We have used laser absorption spectroscopy to study the collisional broadening and shift of the 254-nm $6{}^1{S}_0\to 6{}^3{P}_1$ intercombination line of Hg in the presence of ${\mathrm{N}}_2$ for pressures below 400 Torr. This study comprises the first measurements of the proportionality constants for pressure broadening and shift of Hg due to ${\mathrm{N}}_2$ in this pressure range, and the first high-precision measurements of these pressure effects on Hg for any foreign gas. We obtain $-2.54\left(2\right)\mathrm{}\mathrm{M}\mathrm{H}\mathrm{z}/\mathrm{T}\mathrm{o}\mathrm{r}\mathrm{r}$ for the shift and 9.01(4) MHz/Torr for the broadening (full width at half maximum) at $21°\mathrm{C}$ (95% confidence interval). These results are important for ongoing experiments using optical pumping of mercury in tests of fundamental symmetries, as well as for characterization of interatomic forces and tests of the theory of collisional line broadening.

The accuracy of Bohr’s and more recent analytical calculations of the electronic stopping of heavy charges by a classical harmonic oscillator is analyzed. According to results in this paper, for $|\xi |>~100$ (ξ being the Bohr stopping parameter) the present simulations agree with previous theoretical calculations, whereas for smaller |ξ| values, discrepancies are evident. In fact, for $|\xi |<100\mathrm{}$ the stopping cross section seems to be sensitive to the sign of the ion charge. The so-called Barkas effect is unambiguously observed and positively charged projectiles appear to have a larger stopping compared to that of negative ones at the same ξ. Bohr’s calculations, however, seem to reproduce the stopping of negative charges relatively well, but those of positive ions are consequently underestimated. By giving the electron an initial velocity, the so-called inner-shell effect on the stopping can be readily studied. The present simulations show that previous analytical predictions of this effect do not account for the present results.

The electron dynamics of rare-gas clusters in laser fields is investigated quantum mechanically by means of time-dependent density-functional theory. The mechanism of early inner and outer ionization is revealed. The formation of an electron wave packet inside the cluster shortly after the first removal of a small amount of electron density is observed. By collisions with the cluster boundary the wave-packet oscillation is driven into resonance with the laser field, hence leading to higher absorption of laser energy. Inner ionization is increased because the electric field of the bouncing electron wave packet adds up constructively to the laser field. The fastest electrons in the wave packet escape from the cluster as a whole so that outer ionization is increased as well.

We investigate photodetachment from negative ions in a homogeneous 1.0-T magnetic field and a parallel ac electric field of $\sim 10\mathrm{V}/\mathrm{c}\mathrm{m}.$ A theoretical model for detachment in combined fields is presented. Calculations show that a field of 10 V/cm or more should considerably diminish the Landau structure in the detachment cross section. The ions are produced and stored in a Penning ion trap and illuminated by a single-mode dye laser. We present preliminary results for detachment from ${\mathrm{S}}^{-}$ showing qualitative agreement with the model. Future directions of the work are also discussed.

We present a detailed study of the low-lying collective excitations of a spherically trapped Bose-Fermi mixture at finite temperature in the collisionless regime. The excitation frequencies of the condensate are calculated self-consistently using the static Hartree-Fock-Bogoliubov theory within the Popov approximation. The frequency shifts and damping rates due to the coupled dynamics of the condensate, noncondensate, and degenerate Fermi gas are also taken into account by means of the random-phase approximation and linear-response theory. In our treatment, the dipole excitation remains close to the bare trapping frequency for all temperatures considered, and thus is consistent with the generalized Kohn theorem. We discuss in some detail the behavior of monopole and quadrupole excitations as a function of the Bose-Fermi coupling. At nonzero temperatures we find that, as the mixture moves towards spatial separation with increasing Bose-Fermi coupling, the damping rate of the monopole (quadrupole) excitation increases (decreases). This provides us a useful signature to identify the phase transition of spatial separation.

