We study numerically the dynamics of a multimode laser diode subject to a mode-selective optical feedback by using a generalization of the Lang-Kobayashi equations. In this configuration, only one longitudinal mode of the laser is reinjected into the laser cavity; the other modes are free. When the laser operates in the low-frequency fluctuation regime, our model predicts intensity bursts in the free modes simultaneously with dropouts in the selected mode, in good agreement with recent experiments. In the frame of our model, intensity bursts and dropouts are associated with collisions of the system trajectory in phase space with saddle-type antimodes.

We study unidirectionally coupled multimode semiconductor lasers. The transmitter is subject to a weak-to-moderate optical feedback. Multimode chaotic output exhibits antiphase dynamics and multiple coexisting attractors. As a result of antiphase dynamics, total output power synchronization does not require modal output synchronization. The route to synchronization is found to be via intermittency. We also report on synchronization collapse, even for identical lasers.

Consider a system consisting of n d-dimensional quantum particles (qudits), and suppose that we want to optimize the entanglement between each pair. One can ask the following basic question regarding the sharing of entanglement: what is the largest possible value $E_{\max }\mathrm{}(n,d)\mathrm{}$ of the minimum entanglement between any two particles in the system? (Here we take the entanglement of formation as our measure of entanglement.) For $n=3\mathrm{}$ and $d=2,$ that is, for a system of three qubits, the answer is known: $E_{\max }\mathrm{}(3,2)=\mathrm{0.550.}\mathrm{}$ In this paper we consider first a system of d qudits and show that $E_{\max }\mathrm{}(d,d)\mathrm{}>~1.$ We then consider a system of three particles, with three different values of d. Our results for the three-particle case suggest that as the dimension d increases, the particles can share a greater fraction of their entanglement capacity.

Vibronic close-coupling calculations of charge transfer and ${\mathrm{H}}_2$ vibrational excitation total cross sections in ${\mathrm{H}}^{+}+{\mathrm{H}}_2{(}^1{\Sigma }_g^{+},\nu =0)$ collisions are presented and compared with experimental data in the energy range $50\hspace{0.5em}\mathrm{eV}<E<\mathrm{}2\mathrm{}\hspace{0.5em}\mathrm{keV}$. It is shown that the sudden approximation for vibration is inappropriate for this system at low velocities, where the dominant mechanism involves transitions between quasidegenerate vibronic levels.

We present a scheme to store unitary operators with self-inverse generators in quantum states and a general circuit to retrieve them with definite success probability. The continuous variable of the operator is stored in a single-qubit state and the information about the kind of the operator is stored in classical states with finite dimension. The probability of successful retrieval is always 1/2 irrespective of the kind of the operator, which is proved to be maximum. In case of failure, the result can be corrected with additional quantum states. The retrieving circuit is almost as simple as that which handles only the single-qubit rotations and CNOT as the basic operations. An interactive way to transfer quantum dynamics, that is, to distribute naturally copy-protected programs for quantum computers is also presented using this scheme.

We propose a scheme for generating multipartite entangled coherent states via entanglement swapping, with an example of a physical realization in ion traps. Bipartite entanglement of these multipartite states is quantified by the concurrence. We also compute multipartite entanglement for certain systems. Finally we establish that these results for entanglement can be applied to more general multipartite entangled nonorthogonal states.

In this paper, we present a formalism for the exact calculation of spectral profile of an isolated line undergoing speed-dependent collisional broadening and shifting and Dicke narrowing. It is couched in the language of the density matrix as in the closely related fields of transport phenomena, laser physics, and nonlinear optics. The formalism is ab intio in spirit, permitting the calculation of the line shape starting only from a given interaction potential. The line shape is obtained by solving a transport/relaxation equation for the off-diagonal element of the density matrix, a macroscopic quantity. The collision operator in the transport/relaxation equation, which is determined from the (microscopic) interaction between the active molecule and the perturber, is presumed known.

We apply the formalism presented in a previous paper to calculate spectra of an isolated line with speed-dependent broadening. The translational motion is modeled on a rigid-sphere interaction and includes the effect of the mass of the perturber on the profile. We show that the density, the mass of the perturber, and the ratio of the optical to kinetic cross section all play a role in revealing or concealing (in the profile) the effects of broadening, which depend upon the speed of the active molecule.

We report calculations of diffusion, excitation transfer, width, and shift cross sections. We use these cross sections to obtain the diffusion coefficient and the width and line shift due to collisions of alkali-metal atoms in the impact approximation. The results are compared to analytical expressions obtained from semiclassical treatments, and with measured widths for sodium atoms. Extension to other alkali metal atoms is also given.

