Imagine two parties, Alice and Bob, who share an entangled quantum state. A well-established result that if Alice performs a two-outcome measurement on the portion of the state in her possession and Bob does likewise, they are able to produce correlations that cannot be reproduced by any classical theory. The allowed classical correlations can be expressed quantitatively by the Bell inequalities. Here we propose new families of Bell inequalities, as a generalization of the Clauser-Horne-Shimony-Holt inequality and show that the maximum violation of these Bell inequalities allowed by quantum theory cannot be attained by a bipartite quantum system having support on a qubit at each site.

We present a way for entanglement purification based on two parametric down-conversion (PDC) sources with cross-Kerr nonlinearities. It is comprised of two processes. The first one is a primary entanglement purification protocol for PDC sources with nondestructive quantum nondemolition (QND) detectors by transferring the spatial entanglement of photon pairs to their polarization. In this time, the QND detectors act as the role of controlled-NOT (CNOT) gates. Also they can distinguish the photon number of the spatial modes, which provides a good way for the next process to purify the entanglement of the photon pairs kept more. In the second process for entanglement purification, new QND detectors are designed to act as the role of CNOT gates. This protocol has the advantage of high yield and it requires neither CNOT gates based on linear optical elements nor sophisticated single-photon detectors, which makes it more convenient in practical applications.

Measured and calculated differential cross sections for elastic (rotationally unresolved) electron scattering from two primary alcohols, methanol $\left(\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}\right)$ and ethanol $\left({\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}\right)$, are reported. The measurements are obtained using the relative flow method with helium as the standard gas and a thin aperture as the collimating target gas source. The relative flow method is applied without the restriction imposed by the relative flow pressure conditions on helium and the unknown gas. The experimental data were taken at incident electron energies of 1, 2, 5, 10, 15, 20, 30, 50, and $100\mathrm{eV}$ and for scattering angles of 5°–130°. There are no previous reports of experimental electron scattering differential cross sections for $\mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$ and ${\mathrm{C}}_2{\mathrm{H}}_5\mathrm{O}\mathrm{H}$ in the literature. The calculated differential cross sections are obtained using two different implementations of the Schwinger multichannel method, one that takes all electrons into account and is adapted for parallel computers, and another that uses pseudopotentials and considers only the valence electrons. Comparison between theory and experiment shows that theory is able to describe low-energy electron scattering from these polyatomic targets quite well.

Cross sections for electron collisions with ${\text{NeH}}^{+}$ resulting in dissociative recombination, resonance enhanced dissociative excitation, and vibrational excitation are calculated in the 1 to 19.5 eV energy region. In this energy region, the cross sections are dominated by a series of doubly excited resonance states converging to the first excited state of the ion. The resonance state energies and widths are calculated using the complex Kohn variational method and are used as input to a time-dependent treatment of the nuclear dynamics. The calculated cross sections for dissociative recombination of ${\text{NeH}}^{+}$ and ${\text{NeD}}^{+}$ are compared to recent measurements at the ASTRID storage ring. Cross sections for dissociative excitation and vibrational excitation are also reported for the ${\text{NeH}}^{+}$ and ${\text{NeD}}^{+}$ molecular ions and compared to recent cross beam experiments.

Atomic vapor of four different paramagnetic species: gold, silver, lithium, and rubidium, is produced and studied inside several buffer gases: helium, nitrogen, neon, and argon. The paramagnetic atoms are injected into the buffer gas using laser ablation. Wires with diameters 25, 50, and $100\mu \text{m}$ are used as ablation targets for gold and silver; bulk targets are used for lithium and rubidium. The buffer gas cools and confines the ablated atoms, slowing down their transport to the cell walls. Buffer gas temperatures between 20 and 295 K and densities between ${10}^{16}{\text{cm}}^{-3}$ and $2\times {10}^{19}{\text{cm}}^{-3}$ are explored. Peak paramagnetic atom densities of ${10}^{11}{\text{cm}}^{-3}$ are routinely achieved. The longest observed paramagnetic vapor density decay times are 110 ms for silver at 20 K and 4 ms for lithium at 120 K. The candidates for the principal paramagnetic-atom loss mechanism are impurities in the buffer gas, dimer formation, and atom loss on sputtered clusters.

