The relativistic coupled-cluster method is applied to calculate the magnetic dipole hyperfine constant $“A”$ of the ${6s}_{1/2},$ ${6p}_{1/2},$ ${6p}_{3/2},$ and ${5d}_{3/2}$ states of singly ionized barium. After the inclusion of two-body correlation effects into the computation of the hyperfine matrix elements, the accuracy of the obtained values was significantly increased compared to earlier computations. Based on these numbers and earlier calculations of the electric dipole transitions and excitation energies, an estimate for the accuracy of the $|\left[{5p}^6\right]{6s}_{1/2}\overrightarrow{〉}|\left[{5p}^6\right]{5d}_{3/2}〉$ parity-nonconserving electric dipole transition amplitude is carried out. The results suggest that for the first time, to our knowledge, a precision of better than 1% is feasible for this transition amplitude.

Electromagnetically induced transparency in an optically thick, cold medium creates a unique system where pulse-propagation velocities may be orders of magnitude less than c and optical nonlinearities become exceedingly large. As a result, nonlinear processes may be efficient at low-light levels. Using an atomic system with three, independent channels, we demonstrate a quantum interference switch where a laser pulse with an energy per area of $\sim 23$ photons per ${\lambda }^2/\left(2\mathrm{}\pi \right)$ causes a $1/e$ absorption of a second pulse.

We derive a model to describe decoherence of atomic clouds in atom-chip traps taking the excited states of the trapping potential into account. We use this model to investigate decoherence for a single trapping well and for a pair of trapping wells that form the two arms of an atom interferometer. Including the discrete spectrum of the trapping potential gives rise to a decoherence mechanism with a decoherence rate $\Gamma$ that scales like $\Gamma \sim {1/r}_0^4$ with the distance $r_0$ from the trap minimum to the wire.

We show that for an m-partite quantum system, there is a ball of radius $2^{-(m/2\mathrm{}-1)}$ in Frobenius norm, centered at the identity matrix, of separable (unentangled) positive semidefinite matrices. This can be used to derive an ε below which mixtures of ε of any density matrix with $1-\epsilon$ of the maximally mixed state will be separable. The ε thus obtained is exponentially better (in the number of systems) than existing results. This gives a number of qubits below which nuclear magnetic resonance with standard pseudopure-state preparation techniques can access only unentangled states; with parameters realistic for current experiments, this is 23 qubits (compared to 13 qubits via earlier results). A ball of radius 1 is obtained for multipartite states separable over the reals.

We propose a simple scheme for complete Bell-state measurement of photons using hyperentangled states—entangled in multiple degrees of freedom. In addition to hyperentanglement, our scheme requires only linear optics and single photon detectors, and is realizable with current technology. At the cost of additional classical communication, our Bell-state measurement can be implemented nonlocally. We discuss the possible application of these results to quantum dense coding and quantum teleportation.

We provide a general approach for the analysis of optical state evolution under conditional measurement schemes, and identify the necessary and sufficient conditions for such schemes to simulate unitary evolution on the freely propagating modes. If such unitary evolution holds, an effective photon nonlinearity can be identified. Our analysis extends to conditional measurement schemes more general than those based solely on linear optics.

A technique is devised to perform orthogonal state quantum key distribution. In this scheme, entangled parts of a quantum information carrier are sent from Alice to Bob through two quantum channels. However, before the transmission, the order of the quantum information carrier in one channel is reordered so that Eve cannot steal useful information. At the receiver’s end, the order of the quantum information carrier is restored. The order rearrangement operation in both parties is controlled by a prior shared control key which is used repeatedly in a quantum key distribution session.

We present a simple technique that can, without the need for frequency-chirped lasers or varying Stark shifts, produce a complete adiabatic passage between two atomic or molecular bound states. The technique uses a single laser beam, which crosses the particle beam slightly tilted away from normal incidence, thereby inducing a controlled Doppler shift. A subsequent retroreflection produces effectively a two-pulse bichromatic field in the rest frame of each particle. The retroreflected beam is parallel to the incident beam, but is slightly offset in space and has a reduced intensity. Using the adiabatic Floquet theory developed recently by Guérin et al. [Phys. Rev. 63, 031403 (2001)] we show that such a field is capable of producing complete and robust population transfer between two quantum states. We derive the conditions for such transfer and illustrate the potential success with model simulations.

Recently, the gauge-P representation has been introduced to eliminate boundary term problems arising in some cases with the positive-P representation. We show that the gauge-P representation does not solve the problem in the case of a many-body system with binary interactions, unless the number of atoms is definite. In this case we find a set of stochastic equations for the gauge-P representation that reduces the statistical errors with respect to the equations suggested in a previous work. We also analyze the possibility to reduce the statistical errors with appropriate decompositions of the diffusion matrix. Finally, we study the relation between the gauge-P and alternative representations.

Quasi-one-dimensional solitons that may occur in an elongated Bose-Einstein condensate become unstable at high particle density. We study two basic modes of instability and the corresponding bifurcations to genuinely three-dimensional solitary waves such as axisymmetric vortex rings and nonaxisymmetric solitonic vortices. We calculate the profiles of the above structures and examine their dependence on the velocity of propagation along a cylindrical trap. At sufficiently high velocity, both the vortex ring and the solitonic vortex transform into an axisymmetric soliton. We also calculate the energy-momentum dispersions and show that a Lieb-type mode appears in the excitation spectrum for all particle densities.

