Correlations in an Einstein-Podolsky-Rosen-Bohm experiment can be made stronger than quantum correlations by allowing a single bit of classical communication between the two sides of the experiment.

The decoherence of an asymmetric two-qubit system that is coupled via a variable interaction term to a common bath or two individual baths of harmonic oscillators is examined. The dissipative dynamics are evaluated using the Bloch-Redfield formalism. It is shown that the behavior of the decoherence effects is affected mostly by the symmetries between the qubit operator that is coupled to the environment and the temperature, whereas the differences between the two bath configurations are very small. Moreover, it is elaborated that small imperfections of the qubit parameters do not necessarily lead to a drastic enhancement of the decoherence rates.

The variational two-electron reduced-density-matrix (2-RDM) method computes the ground-state energy and 2-RDM of an atom or molecule without calculation of the many-electron wave function. Recently, the computational efficiency of the 2-RDM method has been significantly enhanced through the use of a first-order algorithm for semidefinite programming [Mazziotti, Phys. Rev. Lett. 93, 213001 (2004)]. In this paper we develop a spin- and symmetry-adapted formulation of the method that further improves its efficiency by incorporating both the spin and spatial symmetries of many-electron atoms and molecules. While previous work on density-matrix symmetry focused on only one form of the 2-RDM, the variational method employs three different forms of the 2-RDM, known as the $D$, $Q$, and $G$ matrices, to restrict the 2-RDM to be approximately $N$-representable, that is representable by an $N$-electron wave function. We apply spin symmetries to the three forms of the 2-RDM, each of which breaks into four diagonal spin-blocks, namely one singlet and three triplet blocks. If the molecules have point-group symmetry, each of the 2-RDMs may be further subdivided into smaller diagonal blocks according to the spatial symmetry of the basis functions. The subdivision of the 2-RDMs into diagonal blocks generates significant computational savings in both floating-point operations and memory storage. Calculations illustrate the computational savings. Spin adaptation also enforces the correct expectation value of the ${\widehat{S}}^2$ operator, which in earlier work is applied as a separate constraint.

This study reports static electric dipole polarizabilities of the group 13 atoms in their lowest $J=\frac{1}{2},\frac{3}{2}$ states including resolution of the property in the corresponding $M_J$ components. The polarizabilities are obtained by a numerical finite-field technique applying weak external electric fields. Special relativity is accounted for in the four-component Dirac framework. Two large-scale configuration interaction programs which can be applied either including or neglecting spin-dependent terms in the Dirac Hamiltonian, respectively, are used for the treatment of dynamic electron correlation. Coupled cluster calculations are performed for obtaining highly accurate $J=\frac{1}{2}$ ground-state polarizabilities and for calibrating the configuration interaction calculations. The heavier atoms show large differences in the properties for different $J$ states, and the effect of spin-orbit coupling is elucidated by separating it off from scalar relativistic contributions. The impact of spin-orbit interaction is also demonstrated for polarizability anisotropy components.

Gaussian basis functions, routinely employed in molecular electronic structure calculations, can be combined with numerical grid-based functions in a discrete variable representation to provide an efficient method for computing molecular continuum wave functions. This approach, combined with exterior complex scaling, obviates the need for slowly convergent single-center expansions, and allows one to study a variety of electron-molecule collision problems. The method is illustrated by computation of various bound and continuum properties of $\mathrm{H}_2{}^{+}$.

Charge-transfer processes in collisions of ${\mathrm{H}}^{+}$ ions with ${\mathrm{C}}_2{\mathrm{H}}_6$ molecules are investigated theoretically below $10\text{−}\mathrm{keV}$ collision energies within a molecular representation. Converged total as well as differential cross sections are obtained in this energy range within a discrete basis of electronic states computed by ab inito methods. The present collision system suggests that the combination of the Demkov-type and Landau-Zener–type mechanisms primarily governs the scattering dynamics for the flux exit from the initial channel. The present charge-transfer cross sections determined are found to agree very well with all available experimental data below a few keV, but begin to deviate above $3\mathrm{keV}$, in which the present results slowly decrease, while measurements stay nearly constant. From the study of the electronic state calculation, we provide some information on fragmented species, which should help shed some light on the fragmentation mechanism and process of ${\mathrm{C}}_2{\mathrm{H}}_6^{+}$ ions produced after charge transfer. In addition, the vibrational effect of the initial target to charge transfer is examined.

