We investigate the time evolution of entanglement of various entangled states of a two-qubit system exposed to either thermal or squeezed reservoirs. We show that, except for the vacuum reservoir, the sudden-death of entanglement always exists in the thermal and squeezed reservoirs. We present explicit expression for the sudden-death time of entanglement for various entangled states. We find that the sudden-death of entanglement results from the portion of the double excitation component in the initial entangled state. In this sense, the maximally entangled states of a two-qubit system that do not have the double excitation component is more robust against the quantum fluctuations of the vacuum reservoir.

We introduce the idea of using adiabatic rotation to generate superpositions of a large class of quantum states. For quantum computing this is an interesting alternative to the well-studied “straight line” adiabatic evolution. In ways that complement recent results, we show how to efficiently prepare three types of states: Kitaev’s toric code state, the cluster state of the measurement-based computation model, and the history state used in the adiabatic simulation of a quantum circuit. We also show that the method, when adapted for quantum search, provides quadratic speedup as other optimal methods do with the advantages that the problem Hamiltonian is time independent and that the energy gap above the ground state is strictly nondecreasing with time. Likewise the method can be used for optimization as an alternative to the standard adiabatic algorithm.

We propose a new protocol of universal entanglement concentration, which converts many copies of an unknown pure state to an exact maximally entangled state. The yield of the protocol, which is outputted as a classical information, is probabilistic, and achieves the entropy rate with high probability, just as nonuniversal entanglement concentration protocols do.

We have investigated cold ionizing heteronuclear collisions in dilute mixtures of metastable $\left(2{}^{3}S_1\right)$ ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$ atoms, extending our previous work on the analogous homonuclear collisions [R. J. W. Stas et al., Phys. Rev. A 73, 032713 (2006)]. A simple theoretical model of such collisions enables us to calculate the heteronuclear ionization rate coefficient, for our quasiunpolarized gas, in the absence of resonant light $(T=1.2\mathrm{mK})$: $K_{34}^{\left(\mathrm{th}\right)}=2.4\times {10}^{-10}{\mathrm{cm}}^3∕\mathrm{s}$. This calculation is supported by a measurement of $K_{34}$ using magneto-optically trapped mixtures containing about $1\times {10}^8$ atoms of each species, $K_{34}^{\left(\exp \right)}=2.5\left(8\right)\times {10}^{-10}{\mathrm{cm}}^3∕\mathrm{s}$. Theory and experiment show good agreement.

We study the excitation of heavy hydrogenlike ions occurring in high-energy collisions with many-electron atoms by considering three theoretical approaches. In all of them the initial and final undistorted states of the electron in the ion are described by relativistic Coulomb-Dirac wave functions. In two of these approaches the interaction between the electron of the ion and the atom is described within the first order perturbation theory. In the first approach the presence of the atomic electrons is neglected whereas the second approach takes them into account. The comparison of results of these two approaches allows one to establish the range of collision energies where the effect of the electrons of the atom on the excitation process is weak and can be neglected. At these energies, however, the interaction between the electron of the ion and the nucleus of the atom may become too strong for the first order theory to be a good approximation. In order to deal with this point we present the third approach which is based on the symmetric eikonal approximation. Theoretical results are compared with available experimental data.

We present results from a joint experimental and theoretical investigation of superelastic electron scattering from laser-excited Cs atoms in the ${\left(6p\right)}^2{P}_{3∕2}$ state. Comparison of the measured pseudo-Stokes parameters ${\overline{P}}_1$, ${\overline{P}}_2$, and ${\overline{P}}_3$ and the total degree of polarization $P^{+}$ for incident energies of $5.5\mathrm{eV}$ and $13.5\mathrm{eV}$, respectively, with theoretical predictions based upon a nonrelativistic convergent close-coupling method and a 24-state semirelativistic Breit-Pauli $R$-matrix approach indicates that driving channel coupling to convergence for these observables is more important than accounting for relativistic effects.

