Photorefractive simple and double phase-conjugate mirrors very often exhibit strong instabilities in their reflectivities. One of their possible origins can be the competition between phase conjugation and its generating process, photorefractive beam fanning. However, by suitably choosing the incident beam widths and directions, we suppressed this competition and replaced it by the enhancement of each process by the other. This technique allows to obtain a double phase-conjugate mirror in ${\mathrm{BaTiO}}_3$ that establishes itself in seconds for beam intensities on the order of the tenth of a milliwatt. The reflectivities reached are on the order of 30% and stable within 3% for hours.

Interest in lossless nonlinearities has focussed on the dispersive properties of $\Lambda$ systems under conditions of electromagnetically induced transparency (EIT). We generalize the $\Lambda$ system by introducing further degenerate states to realize a “chain $\Lambda ”$ atom where multiple coupling of the probe field significantly enhances the intensity-dependent dispersion without compromising the EIT condition.

We present two- and three-dimensional model calculations of high-order harmonic generation in ${\mathrm{H}}_2^{+}.$ The harmonic spectra exhibit clear signatures of intramolecular interference. An interference minimum appears at a harmonic order that depends on the molecular orientation. Harmonic generation in three-center molecules is studied on the basis of two-dimensional calculations for a ${\mathrm{H}}_3^{2+}$ model system. From analytical considerations, the orientation dependence of the harmonic intensities in three-center molecules exhibits a double minimum due to intramolecular interference. In the numerical results, the double minimum is broadened into a single wide minimum. The effect of nonzero laser ellipticity on harmonic generation is investigated by means of two-dimensional simulations for ${\mathrm{H}}_2^{+}.$ We find that harmonic generation with elliptical polarization is governed by interference effects similar to linear polarization.

We study solutions of Dirac equation in the field of Aharonov-Bohm solenoid and a collinear uniform magnetic field. On this base we construct self-adjoint extensions of the Dirac Hamiltonian using von Neumann’s theory of deficiency indices. We reduce $\mathrm{}(3+1)\mathrm{}$-dimensional problem to $\mathrm{}(2+1)\mathrm{}$-dimensional one by a proper choice of spin operator. Then we study the problem doing a finite radius regularization of the solenoid field. We exploit solutions of the latter problem to specify boundary conditions in the singular case.

Geometric quantum computation is extended to the noncyclic case. Adiabatic nuclear magnetic resonance implementations of geometric phase-shift gates at any precession angle and their concomitant fault tolerance with respect to the amplitude of the oscillating field are analyzed. Nonadiabatic one-qubit geometric quantum computation is considered from the noncyclic perspective, with particular emphasis on errors in precession angles.

We propose a scheme to implement the $\overrightarrow{1}2$ universal quantum cloning machine of Buzek and Hillery [Phys. Rev. A $54,$ 1844 (1996)] in the context of cavity QED. The scheme requires cavity-assisted collision processes between atoms, which cross through nonresonant cavity fields in the vacuum states. The cavity fields are only virtually excited to face the decoherence problem. That’s why the requirements on the cavity quality factor can be loosened.

We investigate experimentally the polarization dynamics in vertical-cavity surface-emitting lasers exposed to isotropic time-delayed optical feedback. The existence of low-frequency fluctuations with and without the simultaneous excitation of polarization degrees of freedom is demonstrated. The appearance of polarization dynamics is shown to depend on the magnitude of the intrinsic dichroism of the device under study. If there is no polarization dynamics, low-frequency fluctuations of a single mode are observed.

We present a detailed quantum-mechanical study for dissociation of vibrationally excited molecular diatomic target, of ${\mathrm{H}}_2\left({\nu }_i\right)$ by proton impact and ${\mathrm{H}}_2^{+}\left({\nu }_i\right)$ by hydrogen-atom impact, in the range of center-of-mass collision energies from threshold to 9.5 eV. The dominant dissociation mechanisms in this three-atomic collision system are identified and their effectiveness analyzed for different collision geometries. The cross section calculations for direct and charge-transfer dissociation are performed by solving the Schrödinger equation for the nuclear and electronic motions on the two lowest diabatic electronic surfaces of ${\mathrm{H}}_3^{+},$ and by using an expansion of nuclear wave function in a vibrational basis containing all discrete ${\mathrm{H}}_2$ and ${\mathrm{H}}_2^{+}$ states and a large number of pseudostates from each of the corresponding discretized continua. The energy and angular spectra of the fragments are also calculated and analyzed.