We demonstrate enhancement in the number of trapped cesium atoms in a magneto-optical trap (MOT) using a control laser that illuminates only a small faction of the capture region of the trap without interacting with the cold cloud of atoms. The enhancement is maximized when the laser is slightly blue detuned with respect to the cooling transition. Trap loading curves point to approximately a twofold increase in the capture rate, which as a consequence results in the increase in the steady state number of trapped atoms. Enhanced loading is confirmed by MOT loading and decay curves taken under the modulation of the control laser beam. Optical pumping of the inaccessible Zeeman states into the stretched states is suggested as a possible mechanism.

We investigate theoretically the mechanical effects of light on atoms trapped by an external potential, whose dipole transition couples to the mode of an optical resonator and is driven by a laser. We derive an analytical expression for the quantum center-of-mass dynamics, which is valid in presence of a tight external potential. This equation has broad validity and allows for a transparent interpretation of the individual scattering processes leading to cooling. We show that the dynamics is a competition of the mechanical effects of the cavity and of the laser photons, which may mutually interfere. We focus on the good-cavity limit and identify novel cooling schemes, which are based on quantum interference effects and lead to efficient ground-state cooling in experimentally accessible parameter regimes.

I analyze an atomic Fermi gas with a planar $p$-wave interaction, motivated by the experimentally observed anisotropy in $p$-wave Feshbach resonances. An axial superfluid state is verified. A domain wall object is discovered to be a new topological defect of this superfluid and an explicit solution has been found. Gapless quasiparticles appear as bound states on the wall, dispersing in the continuum of reduced dimensions. They are chiral, deeply related to fermion zero modes in quantum chromodynamics. The chirality of the superfluid is manifested by a persistent anomalous mass current of atoms in the ground state. This quantum phenomenon is a prediction for future experiments.

In the above-mentioned paper by Dunkel and Trigger [Phys. Rev. A 71, 052102 (2005)] a hypothesis has been pursued that the loss of information associated with the quantum evolution of pure states, quantified in terms of an increase in the time of so-called Leipnik’s joint entropy, could be a rather general property shared by many quantum systems. This behavior has been confirmed for the unconfined model systems and properly tuned initial data (maximally classical states). We provide two particular examples which indicate a complexity of the quantum evolution. In the presence of a confining (harmonic) potential Leipnik’s entropy there may be nonincreasing for maximally classical initial data. Another choice of initial data implies periodicity in the time of the Leipnik entropy.

We show that a dilute harmonically trapped two-component gas of fermionic atoms with a weak repulsive interaction has a pronounced super-shell structure: The shell fillings due to the spherical harmonic trapping potential are modulated by a beat mode. This changes the “magic numbers” occurring between the beat nodes by half a period. The length and amplitude of this beating mode depend on the strength of the interaction. We give a simple interpretation of the beat structure in terms of a semiclassical trace formula for the symmetry breaking $\mathrm{U}\left(3\right)\to \mathrm{SO}\left(3\right)$.

We discuss an alternative to relative entropy as a measure of distance between mixed quantum states. The proposed quantity is an extension to the realm of quantum theory of the Jensen-Shannon divergence (JSD) between probability distributions. The JSD has several interesting properties. It arises in information theory and, unlike the Kullback-Leibler divergence, it is symmetric, always well-defined, and bounded. We show that the quantum JSD shares with the relative entropy most of the physically relevant properties, in particular those required for a “good” quantum distinguishability measure. We relate it to other known quantum distances and we suggest possible applications in the field of the quantum information theory.

We compare the performance of various quantum-key-distribution (QKD) systems using a single-photon detector, which combines frequency up-conversion in a periodically poled lithium niobate waveguide and a silicon avalanche photodiode (APD). The comparison is based on the secure communication rate as a function of distance for three QKD protocols: the Bennett-Brassard 1984, the Bennett-Brassard-Mermin 1992, and the coherent differential-phase-shift keying protocols. We show that the up-conversion detector allows for higher communication rates and longer communication distances than the commonly used $\mathrm{In}\mathrm{Ga}\mathrm{As}∕\mathrm{InP}$ APD for all three QKD protocols.

