The ground-state solutions of a dilute Bose condensate with contact and magnetic dipole-dipole interactions are examined. By lowering the value of the scattering length, Goral et al. [Phys. Rev. A 61, 051601 (2000)] numerically predict a region of unstable solutions, accompanied by a neighborhood where the ground-state wave functions have internal structure. On the contrary, we find that the dipolar condensate has an intuitively located stability region, and ground-state solutions near the instability threshold that are without any unusual structure.

Distribution functions defined in accordance with the quantum theory of measurement are combined with results obtained from the quantum Langevin equation to discuss decoherence in quantum Brownian motion. Closed form expressions for wave packet spreading and the attenuation of coherence of a pair of wave packets are obtained. The results are exact within the context of linear passive dissipation. It is shown that, contrary to widely accepted current belief, decoherence can occur at high temperature in the absence of dissipation. Expressions for the decoherence time with and without dissipation are obtained that differ from those appearing in earlier discussions.

Neutral atoms can be trapped in microscopic potentials resulting from electric and magnetic fields. These microtraps might be used to build simple quantum devices, and ultimately be combined to networks for performing quantum computation. Considering quantum information applications, it is of interest to be able to perform generalized measurements on qubits. A scheme to accomplish this when the qubits are carried by neutral atoms in microtraps is suggested.

We examine the application of the slowly varying envelope approximation (SVEA) to the study of the laser with optical feedback, comparing the monochromatic modes obtained using a SVEA approximation with those emerging from a full treatment of the Maxwell-Bloch equations in a coupled cavity formulation of the laser with optical feedback [A. A. Duarte and H. G. Solari Phys. Rev. A 58, 614 (1998)]. While the SVEA approximation in the body of laser produces reliable results, the same approximation applied to the boundary conditions completely distorts the metamorphosis of the spectrum present in the original model, yet far from the transition region the SVEA gives acceptable results. The failure of the SVEA at the metamorphosis is related to the high sensitivity of the dispersion relations $k\left(w\right)\mathrm{}$ with respect to frequency changes.

Total electron yields and the most probable electron energy have been measured for impact of ${\mathrm{O}}^{q+}$ $(q=1\mathrm{}–7)$ ions at near-normal incidence on a clean Al(110) single-crystal target. The projectile velocities were varied in the range from 0.08 a.u. to 0.6 a.u. The contributions from potential emission (PE) and kinetic emission (KE) to the total yields have been studied. The results support the procedure described by Hughes et al. [Phys. Rev. Lett. 71, 291 (1993)] for the separation of PE from the total electron yield where the KE contribution is more prominent.

The theoretical analysis of C. Rangan and A. R. P. Rau [Phys. Rev. A 61, 033405 (2000)] on the process of photodetachment of ${\mathrm{H}}^{-}$ in a strong static electric field, which calls into question the predictions of B. Gao and A. F. Starace [Phys. Rev. A 42, 5580 (1990)] and also of M. Q. Bao et al. [Phys. Rev. A 58, 411 (1998)], is shown in this Comment to be incorrect. First, we point out that a number of assumptions of Rangan and Rau’s analysis rest on tenuous theoretical grounds. Second, we adduce two completely independent and different analyses of the problem which precisely confirm the results of Gao and Starace. These independent analyses also provide the interpretation that Gao and Starace’s predicted strong-field effects are due to the exact account of the influence of the static electric field, not only on the final state of the detached electron, but also on the initial bound-electron state.

While distortion of the initial negative-ion state by a strong static electric field can have observable effects, the effect attributed by the authors of the preceding Comment [Phys. Rev. A 64, 037401 (2001)] to a cross term between the detaching laser field and the static field is spurious, an artifact of their procedures. Other points of dispute are also clarified.

We generalize our previous results of universal linear manipulations [Phys. Rev. A 63, 032304 (2001)] to investigate three types of nonlinear qubit transformations using measurement and quantum-based schemes. First, nonlinear rotations are studied. We rotate different parts of a Bloch sphere in opposite directions about the z axis. The second transformation is a map that sends a qubit to its orthogonal state. We consider the case in which the orthogonal state is applied to only a partial area of a Bloch sphere. We also study nonlinear general transformation, i.e., $(\vartheta ,\phi )\to (\vartheta -\alpha ,\phi ),$ again applied only to part of the Bloch sphere. In order to achieve these three operations, we consider different measurement preparations and derive the optimal average (instead of universal) quantum unitary transformations. We also introduce a simple method for a qubit measurement and its application to other cases.

We show that quantum operations on multiparticle systems have a nonlocal content; this mirrors the nonlocal content of quantum states. We introduce a general framework for discussing the nonlocal content of quantum operations, and give a number of examples. Quantitative relations between quantum actions and the entanglement and classical communication resources needed to implement these actions are also described. We also show how entanglement can catalyze classical communication from a quantum action.

We extend a sum rule, resulting from the covariance properties of the Schroedinger equation under transformation in an accelerated frame and originally derived for an inhomogeneous electron system, to the linear-density response functions of a confined Bose-condensed gas at finite temperature. We apply the sum rule to test some approximate theories and to show that in the case of a harmonic confinement the random-phase approximation includes the Kohn modes in its spectrum.