We studied, by density functional theory, a group of 16 clusters having the general formula $A_4{B}_{12}$, where “$A$” is divalent and “$B$” monovalent. Global optimization was done in each case followed by calculation of energy second derivatives and vibrational frequencies. The clusters have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps ranging from $1.2\text{to}2.6\mathrm{eV}$ and other features suggesting special stability. This is consistent with the jellium model and 20 electron count. A $T_d$ symmetry cage structure is found as the putative global minimum for ${\mathrm{Mg}}_4{\mathrm{Ag}}_{12}$, ${\mathrm{Mg}}_4{\mathrm{Au}}_{12}$, ${\mathrm{Cd}}_4{\mathrm{Au}}_{12}$, and ${\mathrm{Ca}}_4{\mathrm{Na}}_{12}$. It is also a low energy isomer (less than $0.1\mathrm{eV}$ above the global minimum) for ${\mathrm{Zn}}_4{\mathrm{Au}}_{12}$, ${\mathrm{Be}}_4{\mathrm{Au}}_{12}$, and ${\mathrm{Be}}_4{\mathrm{Ag}}_{12}$. The $T_d$ cage structure has ions arranged in shells, with charge $+8$ and $+12$, that match two jellium electronic shell closings, $(1s^2,1p^6)$ and $(1d^{10},2s^2)$ with 8 and 12 electrons, respectively. The importance of symmetry and ion shells is shown by the relative stability of homotops of ${\mathrm{Mg}}_4{\mathrm{Ag}}_{12}$.

A semiclassical theory of chaotic atomic transport in a one-dimensional nondissipative optical lattice, when the standing-wave field and center-of-mass motion are treated classically, is developed. Using the basic equations of motion for the Bloch and translational atomic variables, we derive a stochastic map for the synchronized component of the atomic dipole moment that determines the center-of-mass motion. We find the analytical relations between the atomic and lattice parameters under which atoms typically alternate between flying through the lattice and being trapped in the wells of the optical potential. We use the stochastic map to derive formulas for the probability density functions (PDFs) for the flight and trapping events. Statistical properties of chaotic atomic transport strongly depend on the relations between the atomic and lattice parameters. We show that there is a good quantitative agreement between the analytical PDFs and those computed with the stochastic map and the basic equations of motion for different ranges of the parameters. Typical flight and trapping PDFs are shown to be broad distributions with power law segments with the slope $-1.5$ and exponential “tails.” The lengths of the power law and exponential segments of the PDFs depend on the values of the parameters. We find analytical conditions under which deterministic atomic transport has fractal properties and explain a hierarchical structure of the dynamical fractals.

We consider a homogeneous two-dimensional Bose gas with repulsive dipole-dipole interactions. The ground-state equation of state, calculated using the diffusion Monte Carlo method, shows quantitative differences from the predictions of the commonly used Gross-Pitaevskii mean-field theory. The static structure factor, pair distribution function, and condensate fraction are calculated in a wide range of the gas parameter. Differences from mean-field theory are reflected in the frequency of the lowest “breathing” mode for harmonically trapped systems.

We study numerically the low-temperature behavior of a one-dimensional Bose gas trapped in an optical lattice. For a sufficient number of particles and weak repulsive interactions, we find a clear regime of temperatures where density fluctuations are negligible but phase fluctuations are considerable, i.e., a quasicondensate. In the weakly interacting limit, our results are in very good agreement with those obtained using a mean-field approximation. In coupling regimes beyond the validity of mean-field approaches, a phase-fluctuating condensate also appears, but the phase-correlation properties are qualitatively different. It is shown that quantum depletion plays an important role.

This paper shows experimental evidence of photorefractive steady state self-focusing in InP:Fe for a wide range of intensities, at both 1.06 and $1.55\mu \mathrm{m}$. To explain those results, it is shown that despite the bipolar nature of InP:Fe where one photocarrier and one thermal carrier are to be considered, the long standing one photocarrier model for photorefractive solitons can be usefully applied. The relationship between the dark irradiance stemming out of this model and the known resonance intensity is then discussed.

We demonstrate, both theoretically and experimentally, how to acquire nonlinear control over the angular momentum of a cluster of solitary waves. Our results, stemming from a universal theoretical model, show that the angular momentum can be adjusted by acting on the global energy input in the system. The phenomenon is experimentally ascertained in nematic liquid crystals by observing a power-dependent rotation of a two-soliton ensemble.