In this paper, we study the correlations that arise when two separated parties perform measurements on systems they hold locally. We restrict ourselves to those correlations with which arbitrarily fast transmission of information is impossible. These correlations are called nonsignaling. We allow the measurements to be chosen from sets of an arbitrary size, but promise that each measurement has only two possible outcomes. We find the structure of this convex set of nonsignaling correlations by characterizing its extreme points. Taking an information-theoretic view, we prove that all of these extreme correlations are interconvertible. This suggests that the simplest extremal nonlocal distribution (called a PR box) might be the basic unit of bipartite nonlocality. We also show that this unit of nonlocality is sufficient to simulate all bipartite quantum states when measured with two outcome measurements.

We present a protocol for the teleportation of the quantum state of a pulse of light onto the collective spin state of an atomic ensemble. The entangled state of light and atoms employed as a resource in this protocol is created by probing the collective atomic spin, Larmor precessing in an external magnetic field, off resonantly with a coherent pulse of light. We take here full account of the effects of Larmor precession and show that it gives rise to a qualitatively different type of multimode entangled state of light and atoms. The protocol is shown to be robust against the dominating sources of noise and can be implemented with an atomic ensemble at room temperature interacting with free-space light. We also provide a scheme to perform the readout of the Larmor precessing spin state enabling the verification of successful teleportation as well as the creation of spin squeezing.

Excitation energies, normal modes of vibration and geometries of the electronically excited states of the water molecule are calculated using time-dependent density functional theory with ab initio pseudopotentials and the local density approximation. We express electronic orbitals on a real-space grid with no explicit basis set; as such, no polarization functions are required. Convergence is controlled by a single parameter, the grid spacing. It is found that for the first and second excited states the time-dependent local density approximation predicts a metastable configuration, in disagreement with published results using asymptotically corrected functionals that correctly predict the photodissociation of the molecule. It is shown that applying the real space method with the asymptotically corrected local density approximation reproduces the previous results. The polarizability of the water molecule is calculated using the time dependent local density approximation as well as using the asymptotically corrected local density approximation.

Similarly to differential cross sections for one-electron photoionization, the doubly differential cross section for double photoionization (DPI) may be conveniently described by four parameters: the singly differential (with respect to energy sharing) cross section $\left({\sigma }_0\right)$, the dipole asymmetry parameter $\left(\beta \right)$, and two nondipole asymmetry parameters ($\gamma$ and $\delta$). Here we derive two model-independent representations for these parameters for DPI from a ${}^{1}S_0$ atomic bound state: (i) in terms of one-dimensional integrals of the polarization-invariant DPI amplitudes and (ii) in terms of the exact two-electron reduced matrix elements. For DPI of He at excess energies, $E_{\mathrm{exc}}$, of 100 eV, 450 eV, and 1 keV, we present numerical results for the asymmetry parameters within the framework of the convergent close-coupling theory and compare them with results of lowest-order (in the interelectron interaction) perturbation theory (LOPT). The results for $E_{\mathrm{exc}}=1\mathrm{keV}$ exhibit a nondipole asymmetry that is large enough to be easily measured experimentally. We find excellent agreement between our LOPT results and other theoretical predictions and experimental data for total cross sections and ratios of double to single ionization cross sections for $K$-shell DPI from several multielectron atoms.

The modification due to an external linearly polarized monochromatic laser field in the dynamics of the ionization process of a hydrogenic ion by electron and positron impact is studied theoretically for coplanar geometry. Significant changes are noted both in the shape and magnitude of the triple-differential cross sections (TDCS) by the application of the laser field. The net effect of the laser field is to suppress the field-free (FF) cross sections in the zeroth-order approximation of the ejected electron wave function, while in the first order the cross sections are found to be enhanced or suppressed depending on the kinematics of the process. The TDCS is found to be quite sensitive with respect to the initial phase as well as the frequency of the laser field. For positron impact, the effect of the laser field on the dynamics of the electron capture to continuum (ECC) phenomena is also studied. No significant change is noted in the qualitative behavior of the FF ECC peak with the application of the laser field.

We analyze a microscopic decoherence model in which the total system is described as a spin gas. A spin gas consists of $N$ classically moving particles with additional, interacting quantum degrees of freedom (e.g., spins). For various multipartite entangled probe states, we analyze the decoherence induced by interactions between the probe and environmental spins in such spin gases. We can treat mesoscopic environments ($\approx {10}^5$ particles). We present results for a lattice gas, which could be realized by neutral atoms hopping in an optical lattice, and show the effects of non-Markovian and correlated noise, as well as finite-size effects.