Experimental preparation of multispecies Bose-Einstein condensates should permit the creation of topologically stable defects beyond the superfluid vortex. But the coldness and isolation of condensates should also permit the survival for observable durations of “pseudodefects,” such as the one-dimensional dark soliton: localized structures related to a defect but not topologically stable. In this paper we investigate the viability of pseudodefects beyond one dimension, by examining “wave-function monopoles” in two-species condensates in two dimensions. We identify interesting instabilities, including a “dancing mode” for monopoles of higher winding number, and (in a one-dimensional limit) “superfluid roulette.”

A recently proposed photonic blockade mechanism using four-level atoms inside an optical cavity is reviewed and analyzed. Various possible theoretical approaches are discussed, including adiabatic elimination of atomic variables, linearization, and numerical calculations in the weak excitation regime.

We demonstrate lasing on the nickel-like ${3d}^{9}4f{}^1{P}_1\to {3d}^{9}4d{}^1{P}_1$ x-ray laser line in Zr $\mathrm{}(Z=40\mathrm{}),$ Nb $\mathrm{}(Z=41\mathrm{}),$ and Mo $\mathrm{}(Z=42\mathrm{})\mathrm{}$ and present measured wavelengths for these ions as well as predicted values for ions from $Z=36\mathrm{}$ to 54. Lasing on this line was first predicted two years ago and has now been achieved. Unlike the usual collisional excitation laser, this line lases because radiation trapping allows a large radiation field to build up on the ${3d}^{10}{}^1{S}_0\to {3d}^{9}4f{}^1{P}_1$ resonance line and populate the $4f$ upper laser state by the self-photopumping process. For Ni-like Mo a gain of 13 ${\mathrm{cm}}^{\mathrm{-}1}$ is measured on this transition at 226 Å for targets up to 1 cm long.

We consider the possibility of wave propagation in the normally disallowed (band-gap) region of a resonant optical medium, that is, in the band of frequencies near resonance where the real part of the frequency-dependent dielectric function is negative. We demonstrate that wave propagation can become allowed by the application of a strong electromagnetic field resonant with some additional transition of the material system. The frequencies at which these effects can occur are strongly influenced by local-field effects.

We demonstrate the applicability of the time-dependent semiclassical initial-value formalism to the problem of population transfer between adiabatic potential surfaces which are coupled through a pulsed laser field. Despite the fact that solutions of highly nonlinear classical equations are entering the semiclassical expression, our results for ultrashort pulse excitation are in good agreement with full quantum solutions. Even in the case where the Rosen-Zener approach to population transfer breaks down, the semiclassical results are still close to the quantum ones.

Recently a physically transparent definition of classical-like states of a quantum one-dimensional particle in a potential has been proposed [D. M. Davidović and D. Lalović, J. Phys. A. 31, 2281 (1998)]; such states have been explicitly constructed and studied. In the present paper we formulated an analogous definition of the classical-like states of a quantum system with no underlying classical model, namely the spin $s=\frac{1}{2}$. The definition leads to a natural and consistent measure of the classical characteristics of a quantum state. We then constructed such classical-like states explicitly, and discussed their properties.

For time (t)-dependent wave functions, we derive rigorous conjugate relations between analytic decompositions (in the complex t plane) of phases and log moduli. We then show that reciprocity, taking the form of Kramers-Kronig integral relations (but in the time domain), holds between observable phases and moduli in several physically important instances. These include the nearly adiabatic (slowly varying) case, a class of cyclic wave functions, wave packets, and noncyclic states in an “expanding potential”. The results define a unique phase through its analyticity properties, and exhibit the interdependence of geometric phases and related decay probabilities. Several known quantum-mechanical applications possess the reciprocity property obtained in the paper.

We study the use of the Faraday effect as a quantum clock for measuring traversal times of evanescent photons through magnetorefractive structures. The Faraday effect acts both as a phase shifter and as a filter for circular polarizations. Only measurements based on the Faraday phase-shift properties are relevant to the traversal time measurements. The Faraday polarization filtering may cause the loss of nonlocal (Einstein-Podolsky-Rosen) two-photon correlations, but this loss can be avoided without sacrificing the clock accuracy. We show that a mechanism of destructive interference between consecutive paths is responsible for superluminal traversal times measured by the clock.

