We suggest a method to prepare any chosen superposition $a_0|0〉{+a}_1|1〉$ of the vacuum and one-photon states. The method is based on a conditional double interferometer fed by a one-photon state and a coherent state. The scheme involves only linear optical elements and avalanche photodetectors, and therefore it should be realizable with current technology. A realistic description of the triggering photodetectors is employed, i.e., we assume that they can only check, with a certain efficiency, whether or not any photon is present. We discuss two working regimes, and show that output states with fidelity arbitrarily close to unit may be obtained, with nonvanishing conditional probability, also for low quantum efficiency at the photodetectors.

A discussion on the roll-off in the emitted intensity in transient collisional excitation (TCE) schemes for x-ray lasers is presented. The high gain in TCE lasers results in a group velocity significantly below the light velocity for the amplified pulse. This effect is proven to be the main reason that the measured gain coefficients fall considerably short of theoretical predictions. Is is outlined how a more effective excitation can be achieved when using the TCE scheme.

We propose a scheme for the reconstruction of the motional state of an ion trapped in a one-dimensional harmonic potential. In the scheme the ion is multichromatically excited by three lasers. Then the measurement of the population of the lower internal state directly yields the Weyl characteristic function for the motional state. The scheme is easily generalized to the two-dimensional case. The scheme operates in the Lamb-Dicke limit.

We present a proposal for the estimation of Büttiker-Landauer traversal time based on the visibility of transmission current. We analyze the tunneling phenomena with a time-dependent potential and obtain the time-dependent transmission current. We found that the visibility was directly connected to the traversal time. Furthermore, this result is valid not only for rectangular potential barrier but also for general form of potential to which the WKB approximation is applicable. We compared these results with the numerical values obtained from the simulation of Nelson’s quantum mechanics. Both of them fit together and it shows that our method is very effective in measuring experimentally the traversal time.

We give a detailed description of the entanglement purification protocol which generates maximally entangled states with high efficiencies from realistic Gaussian continuous variable entangled states. The physical implementation of this protocol is extensively analyzed using high finesse cavities and cavity enhanced cross Kerr nonlinearities. In particular, we take into account many imperfections in the experimental scheme and calculate their influences. Quantitative requirements are given for the relevant experimental parameters.

We report a measurement of the lifetime of the $3d{}^2{D}_{5/2}$ metastable level in ${}^{40}{\mathrm{Ca}}^{+},$ using quantum jumps of a single cold calcium ion in a linear Paul trap. The $4s{}^2{S}_{1/2}–3d{}^2{D}_{5/2}$ transition is significant for single-ion optical frequency standards, astrophysical references, and tests of atomic structure calculations. We obtain $\tau =1.168\mathrm{}\pm 0.007$ s from observation of nearly $64000$ quantum jumps during $\sim 32$ h. Our result is more precise and significantly larger than previous measurements. Experiments carried out to quantify systematic effects included a study of a previously unremarked source of systematic error, namely, excitation by the broad background of radiation emitted by a semiconductor diode laser. Combining our result with atomic structure calculations yields $1.20\pm 0.01$ s for the lifetime of $3d{}^2{D}_{3/2}.$ We also use quantum jump observations to demonstrate photon antibunching, and to estimate background pressure and heating rates in the ion trap.

We estimate the ratio ${R=g}_3{/g}_2$ of the critical coupling constants $g_2$ and $g_3$ that are required to achieve binding of two or three bosons, respectively, with a short-range interaction, and examine how this ratio depends on the shape of the potential. Simple monotonous potentials give $R\simeq \mathrm{0.8.}$ A wide repulsive core pushes this ratio close to $R=1.\mathrm{}$ On the other hand, for an attractive well protected by an external repulsive barrier, the ratio approaches the rigorous lower bound $R=2/3.\mathrm{}$ We also present results for $N=4\mathrm{}$ bosons, sketch the extension to $N>\mathrm{}4,$ and discuss various consequences.

Radiative lifetimes of two levels in Ce I, eight levels in Ce II, and nine levels in Ce III have been measured using the time-resolved laser-induced fluorescence technique. Free cerium atoms and singly and doubly ionized ions were obtained in a laser-produced plasma. A narrow bandwidth UV laser pulse was employed to selectively populate the short-lived upper levels and the lifetime values were evaluated from the time-resolved fluorescence signals recorded by a fast detection system. Transition probabilities for Ce III were obtained from branching fractions calculated by the Cowan code and the experimental lifetimes. The results are compared with previous measurements and calculations. Spectral lines of Ce III were identified in the spectrum of the magnetic chemically peculiar star ${\alpha }^2\mathrm{CVn}$ and the abundance of cerium was determined from synthetic spectrum fitting to be 800 times greater than the solar abundance.

Using the average method, we derive a closed set of linear equations that describes the spinning up of an harmonically trapped gas by a rotating anisotropy. We find explicit expressions for the time needed to transfer angular momentum as well as the decay time induced by a static residual anisotropy. These different time scales are compared with the measured nucleation time and lifetime of vortices by Madison et al. We find a good agreement that may emphasize the role played by the noncondensed component in those experiments.