Temporal optical soliton molecules were recently demonstrated; they potentially allow further increase of data rates in optical telecommunication. Their binding mechanism relies on the internal phases, but these have not been experimentally accessible so far. Conventional frequency-resolved optical gating techniques are not suited for measurement of their phase profile: Their algorithms fail to converge due to zeros both in their temporal and their spectral profile. We show that the VAMPIRE (very advanced method of phase and intensity retrieval of $E$-fields) method performs reliably. With VAMPIRE the phase profile of soliton molecules has been measured, and further insight into the mechanism is obtained.

A recent paper by [Nascimento et al., Phys. Rev. A 74, 054501 (2006)], and a previous paper from the same group [de Oliveira et al., Phys. Rev. A 65 031401(R) (2002)], report measurements of Rydberg lifetimes in rubidium in the range $27\le n\le 44$. In this Comment, we point out one apparent flaw of experimental technique in these papers that may lead to systematic errors in the results. We also clarify a point of theoretical interpretation used in these papers which suggests that the agreement between experiment and theory may not be as good as it seems.

In response to the preceding Comment, we present important experimental information on our $S$, $P$, and $D$ Rydberg lifetime measurements for ${}^{85}\mathrm{Rb}$ using a sample of cold Rydberg atoms. We analyze and discuss the implications of blackbody radiation transfer on our results. We also discuss the limitation of the theory compared with our results.

The Casimir force, which results from the confinement of the quantum-mechanical zero-point fluctuations of electromagnetic fields, has received significant attention in recent years for its effect on micro- and nanoscale mechanical systems. With few exceptions, experimental observations have been limited to interacting conductive bodies separated by vacuum or air. However, interesting phenomena, including repulsive forces, are expected to exist in certain circumstances between metals and dielectrics when the intervening medium is not vacuum. In order to better understand the effect of the Casimir force in such situations and to test the robustness of the generalized Casimir-Lifshitz theory, we have performed precision measurements of the Casimir force between two metals immersed in a fluid. For this situation, the measured force is attractive and is approximately 80% smaller than the force predicted by Casimir for ideal metals in vacuum. We present experimental results and find them to be consistent with Lifshitz’s theory.

Entanglement measures constructed from two positive, but not completely positive, maps on density operators are used as constraints in placing bounds on the entanglement of formation, the tangle, and the concurrence of $4N$ mixed states. The maps are the partial transpose map and the phi map introduced by Breuer [H.-P. Breuer, Phys. Rev. Lett. 97, 080501 (2006)]. The norm-based entanglement measures constructed from these two maps, called negativity and phi negativity, respectively, lead to two sets of bounds on the entanglement of formation, the tangle, and the concurrence. We compare these bounds and identify the sets of $4N$ density operators for which the bounds from one constraint are better than the bounds from the other. In the process, we present a derivation of the already known bound on the concurrence based on the negativity. We compute bounds on the three measures of entanglement using both the constraints simultaneously. We demonstrate how such doubly constrained bounds can be constructed. We discuss extensions of our results to bipartite states of higher dimensions and with more than two constraints.

We present a general theoretical method to generate maximally entangled mixed states of a pair of photons initially prepared in the singlet polarization state. This method requires only local operations upon a single photon of the pair and exploits spatial degrees of freedom to induce decoherence. We report also experimental confirmation of these theoretical results.

We study the non-Markovian dynamics of a two-mode bosonic system interacting with two uncorrelated thermal bosonic reservoirs. We present the solution to the exact microscopic Master equation in terms of the quantum characteristic function and study in detail the dynamics of entanglement for bipartite Gaussian states. In particular, we analyze the effects of short-time system-reservoir correlations on the separability thresholds and show that the relevant parameter is the reservoir spectral density. If the frequencies of the involved modes are within the reservoir spectral density, entanglement persists for a longer time than in a Markovian channel. On the other hand, when the reservoir spectrum is out of resonance, short-time correlations lead to a faster decoherence and to the appearance of entanglement oscillations.

Recently, it has been realized that nonlocal disentanglement may take a finite time as opposite to the asymptotic decay of local coherences. We find in this paper that a sudden irreversible death of entanglement takes place in a two atom optical Stern-Gerlach model. In particular, the one degree nondissipative environment here considered suddenly destroys the initial entanglement of any Bell’s states $\mid {\varphi }^{\pm }⟩$ superposition.