We propose a spin model which exhibits the main properties of the Kerr medium. The description of the model uses a recently generalized quasiprobability distribution (Wigner function) for the SU(2) group. This function is naturally defined on the sphere, which plays the role of phase space for the spin system. Our model leads to macroscopic quantum superposition states on the sphere.

Spectra and magnetic properties of large spins J (e.g., spins possessed by ions or molecules), placed into a crystal electric field (CEF) of an arbitrary symmetry point group, are shown to change drastically when J changes by $1/2$ or $1.$ At a fixed field symmetry and configuration of its N extrema situated at the p-fold symmetry axis, physical characteristics of the spin depend periodically on J with the period equal to p. The problem of the spectrum and eigenstates of the large spin J is equivalent to an analogous problem for a scalar charged particle confined to a sphere $S^2$ and placed into the magnetic field of the monopole with the charge J. This analogy, as well as the strong difference between close values of J, stems from the Berry phase occurring in the problem. For energies close to the extrema of the CEF, the problem can be formulated as Harper’s equation on the sphere. The $\mathrm{}(2J+1)\mathrm{}$-dimensional space of states is split into smaller multiplets of classically degenerated states. These multiplets in turn are split into submultiplets of states transforming according to specific irreducible representations of the symmetry group determined by J and p. We classify possible configurations and corresponding spectra. Experimental realizations of large spins in a symmetric environment are proposed and physical effects observable in these systems are analyzed.

We study the resonances of the system of a particle in an infinite spherical well with an oscillating wall. Parametric resonances emerge when the oscillation frequency is equal to the energy difference between two eigenstates of the static cavity. Whereas an analytic calculation based on a two-level system approximation reproduces the numerical results at low driving amplitudes $\epsilon$, we observe a drastic change of behavior when $\epsilon >0.1\mathrm{}$, when new resonance states appear bearing no apparent relation to the eigenstates of the static system.

Dynamics of a heavy electrically neutral fluxon moving in the presence of a light charged quantum particle is studied. The adiabatic potentials are derived and the gauge invariance of the system is delineated. The charged particle confined by a harmonic potential and the neutral particle with half a flux unit (semifluxon) is analyzed. It is shown for this setup that the Aharonov-Casher modification of the fluxon’s kinematical momentum leads to a magnetic adiabatic force that breaks time-reversal symmetry. It follows that a conventional adiabatic approach, including the adiabatic reaction forces, is generally inconsistent with time-reversal symmetry.

The Aharonov-Bohm (AB) scattering of charged particles by the magnetic field of an infinitely long and infinitely thin solenoid (magnetic string) in an absorbing medium is studied. We discuss the partial-wave approach to this problem and show that the standard partial-wave method can be adjusted to this case. The effect of absorption leads to oscillations of the AB cross section. Based on this we investigate the scattering of neutral atoms with induced electric dipole moments by a charge wire of finite radius, which is placed in a uniform magnetic field. The physical realistic and practically important case that all atoms that collide with the wire are totally absorbed at its surface is studied in detail. The dominating terms of the scattering amplitude are evaluated analytically for different physical constellations. The remaining terms are written in a form suitable for a numerical computation. We show that if the magnetic field is absent, the absorbing charged wire causes oscillations of the cross section. In the presence of the magnetic field the cross section increases and the dominating Aharonov-Bohm peak appears in the forward direction, suppressing the oscillations.

Tunneling problems are characterized by different quantum time scales of motion. In this paper, we identify a tunneling time scale, which is based on a simple variational principle. The method utilizes the stationary eigenfunctions for a given one-dimensional potential structure, and it provides a truly local definition of the tunneling time, independent of the asymptotic shape of the potential. We express the minimum tunneling time in terms of the more common time scales obtained from the Larmor clock setup. Asymptotic formulas for both the extreme quantum and the semiclassical limit are presented. As an experimental verification of the variational approach we demonstrate that the minimum tunneling time governs the time a particle requires to traverse the barrier in a symmetric double-well structure.