We propose an efficient method for mapping and storage of a quantum state of propagating light in atoms. The quantum state of the light pulse is stored in two sublevels of the ground state of a macroscopic atomic ensemble by activating a synchronized Raman coupling between the light and atoms. We discuss applications of the proposal in quantum information processing and in atomic clocks operating beyond quantum limits of accuracy. The possibility of transferring the atomic state back on light via teleportation is also discussed.

The influence of transverse modes on the intensity fluctuations of the two orthogonally polarized field components of the electric field in a vertical-cavity surface-emitting laser is analyzed both experimentally and theoretically. It is shown that a very high degree of anticorrelation can be obtained in the presence of bistability between two orthogonally polarized first-order modes.

The electron-ion photorecombination (PR) cross section is known to have an asymmetric energy profile, as a result of quantum interference between direct radiative and dielectronic recombination. We introduce a parameter that reflects the degree of this asymmetry and use it to identify PR transitions for which the effects of interference may be observable. Significantly asymmetric PR cross-section profiles are found for a few PR transitions from He-like Ar and Fe via ${1s2l2l}^{\prime }$ autoionizing levels. The profile asymmetry parameter provides a physical measure for the scaling of the interference effect with the ion charge.

Fully relativistic configuration-interaction calculations, based on the Dirac-Coulomb Hamiltonian, were performed on the ground state of HCl, the Cl 2p–ionized ${\mathrm{HCl}}^{+},$ and the $3p(\pi ,\sigma {)}^{-2}$ states of ${\mathrm{HCl}}^{2+}.$ Calculated ionization energies and bond lengths were found to be in good agreement with recent experimental results and previous calculations. The potential-energy curves were used in calculating vibrational profiles of selected Auger transitions. Nonadiabatic effects in the spin-orbit-induced avoided level crossing were investigated using diabatic electronic basis sets.

The energies, fine structures, widths, and radiative rates of core-excited states of a Li isoelectronic sequence $(Z=4\mathrm{}–14)$ are calculated using the saddle point and saddle-point complex-rotation methods. A restricted variational method is used to extrapolate a better nonrelativistic energy. Relativistic effects are included using first-order perturbation theory. The charged effects of the Li isoelectronic sequence are discussed. These results are compared with experimental and theoretical data in the literature.

We show how the finite-element method can be implemented using a discrete variable representation to provide an efficient means for directly solving the time-independent Schrödinger equation on a multidimensional numerical grid. For collision problems, an exterior complex scaling transformation obviates the need for explicit imposition of asymptotic boundary conditions, making the method particularly useful for studying three-body breakup. The method is illustrated by studying an analytically solvable two-dimensional (2D) breakup problem as well as a 2D model problem with exponential potentials.

A detailed analysis of the growth of a Bose-Einstein condensate is given, based on quantum kinetic theory, in which we take account of the evolution of the occupations of lower trap levels, and of the full Bose-Einstein formula for the occupations of higher trap levels, as well as the Bose-stimulated direct transfer of atoms to the condensate level introduced by Gardiner et al. [Phys. Rev. Lett. 79, 1793 (1997); 81, 5266 (1998)]. We find good agreement with experiment at higher temperatures, but at lower temperatures the experimentally observed growth rate is somewhat more rapid. We also confirm the picture of the “kinetic” region of evolution, introduced by Kagan, Svistunov, and Shlyapnikov (Zh. Eksp. Teor. Fiz. 101, 528 (1992) [Sov. Phys. JETP 75, 387 (1992)]), for the time up to the initiation of the condensate. The behavior after initiation essentially follows our original growth equation, but with a substantially increased rate coefficient. Our modeling of growth implicitly gives a model of the spatial shape of the density profile of the condensate-vapor system as the condensate grows, and thus provides an alternative to the present phenomenological fitting procedure, based on the sum of a zero-chemical potential vapor and a Thomas-Fermi-shaped condensate. Our method gives substantially different results for condensate numbers and temperatures obtained from phenomenological fits, but fits the published column density data very well.

We consider a substituting scheme for nonlinear optical couplers operating by means of degenerate parametric down-conversion with strong coherent pumping. The proposed scheme, which provides the same unitary input-output transformation as the original coupler, consists of simple linear and nonlinear optical devices: beam splitter and optical parametric amplifiers. Using a group-theoretical approach, we find analytical formulas for parameters of these optical elements. The scheme allows us to get a better insight into the coupler behavior, because the complex dynamics of the coupler is transformed into a sequence of simpler evolutions governed by the beam splitter and parametric amplifiers, whose properties are well known and understood. We demonstrate that various dynamical regimes of the coupler are clearly reflected by the substituting device.

The static dipole, quadrupole, and octupole polarizabilities of heliumlike ions are calculated for the $1s2p{}^{1}P$ and $1s2p{}^{3}P$ states with $Z$ up to 10, using variational wave functions in Hylleraas coordinates. A general sum rule for the $2^l$-pole oscillator strengths is derived.

We present a decomposition of the general quantum-mechanical evolution operator that corresponds to the path decomposition expansion, and interpret its constituents in terms of the quantum Zeno effect (QZE). This decomposition is applied to a finite-dimensional example and to the case of a free particle in the real line, where the possibility of boundary conditions more general than those hitherto considered in the literature is shown. We reinterpret the assignment of consistent probabilities to different regions of space-time in terms of the QZE. The comparison of the approach of consistent histories to the problem of time of arrival with the solution provided by the probability distribution of Kijowski shows the strength of the latter point of view.