We investigate theoretically the spatial quantum correlations of the light produced by spontaneous emission in type-I second-harmonic generation in the traveling-wave configuration. It is first shown that spontaneous emission occurs at both fundamental and second-harmonic frequency. Considering the quantum fluctuations of the intensities collected in symmetrical parts of the far-field plane, nonclassical correlations below the shot noise are predicted not only at fundamental frequency, but also at second-harmonic frequency. The latter cannot be traced back to any twin-photon emission mechanism, but are generated by a secondary process acting on twin photons. This mechanism also creates correlations between fundamental and second-harmonic field, at a given transverse wave number, and at opposite wave numbers. The analysis of a simplified few-mode model, on a quantum level, provides a good qualitative understanding of these correlations.

The mode-locking properties of a stretched-pulse erbium-doped fiber laser are theoretically investigated. We consider a unidirectional ring cavity composed of a birefringent erbium-doped fiber and a birefringent undoped fiber. An intracavity polarizer is used to achieve mode locking through nonlinear polarization rotation. The model takes into account the orientation of the eigenaxes of both fibers as well as the orientation of the intracavity polarizer. A master mode-locking equation is derived for the envelope of the electric field. This allows an analytic investigation of the stability of the mode-locking solution as a function of the angle between the eigenaxes of the two fibers. It is demonstrated that the optimization of the ranges of mode locking requires the alignment of the eigenaxis of the two fibers.

We report an experimental study of correlated multiple light scattering in an ultracold gas of ${}^{85}\mathrm{Rb}$ confined in a magneto-optic trap. Measurements are made of the polarization dependence of the spatial and spectral profile of light backscattered from the sample. The results show an interferometric enhancement sensitive to coherent multiple scattering in the atomic gas, and strong variations with the polarization of the incident and detected light. The spatial width and peak value of the enhancement are found to be dependent on the sample size. Comparison of all the measurements with realistic quantum Monte Carlo simulations yields a very good agreement.

The main obstacle for coherent control of open quantum systems is decoherence due to different dissipation channels and the inability to precisely control experimental parameters. To overcome these problems we propose to use dissipation-assisted adiabatic passages. These are relatively fast processes where the presence of spontaneous decay rates corrects for errors due to nonadiabaticity while the system remains in a decoherence-free state and behaves as predicted for an adiabatic passage. As a concrete example we present a scheme to entangle atoms by moving them in and out of an optical cavity.

We explain how the anticipation of chaos in a coupled external cavity laser system described by Sivaprakasam, Shahverdiev, Spencer, and Shore [Phys. Rev. Lett. 87, 154101 (2001)] is obtained. We show that the external cavity induces the required symmetry breaking necessary for the existence of a time delay between the synchronized output of the two laser diodes. The inclusion of a detuning between the two lasers causes one laser to anticipate the chaotic dynamics of the other.

We present a direct measurement of the coherence length in the high-order-harmonic generation process up to orders as high as 25. The harmonic signal generated in an argon gas cell by a 805-nm, 6-mJ, 30-fs laser is studied as a function of medium length and shows very high-contrast coherence fringes. From a one-dimensional time-dependent model, we unravel the conditions for which such Maker fringes can appear in high-order harmonic generation.

We describe a quantum phase gate in which the two qubits are represented by the photons in the two modes of the cavity field. The gate is implemented by passing a three-level atom in a cascade configuration through the cavity. The upper levels of the atom are resonant with one of the cavity modes whereas the lower levels are appropriately detuned from the other mode of the cavity. A $\pi$ phase shift is introduced when there is one photon each in the two modes and the atom is initially in the ground state. We also discuss the one-bit unitary gate in such a system and discuss potential applications.

We study relations between bosonic quadrature squeezing and atomic spin squeezing, and find that the latter reduces to the former in the limit of a large number of atoms for even and odd states. We demonstrate this reduction by treating even and odd spin coherent states, for which analytical solutions are readily obtained, and prove that even spin coherent states always exhibit spin squeezing, whereas odd spin coherent states do not, analogous to the squeezing characteristic of even and odd bosonic coherent states. Finally, we examine the squeezing transfer between photons and atoms via the Dicke Hamiltonian, where a perfect transfer of squeezing is demonstrated in the limit of a large number of atoms.

