both schemes are asymptotically $100\%$ in efficiency, hence nearly all the Greenberger-Horne-Zeilinger states in a quantum-secret-sharing process are used to generate shared secret information.

The description of few electron atoms interacting with a slowly varying electromagnetic field including relativistic effects is analyzed. Simple and transparent derivations of relativistic and radiative corrections to various atomic properties such as magnetic moments, radiative decay rates, and shielding of the electric dipole moment are presented.

We describe the spectral characteristics of the radiation scattered by two-level atoms (quantum dots) driven by a strong external field, and coupled to a photonic crystal radiation reservoir. We show that in the presence of strong variations with the frequency of the photonic reservoir density of states, the atomic, Mollow, sideband components of the scattered intensity can be strongly modified. Consequently, a weak optical probe field experiences a substantial differential gain in response to slight variations in the intensity of an optical driving field. We suggest that these effects may be of relevance to all-optical transistor action in photonic crystals. Using a specific photonic crystal heterostructure, we suggest that an all-optical microtransistor based on photonic crystals may operate at less than $100\text{nW}$ switching threshold power. Collective $N$-atom effects substantially enhance this optical switching effect. Near the switching threshold intensity, collective effects are manifest in the $N^2$ scaling of the intensity spectrum (reminiscent of superradiance). Above and below this critical region, the gain spectrum widens (linearly with $N$). This correspondingly reduces the switching time scales of the atomic system in response to external fields. Furthermore, the quantum degree of second-order coherence exhibits unusual features. Scattered photons display a variable degree of antibunching as function of driving laser field intensity and the photonic density of states discontinuity. We analyze the effects of the inhomogeneous atomic line broadening on the amplification process. We show, using suitable photonic density of states engineering, that it is possible to select a narrow spectral range around the central frequency of the atomic frequency distribution over which amplification and switching occur. This is done either by spectral decoupling of the active elements from the electromagnetic field (through the introduction of band gaps at specified spectral locations) or through incoherent pumping to selectively saturate atoms outside the spectral region chosen for amplification.

The first absolute $\left(e,3e\right)$ measurements, by Lahmam-Bennani et al. [Phys. Rev. A 59, 3548 (1999)], have been recently approximately reproduced by Berakdar [Phys. Rev. Lett. 85, 4036 (2000)] and supported by Jones and Madison [Phys. Rev. Lett. 91, 073201 (2003)], but with widely differing conclusions. The former indirectly implied that the Born-CCC-based calculations of Kheifets et al. [J. Phys. B 32, 5047 (1999)] were inaccurate due to the reliance on the first Born approximation. The latter argued that the first Born approximation was valid, but an insufficiently accurate initial state was used. We investigate these claims by performing calculations with ground states similar to those used by Jones and Madison, as well as including the second Born terms. We find the effect of the second Born terms to be negligible and that the original calculations of Kheifets et al. are reproduced if a ground state similar to that of Jones and Madison is used, but appropriately corrected as done by Le Sech and co-workers [J. Phys. B 23, L739 (1990)].

Non-Markovian evolution of an open quantum system can be “unraveled” into pure state trajectories generated by a non-Markovian stochastic (diffusive) Schrödinger equation, as introduced by Diósi, Gisin, and Strunz. Recently we have shown that such equations can be derived using the modal (hidden variable) interpretation of quantum mechanics. In this paper we generalize this theory to treat jumplike unravelings. To illustrate the jumplike behavior we consider a simple system: a classically driven (at Rabi frequency $\Omega$) two-level atom coupled linearly to a three mode optical bath, with a central frequency equal to the frequency of the atom, ${\omega }_0$, and the two side bands have frequencies ${\omega }_0\pm \Omega$. In the large $\Omega$ limit we observed that the jumplike behavior is similar to that observed in this system with a Markovian (broad band) bath. This is expected as in the Markovian limit the fluorescence spectrum for a strongly driven two level atom takes the form of a Mollow triplet. However, the length of time for which the Markovian-like behavior persists depends upon which jumplike unraveling is used.

Using the Oak Ridge National Laboratory ion-atom merged-beam apparatus, absolute total electron-capture cross sections have been measured for collisions of ${\text{Ne}}^{3+}$ ions with hydrogen (deuterium) atoms at energies between 0.07 and $826\text{eV}∕\mathrm{u}$. Comparison to previous measurements shows large discrepancies between 50 and $400\text{eV}∕\mathrm{u}$. Previously published molecular-orbital close-coupling (MOCC) calculations were performed over limited energy ranges, but show good agreement with the present measurements. Here MOCC calculations are presented for energies between 0.01 and $1000\text{eV}∕\mathrm{u}$ for collisions with both H and D. For energies below $\sim 1\text{eV}∕\mathrm{u}$, an enhancement in the magnitude of both the experimental and theoretical cross sections is observed which is attributed to the ion-induced dipole attraction between the reactants. Below $\sim 4\text{eV}∕\mathrm{u}$, the present calculations show a significant target isotope effect.