We experimentally investigate the dependence of the transverse pattern formation in a solid-state microchip laser on the Fresnel number. Controlling the transverse-mode spacing and the mode size can generate the stable transverse pattern of optical vortex lattices. A spontaneous process of transverse-mode locking within almost-degenerated mode families is found in the formation of vortex lattices. The frequency of self-induced oscillation in vortex lattices agrees well with the previous theoretical prediction.

Continuous wave lasing without population inversion based on interference of the spontaneous emission is shown to be possible in a closed four-level system coupled with field reservoir, in the absence of any coherent driving fields. Practical implementation of the scheme based on a pseudophotonic band-gap structure doped by three-level ions or semiconductor quantum dots is suggested.

We consider a typical setup of cavity QED consisting of a two-level atom interacting strongly with a single resonant electromagnetic-field mode inside a cavity. The cavity is resonantly driven and the output undergoes continuous homodyne measurements. We derive an explicit expression for the state of the system conditional on a discrete photocount record. This expression takes a particularly simple form if the system is initially in the steady state. As a byproduct, we derive a formula for the steady state that had been conjectured before in the strong driving limit.

The dynamics of coupled semiconductor lasers is investigated by numerical simulations. A realistic laser simulation engine is used to study the synchronization and dynamical regime in two mutually coupled Fabry-Pérot and/or distributed feedback lasers. Both single- and multimode operation regimes are studied, with emphasis on the role of the multiple laser-cavity modes. Our findings indicate that the two lasers synchronize within each laser-cavity mode, while the synchronization across different cavity modes is significantly weaker.

In this paper we derive an analytical solution for long pulse (order of nanoseconds) filaments at a wavelength of 248.6 nm. This is achieved by solving Maxwell’s equations in the presence of a quasi-steady-state nonlinear polarization. The resulting solution reproduces the experimental data seen in the case of short pulse (picosecond) uv filaments and offers a quick and intuitive understanding of the complex phenomenon of filamentation.

We extend our previous analysis of the motion of vortex lines [I. Bialynicki-Birula, Z. Bialynicka-Birula, and C. Śliwa, Phys. Rev. A 61, 032110 (2000)] from linear to a nonlinear Schrödinger equation with harmonic forces. We also argue that under certain conditions, the influence of the contact nonlinearity on the motion of vortex lines is negligible. The present analysis adds new weight to our previous conjecture that the topological features of vortex dynamics are to a large extent universal.

Conditions for achieving “optical asymmetric synthesis,” an example of controlled chiral symmetry breaking, using the electric-dipole light-field interaction are derived. These include scenarios in which neither the medium nor the light is chiral by itself. Specifically, parity requirements are used to show that any optical scenario in which the dynamics of the molecule depends on the overall sign of the electric field allows for control over the production of one chiral species in preference to its mirror image. A sample laser-molecule scenario is used to demonstrate these conditions.

We use a dark-spot Zeeman-tuned slowing technique to measure the capture velocity in a sodium magneto-optical trap as a function of the trapping laser intensity. We expand on previous work by measuring the capture velocity over a broad range of light intensities. We observe that the capture velocity reaches a maximum value and then decreases with increasing light intensities, which might imply a minimum in the trap-loss rate. This observation supports a recently published explanation of the dependence of the trap-loss rates, at low intensities, based mainly on the escape velocity.

A recent experiment by Zbinden et al. [Phys. Rev A 63, 02111 (2001)] purports to test compatibility between relativity and quantum mechanics in the classic Einstein-Podolsky-Rosen (EPR) setting. We argue that relativity has no role in the EPR argument based solely on nonrelativistic quantum formalism. It is suggested that this interesting experiment may have significance to address fundamental questions on quantum probability.

We report the creation of a wide range of quantum states with controllable degrees of entanglement and entropy using an optical two-qubit source based on spontaneous parametric down-conversion. The states are characterized using measures of entanglement and entropy determined from tomographically determined density matrices. The tangle-entropy plane is introduced as a graphical representation of these states, and the theoretic upper bound for the maximum amount of entanglement possible for a given entropy is presented. Such a combination of general quantum state creation and accurate characterization is an essential prerequisite for quantum device development.

We formulate the dynamics of three coupled Bose-Einstein condensates within a semiclassical scenario based on the standard boson coherent states. We compare such a picture with that of K. Nemoto et al. [Phys. Rev. A 63, 013604 (2001)] and show how our approach entails a simple formulation of the dimeric regime therein studied. This allows us to recognize the parameters that govern the bifurcation mechanism causing self-trapping, and paves the way to the construction of analytic solutions.