We investigate experimentally and theoretically the polarization dynamics in vertical-cavity surface-emitting lasers in the vicinity of the lasing threshold. If the net gain anisotropy between the two orthogonally polarized modes is close to zero, the laser operates in a partially polarized state. This state occurs at and slightly above the lasing threshold. It is characterized by anticorrelated dynamics of the two modes which result from completely anticorrelated dynamics at low frequencies and correlated dynamics on the time scale of the relaxation oscillations. If the net anisotropy is changed by a change of the detuning between the cavity resonance and the gain maximum, one of the two linear polarization states is selected depending on the sign of the net gain. For one sign the well-known phenomenon of polarization switching is recovered at higher currents. The experimental results show very good agreement with simulations based on a model that takes into account nonlinear spin dynamics and linear temperature-dependent detuning effects.

Quantum operations arise naturally in many fields of quantum information theory and quantum computing. One of the simplest examples of quantum operation is the von Neumann–Lüders measurement. Destruction of states in quantum mechanics can be treated as a supplement to the von Neumann–Lüders measurement [P. Caban, J. Rembieliński, K. A. Smoliński, and Z. Walczak, J. Phys. A 35, 3265 (2002)]. We show that destruction of states in one-qudit system is a quantum operation by finding its Kraus representation.

We experimentally implemented an eavesdropping attack against the Ekert protocol for quantum key distribution based on the Wigner inequality. We demonstrate a serious lack of security of this protocol when the eavesdropper gains total control of the source. In addition we tested a modified Wigner inequality which should guarantee a secure quantum key distribution.

We consider two characteristic temperatures for a Bose-Einstein condensate, which are related to certain properties of the condensate statistics. We calculate them for an ideal gas confined in power-law traps and show that they approach the critical temperature in the limit of large number of particles. The considered characteristic temperatures can be useful in the studies of Bose-Einstein condensates of a finite number of atoms indicating the point of a phase transition.

Absolute doubly differential cross sections for the electron-impact ionization of atomic hydrogen have been measured from near threshold to intermediate energies. The measurements are calibrated to the well-established, accurate differential cross section for electron-impact excitation of the atomic hydrogen transition $\mathrm{H}{(1\mathrm{}}^2\overrightarrow{S}{2}^2{S+2\mathrm{}}^{2}P).\mathrm{}$ In these experiments background secondary electrons are suppressed by moving the atomic hydrogen target source to and from the collision region. Measurements cover the incident electron energy range of 14.6–40 eV, for scattering angles of $10°–120°$ and are found to be in very good agreement with the results of the most advanced theoretical models—the convergent close-coupling model and the exterior complex scaling model.

The irreversible motion of an open quantum system can be represented through an ensemble of state vectors following a stochastic dynamics with piecewise deterministic paths. It is shown that this representation leads to a natural definition of the rate of quantum entropy production. The entropy production rate is expressed in terms of the von Neumann entropy and of the numbers of quantum jumps corresponding to the various decay channels of the open system. The proof of the positivity and of the convexity of the entropy production rate is given. Monte Carlo simulations of the stochastic dynamics of a driven qubit and of a $\Lambda$ configuration involving a dark state are performed in order to illustrate the general theory.

There are presently two models for quantum walks on graphs. The “coined” walk uses discrete-time steps, and contains, besides the particle making the walk, a second quantum system, the coin, that determines the direction in which the particle will move. The continuous walk operates with continuous time. Here a third model for quantum walks is proposed, which is based on an analogy to optical interferometers. It is a discrete-time model, and the unitary operator that advances the walk one step depends only on the local structure of the graph on which the walk is taking place. This type of walk also allows us to introduce elements, such as phase shifters, that have no counterpart in classical random walks. Several examples are discussed.