We investigate realistic atomic systems whose dispersion at line center switches from positive to negative and vice versa, as a particular experimental parameter is changed. We concentrate on those systems which have low probe absorption or gain at line center leading to a high figure of merit, and simultaneously have low pump absorption. For this purpose, we study both open and artificially closed $F_g=\overrightarrow{1}{F}_e=1\mathrm{}$ and $F_g=\overrightarrow{1}{F}_e=2\mathrm{}$ transitions in ${}^{87}\mathrm{Rb},$ in the absence and presence of an applied magnetic field. We find that the preferred system for observing switchover in the sign of the dispersion from positive to negative, as the pump Rabi frequency increases, is the open near-degenerate $F_g=\overrightarrow{1}{F}_e=1\mathrm{}$ system. In the presence of Doppler broadening, the dispersion is positive for all realistic experimental parameters. Thus the switchover in the sign of the dispersion is only observable in the absence of Doppler broadening.

We present an experimental scheme that realizes joint measurement of photon-number sum and phase-difference operators on a two-mode field. The proposed scheme only involves linear optical elements and photon detectors with single-photon sensitivity. Furthermore, we demonstrate that such a measurement setup can be applied to generate two-mode N-photon entangled states from a pair of squeezed vacuum states. These N-photon entangled states are useful resources for quantum-information processing, high-precision frequency measurement, and quantum optical lithography.

We emphasize that there is no spin-statistics connection in nonrelativistic quantum mechanics. In several recent papers [including Phys. Rev. A 67, 042102 (2003)], quantum mechanics is modified so as to force a spin-statistics connection, but the resulting theory is quite different from standard physics.

The preceding Comment states that I have assumed the conclusion in my proof from nonrelativistic quantum mechanics that spin-zero particles must be bosons, and that the theory presented is different from standard quantum mechanics. I show here that neither of those statements is true.

A recent discussion of quantum limitations to the fidelity with which superpositions of internal atomic energy levels can be generated by an applied, quantized, laser pulse, is shown to be based on unrealistic physical assumptions. This discussion assumed the validity of Jaynes-Cummings dynamics for an atom interacting with a laser field in free space, that is, when the atom is not surrounded by a resonant cavity. If the laser field is a multimode quantum coherent state, and the Rabi frequency is much greater than the spontaneous decay rate, then the total atomic decoherence rate is on the order of the spontaneous decay rate. With the use of a unitary transformation of the field states due to Mollow, it can be shown that the atomic decoherence rate is the same as if the laser field were treated classically, without any additional contribution due to the quantum nature of the laser field.

We show that Itano’s arguments confirm our conclusion, that entanglement of the atom with the laser field contributes a small but nonzero amount to the total decoherence of an atom in a laser field. This small effect scales as $1/\mathrm{n̄}$ with $\mathrm{n̄}$ the average number of photons in the laser pulse.

I show that there is no contradiction between the results in my original paper [Phys. Rev. A 65, 022308 (2002)] and Itano’s claims in the previous Comment. The error probabilities I calculated from quantum field fluctuations can equivalently be obtained by considering the probability of spontaneous emission by the atom into the field modes that make up the laser pulse.

Electron correlation effects in the production of two K-shell vacancies in lithium are investigated by bombarding atomic Li with 5-keV electrons. High-resolution Auger electron spectra show that discrete doubly vacant K-shell states are predominantly formed from ionization plus excitation, giving rise to the $2sns{}^{1,3}S$ and $2snp{}^{1,3}P$ states of ${\mathrm{Li}}^{+}.$ For these fast electron projectiles, the shake-up and dielectronic manifestations of the electron-electron interaction are shown to be the only mechanisms for creating the observed hollow states. The formation of the $2snp{}^{1,3}P$ states is found to be mediated only by the dielectronic process, permitting this aspect of the electron-electron interaction to be separately identified and its contribution characterized.

The influence of the half-cycle pulse (HCP) kick on the asymptotic velocity of the ejected electron has been studied for excited xenon atoms ${(n}^{*}=34)$ in the presence of a static electric field $\left(220\mathrm{}\mathrm{V}{\mathrm{cm}}^{-1}\right).\mathrm{}$ We find that the HCP does not change the momentum distribution perpendicular to the kick direction. Therefore half-cycle pulse ionization and electron velocity map imaging can be used to obtain two-dimensional momentum distributions of atomic Rydberg states. Semiclassical and quantum mechanical calculations complement the experimental results.

We study to what extent propagation of wave packets through potential barriers can be described semiclassically in real time. By analyzing the semiclassical approximation to the exact path-integral representation of the quantum-mechanical time evolution operator, we show that in the long-time sector it is dominated by quasiclassical fluctuations associated with caustics. Contrary to the description of tunneling in imaginary times, simple tunneling paths even for complex energies do not exist. Our findings lead to an extended semiclassical propagation scheme for wave-packet dynamics which is numerical efficient and accurately describes deep incoherent tunneling through static and externally driven barrier potentials.