We study electromagnetically induced transparency for a probe traveling-wave (TW) laser field in closed Doppler-broadened three-level systems driven by a standing-wave (SW) laser field of moderate intensity (its Rabi frequencies are smaller than the Doppler width of the driven transition). We show that probe windows of transparency occur for values of the probe to drive field frequency ratio R close to half-integer values. For optical transitions and typical values of Doppler broadening for atoms in a vapor cell, we show that for $R>\mathrm{}1\mathrm{}$ a SW drive field is appreciably more efficient than a TW drive in inducing probe transparency. As examples, we consider parameters for real cascade schemes in barium atoms with $R\approx 1.5$ and in beryllium atoms with $R\approx 3.5$ showing that probe transmission values well above 50% are possible for conditions in which it is almost negligible either without driving field or with only one of the TW components of the drive. We show that a strongly asymmetric drive having two TW components with unequal intensities is even more eficient than a symmetric SW drive in inducing probe transparency. The case of arbitrary probe intensity is also considered.

We present a unified approach to quantum teleportation in arbitrary dimensions based on the Wigner-function formalism. This approach provides us with a clear picture of all manipulations performed in the teleportation protocol. In addition within the framework of the Wigner-function formalism all the imperfections of the manipulations can be easily taken into account.

Using standard bound-state methodology, variational calculations of molecular R matrices can severely underestimate polarization response in the near-target region inside the R-matrix boundary. An $“\Delta R”$ procedure is proposed here as an easily implemented but significant improvement of such R matrices. The efficacy of this procedure is demonstrated in calculations of differential and integral cross sections for vibrationally elastic $e-{\mathrm{CO}}_2$ scattering at and below 1 eV.

The stability of a number of exotic systems consisting of $N=5\mathrm{}$ unit charge particles is investigated using the stochastic variational method. Several interesting exotic molecules are found to be stable. The properties of the most intriguing systems consisting of two electrons and two positrons (e.g., $e^{+}\mathrm{PsH}$ or ${\mathrm{Li}}^{+}{\mathrm{Ps}}_2)$ are investigated in great detail.

We present experimental results for the dynamics of cold atoms in a far detuned amplitude-modulated optical standing wave. Phase-space resonances constitute distinct peaks in the atomic momentum distribution containing up to 65% of all atoms resulting from a mixed quantum chaotic phase space. We characterize the atomic behavior in classical and quantum regimes and we present the applicable quantum and classical theory, which we have developed and refined. We show experimental proof that the size and the position of the resonances in phase space can be controlled by varying several parameters, such as the modulation frequency, the scaled well depth, the modulation amplitude, and the scaled Planck’s constant of the system. We have found a surprising stability against amplitude noise. We present methods to accurately control the momentum of an ensemble of atoms using these phase-space resonances which could be used for efficient phase-space state preparation.

The traveling periodic dipole potential of an accelerated optical lattice, created by high-intensity short-pulse lasers, can accelerate polarizable atoms and molecules that are initially at room temperature to hyperthermal velocities (10–100 km/s). We study the acceleration of trapped and untrapped ensembles of particles and show that a significant fraction (30%) of uniformly distributed particles can be accelerated to high velocities over micron-size distances, within nanosecond and subnanosecond time scales.

The spectrum of nominally bound states in barium in an electric field has been measured in a region where singly excited Rydberg states are heavily perturbed by a doubly excited state. Wave-packet and recurrence map analyses are performed and significant deviations are observed from the standard one-electron picture due to the more complex core.

The fundamental question of how Bose-Einstein condensates tunnel into a barrier is addressed. The cubic nonlinear Schrödinger equation with a finite square well potential, which models a Bose-Einstein condensate in a trap of finite depth, is solved for the complete set of localized and partially localized stationary states, which the former evolve into when the nonlinearity is increased. An immediate application of these different solution types is tunable tunneling. Magnetically tunable Feshbach resonances can change the scattering length of certain Bose-condensed atoms, such as ${}^{85}\mathrm{Rb},$ by several orders of magnitude, including the sign, and thereby also change the mean field nonlinearity term of the equation and the tunneling of the wave function. We find both linear-type localized solutions and uniquely nonlinear partially localized solutions where the tails of the wave function become nonzero at infinity when the nonlinearity increases. The tunneling of the wave function into the nonclassical regime (and thus its localization) therefore becomes an external experimentally controllable parameter.

A scheme for generating amplitude squeezed light by means of soliton self-phase modulation is experimentally demonstrated. By injecting 180-fs pulses into an equivalent Mach-Zehnder fiber interferometer, a maximum noise reduction of $4.4\pm 0.3$ dB is obtained $(6.3\mathrm{}\pm 0.6$ dB when corrected for losses). The dependence of noise reduction on the interferometer splitting ratio and fiber length is studied in detail.

We compute the stationary profiles of a coherent beam of Bose-Einstein-condensed atoms propagating through a guide. Special emphasis is put on the effect of a disturbing obstacle present in the trajectory of the beam. The obstacle considered (such as a bend in the guide, or a laser field perpendicular to the beam) results in a repulsive or an attractive potential acting on the condensate. Different behaviors are observed when the beam velocity (with respect to the speed of sound), the size of the obstacle (relative to the healing length), and the intensity and sign of the potential are varied. The existence of bound states of the condensate is also considered.