We formulate a general complementarity relation starting from any Hermitian operator with discrete nondegenerate eigenvalues. We then elucidate the relationship between quantum complementarity and the Heisenberg-Robertson uncertainty relation. We show that they are intimately connected. Finally we exemplify the general theory with some specific suggested experiments.

Previous Born approximation calculations of the photoproduction of singlet (para) positronium are extended to give the cross section to all orders in the atomic number Z, i.e., to include Coulomb corrections. At the same time the cross section for triplet (ortho) positronium production—which does not exist in Born approximation—is obtained to all orders in Z. The calculations and results are given in a form closely related to the well-known high-energy pair production cross section including Coulomb corrections. The relations to pair production are discussed.

We prove a theorem on direct relation between the optimal fidelity $f_{\max }$ of teleportation and the maximal singlet fraction $F_{\max }$ attainable by means of trace-preserving local quantum and classical communication (LQCC) action. For a given bipartite state acting on $C^d\otimes C^d$ we have $f_{\max }{=(F}_{\max }d+1)/(d+1\mathrm{})\mathrm{}$. We assume completely general teleportation scheme (trace preserving LQCC action over the pair and the third particle in unknown state). The proof involves the isomorphism between quantum channels and a class of bipartite states. We also exploit the technique of $U\otimes U^{*}$ twirling states (random application of unitary transformation of the above form) and the introduced analogous twirling of channels. We illustrate the power of the theorem by showing that any bound entangled state does not provide better fidelity of teleportation than for the purely classical channel. Subsequently, we apply our tools to the problem of the so-called conclusive teleportation, then reduced to the question of optimal conclusive increasing of singlet fraction. We provide an example of state for which Alice and Bob have no chance to obtain perfect singlet by LQCC action, but still singlet fraction arbitrarily close to unity can be obtained with nonzero probability. We show that a slight modification of the state has a threshold for singlet fraction, which cannot be exceeded anymore.

We report a particular implementation of a quantum cryptographic device operating at 1540-nm wavelength and involving interference between phase-modulated sidebands produced by a pair of phase modulators in the transmitting and receiving modules. The principle of operation is described in terms of both classical and quantum optics. The method has been demonstrated experimentally using a strongly attenuated semiconductor laser diode. Single photon interference has been obtained with a fringe visibility greater than 90%, indicating that the system can be used for quantum key distribution.

A scheme is proposed to implement microscopic analogs of the Greenberger-Horne-Zeilinger experiment, as described by Lloyd, on a nuclear magnetic resonance (NMR) quantum computer. This scheme includes how to prepare the effective pure spin state |0000〉 (a state with all the spins pointing along the direction of the magnetic field) from a state at thermal equilibrium, how to transform it into states of the form, e.g., $\left(|000〉-|111〉\right)|0〉$ (which comprises the first three spins in an entangled state and the fourth register-spin), and how to measure the eigenvalue of the operator product like ${\sigma }_x^1{\sigma }_y^2{\sigma }_y^3$ for such three-spin entangled states, where ${\sigma }^i$ is the Pauli matrix for the $i\mathrm{th}$ spin, and store the result on the fourth spin. Also proposed is a general method to implement the controlled-not gate, an essential gate for any quantum algorithm, on n-qubit NMR quantum computers with $n>~4.$

We consider the problem of how to manipulate the entanglement properties of a general two-particle pure state, shared between Alice and Bob, by using only local operations at each end and classical communication between Alice and Bob. A method is developed in which this type of problem is found to be equivalent to a problem involving the cutting and pasting of certain shapes along with certain coloring problems. We consider two problems. First, we find the most general way of manipulating the state to obtain maximally entangled states. After such a manipulation, the entangled states $|11〉+|22〉+\cdot \cdot \cdot +|\mathrm{mm}〉$ are obtained with probability $p_m.$ We obtain an expression for the optimal average entanglement obtainable. Also, some results of Lo and Popescu (e-print quant-ph/9707038) pertaining to this problem are given simple geometric proofs. Second, we consider how to manipulate one two-particle entangled state $|\psi 〉$ to another $|{\psi }^{\prime }〉$ with certainty. We derive Nielsen’s theorem (which states a necessary and sufficient condition for this to be possible) using the method of areas.