The dynamics of a metastable attractive Bose-Einstein condensate trapped by a system of laser beams is analyzed in the presence of small fluctuations of the laser intensity. It is shown that the condensate will eventually collapse. The expected collapse time is inversely proportional to the integrated covariance of the time autocorrelation function of the laser intensity and it decays logarithmically with the number of atoms. Numerical simulations of the stochastic three-dimensional Gross-Pitaevskii equation confirm analytical predictions for small and moderate values of mean-field interaction.

We present a theoretical analysis of the experimental scheme for creating maximally entangled states of many atoms in spin-1 Bose-Einstein condensates by driving single atom Raman transition [Li You, Phys. Rev. Lett. 90, 030402 (2003)]. It is shown that, with the help of strong external laser pulses, the model proposed by You can be greatly simplified. Using this simplified interaction, an analytical expression is given for the entangled states between atoms with hyperfine spin $-1$ and 1. We also show how to generate the entangled state between atoms with hyperfine spin 0 and 1.

We predict that the efficiency of the optical phase conjugation generation can be enhanced by more than four orders of magnitude in a ${\chi }^{\left(3\right)}$ nonlinear superlattice, as compared with that in a homogeneous nonlinear medium of the same sample thickness and ${\chi }^{\left(3\right)}$ nonlinearity. Such an effective enhancement utilizes the localized properties of the fields inside the sample at the band-edge state, gap-soliton state, or defect state. Due to the presence of feedback mechanism at each interface of a superlattice, we also predict that the phase conjugation can still be effectively generated when only one pump wave is used.

We use quantum-mechanics formalism to explore the formation of geometrical laser beam trajectories in a hemiconfocal cavity. Theoretical analysis reveals that laser modes localized on geometrical trajectories are formed by a frequency locking of nondegenerate transverse modes with different longitudinal orders. An experiment is performed to validate the theoretical analysis.

In this paper we extend previous hydrodynamic equations, governing the motion of Bose-Einstein-condensed fluids, to include temperature effects. This allows us to analyze some differences between a normal fluid and a Bose-Einstein-condensed one. We show that, in close analogy with superfluid ${}^4\mathrm{He}$, a Bose-Einstein-condensed fluid exhibits the mechanocaloric and thermomechanical effects. In our approach we can explain both effects without using the hypothesis that the Bose-Einstein-condensed fluid has zero entropy. Such ideas could be investigated in existing experiments.

An interaction of a quantized-driven field mode with a two-level atom inside the structured reservoir is considered with the help of the collective operator method. It is shown that the atom-field bound state created via the interaction with the reservoir could be monitored by interaction with the mode. The Markovian damping of the quantized mode does not destroy the atom-field bound state; its existence significantly influences the dynamics of the mode.

We show that the capacity of the Boström-Felbinger (ping-pong) protocol [Phys. Rev. Lett. 89, 187902 (2002)] can be doubled by introducing two additional unitary operations. In message mode, Alice can use one of the four unitary operations to encode two classical bits of information. Bob can decode Alice’s information by using a Bell-basis measurement. Security against eavesdropping attack can be ensured by using two conjugate measurement bases in control mode. A message authentification method can protect this communication against a man-in-the-middle attack without eavesdropping.

We give here the derivation of a Gross-Pitaevskii-type equation for inhomogeneous condensed bosons. Instead of the original Gross-Pitaevskii differential equation, we obtain an integral equation that implies less restrictive assumptions than are made in the very recent study of Pieri and Strinati [Phys. Rev. Lett. 91, 030401 (2003)]. In particular, the Thomas-Fermi approximation and the restriction to small spatial variations of the order parameter invoked in their study are avoided.

An accurate measurement of the tunneling ionization rate for the hydrogenic ground state of helium $\left({\text{He}}^{1+}\right)$ in a circularly polarized, $2\text{−}\text{ps}$, high-intensity laser pulse is presented. The ionizing electric fields are determined with an uncertainty of less than $2\%$ by measuring the energies of electrons ponderomotively accelerated out of the focus. The inferred ionization rate agrees well with semiclassical and exact solutions of the Schrödinger equation.

Recent theoretical developments have highlighted the potential importance of “optical springs” in interferometers for gravitational wave detection as a means for beating the standard quantum limit. We have observed an optical spring effect experimentally in a detuned Fabry-Perot resonator in which one mirror is mounted on a flexure so that it has a significant response to radiation pressure. The main effect of the optical spring, an observed shift in the mechanical resonance frequency of the moveable mirror, agrees well with a simple model.

We study the open system dynamics of a harmonic oscillator coupled with an artificially engineered reservoir. We single out the reservoir and system variables governing the passage between Lindblad-type and non-Lindblad-type dynamics of the reduced system’s oscillator. We demonstrate the existence of conditions under which virtual exchanges of energy between system and reservoir take place. We propose to use a single trapped ion coupled to engineered reservoirs in order to simulate quantum Brownian motion.