We investigate the helium atom embedded in a superstrong magnetic field $\gamma =100\mathrm{}–10000\mathrm{a}.\mathrm{u}.$ All effects due to the finite nuclear mass for vanishing pseudomomentum are taken into account. The influence and the magnitude of the different finite mass effects are analyzed and discussed. Full configuration interaction calculations are performed for singlet and triplet states for the magnetic quantum numbers $M=0,$ $-1,$ $-2,$ $-3,$ as well as positive and negative z parities. Up to six excited states for each symmetry are studied. With increasing field strength the number of bound states decreases rapidly and we remain with a comparatively small number of bound states for $\gamma {=10\mathrm{}}^4\mathrm{a}.\mathrm{u}.\mathrm{}$ within the symmetries investigated here.

Geometrical optics can be regarded both as the short-wavelength approximation of the propagation of electromagnetic waves, and as the exact way in which propagate the surfaces of discontinuity of the classical electromagnetic field. In this work we translate this last idea to quantum mechanics (both relativistic and nonrelativistic). We find that the surfaces of discontinuity of the wave function propagate exactly following the classical trajectories determined by the Hamilton-Jacobi equation. As an example, we consider the lack of diffraction of abrupt wave fronts.

We study the effect of initial correlations on the short-time decoherence of a particle linearly coupled to a bath of harmonic oscillators. We analytically evaluate the attenuation coefficient of the superposition of two wave packets both for a free and a harmonically bound particle, with and without initial thermal correlations between the particle and the bath. While short-time decoherence appears to be independent of the system in the absence of initial correlations, we find on the contrary that, for initial thermal correlations, decoherence becomes system dependent even for times much shorter than the characteristic time of the system. The temperature behavior of this system dependence is discussed.

This paper reports on the effect a perturbing field on an electromagnetically induced transparency. The studies involve inhomogeneously broadened spin transitions of a nitrogen-vacancy color center in diamond and it is observed that the transparency is unaffected, destroyed or split depending on how the perturbing field is coupled to the system. The observations are explained in terms of doubly driven inhomogeneously broadened 4-level systems.

A quantum trajectory analysis of the steady-state photon statistics of a micromaser by Casagrande and collaborators [F. Casagrande et al., Phys. Rev. A 60, 1582 (1999)] shows a significant suppression of the trapping behavior due to the inclusion of two-atom events. We use numerical integration in our investigation and obtain results that disagree with their analysis and show less suppression of the trapping behavior. A comparison with two extreme cases places our results between the two extremes and gives us confidence that our calculations are correct. The results of Casagrande and collaborators do not satisfy this condition.

In the preceding Comment [Phys. Rev. A 67, 027801 (2003)] Johnson and Schieve raise doubts on the validity of a result obtained in our treatment of collective atomic effects in micromaser dynamics [Phys. Rev. A 60, 1582 (1999)]. We fully confirm the validity of our results, showing that the conclusions derived from their analysis are not relevant.

We generalize the notion of relative phase to completely positive (CP) maps with known unitary representation, based on interferometry. Parallel transport conditions that define the geometric phase for such maps are introduced. The interference effect is embodied in a set of interference patterns defined by flipping the environment state in one of the two paths. We show for the qubit that this structure gives rise to interesting additional information about the geometry of the evolution defined by the CP map.

Bell inequalities for number measurements are derived via the observation that the bits of the number indexing a number state are proper qubits. Violations of these inequalities are obtained from the output state of the nondegenerate optical parametric amplifier.

A procedure is formulated, using the quantum teleportation arrangement, that communicates knowledge of an apparatus setting between the wings of the experiment, using statistical fluctuations in a sequence of measurement results. It requires an entangled state, and transmission of classical information totally unrelated to the apparatus setting actually communicated. Our procedure has conceptual interest, and has applications to quantum cryptography.

A protocol for teleportation of the state of a Bose-Einstein condensate trapped in a three-well potential is developed. The protocol uses hard-sphere cross collision between the condensate modes as a means of generating entanglement. As Bell state measurement, it is proposed that a homodyne detection of the condensate quadrature is performed through Josephson coupling of the condensate mode to another mode in a neighboring well.

In this paper we investigate the security of a quantum cryptographic scheme which utilizes balanced homodyne detection and weak coherent pulses (WCP). The performance of the system is mainly characterized by the intensity of the WCP and postselected threshold. Two of the simplest intercept and resend eavesdropping attacks are analyzed. The secure key gain for a given loss is also discussed in terms of the pulse intensity and threshold.

Bounds on average power transfer over a near-field optical path through atmospheric turbulence are used to deduce bounds on the sift and error probabilities of a free-space quantum-key distribution system that uses the Bennett-Brassard 1984 (BB84) protocol. It is shown that atmospheric turbulence imposes at most a modest decrease in the sift probability and a modest increase in the conditional probability of error given that a sift event has occurred.