The ability to control matter on the nanometer scale is greatly influenced by the van der Waals (vdW) interaction. Therefore, understanding and manipulating the vdW interaction is of interest to the fields of nanotechnology and atom optics. We show that near-resonant light can significantly modify atom-surface vdW interactions in the nonretarded regime. A theory based on quantized electromagnetic fields is used to calculate (1) the ordinary vdW interaction, (2) corrections to the ordinary vdW interaction due to thermal radiation, and (3) modifications to the ordinary vdW interaction that result from monochromatic (laser) radiation. Near-resonant laser light with an intensity of $5\text{W}/{\text{cm}}^2$ is predicted to double the vdW interaction strength for sodium atoms, and possible experiments to detect this effect are discussed.

We study the three-body problem for three atomic fermions, in the same spin state, experiencing a resonant interaction in the $\mathit{p}$-wave channel via a Feshbach resonance represented by a two-channel model. The rate of inelastic processes due to recombination to deeply bound dimers is then estimated from the three-body solution using a simple prescription. We obtain numerical and analytical predictions for most of the experimentally relevant quantities that can be extracted from the three-body solution: the existence of weakly bound trimers and their lifetime, the low-energy elastic and inelastic scattering properties of an atom on a weakly bound dimer (including the atom-dimer scattering length and scattering volume), and the recombination rates for three colliding atoms towards weakly bound and deeply bound dimers. The effect of ‘‘background’’ nonresonant interactions in the open channel of the two-channel model is also calculated and allows one to determine which three-body quantities are “universal” and which on the contrary depend on the details of the model.

We elucidate the physics underlying the fact that both bright and dark solitary waves can arise in a one-dimensional spin-polarized gas of fermionic atoms with attractive three-dimensional $p$-wave interactions in a hard-wall trap. This is possible since the one-dimensional fermion system can be mapped to a system of bosons described by the Lieb-Linger model with either repulsive or attractive $\delta$-function interactions which can support solitary waves.

We theoretically discuss sensitivity limitations of a THz receiver which is based on up-conversion of the THz radiation into optical domain using high quality factor crystalline whispering gallery mode resonators. We show that the sensitivity of the receiver operating in the nonlinear regime approaches the sensitivity of an ideal THz photon counter. Thermal noise of the counter can be substantially reduced because of transparency of the nonlinear material in the THz frequency range. We also show that the density of power fluctuations of the receiver operating in the linear regime is given by the THz shot noise.

We introduce a quasianalytic nonlinear Schrödinger equation with beyond mean-field corrections to describe the dynamics of a zero-temperature dilute superfluid Fermi gas in the crossover from the weak-coupling Bardeen-Cooper-Schrieffer (BCS) regime, where $k_F\left|a\right|⪡1$ with $a$ the $s$-wave scattering length and $k_F$ the Fermi momentum, through the unitarity limit $k_{F}a\to \pm \infty$ to the Bose-Einstein condensation (BEC) regime where $k_{F}a>0$. The energy of our model is parametrized using the known asymptotic behavior in the BCS, BEC, and the unitarity limits and is in excellent agreement with accurate Green’s-function Monte Carlo calculations. The model generates good results for frequencies of collective breathing oscillations of a trapped Fermi superfluid.

We demonstrate that the structure of ${\mathrm{CO}}_2$ molecules can be determined by measuring the phases of harmonics from them. This is evidenced by observing a phase jump of harmonics generated in a mixed gas of aligned ${\mathrm{CO}}_2$ molecules and their reference Kr atoms, which serve to extract the net contribution due to the structure of ${\mathrm{CO}}_2$ molecules. Furthermore, in the strong field approximation model, we derive the exact conditions of quantum interference during high-order harmonic generation in ${\mathrm{CO}}_2$ molecules analytically and we propose a method to control harmonic pulse duration.

The ionization of ${\mathrm{H}}_2$ in intense laser pulses is studied by numerical integration of the time-dependent Schrödinger equation for a single-active-electron model including the vibrational motion. The electron kinetic-energy spectra in high-order above-threshold ionization are strongly dependent on the vibrational quantum number of the created $\mathrm{H}_2{}^{+}$ ion. For certain vibrational states, the electron yield in the midplateau region is strongly enhanced. The effect is attributed to channel closings, which were previously observed in atoms by varying the laser intensity.

A CCD array is placed facing a chaotic light source and gated by a photon counting detector that simply counts all randomly scattered and reflected photons from an object. A “ghost” image of the object is then observed in the gated CCD. Differing from all published ghost-imaging experiments, this setup captures ghosts from scattered and reflected light of an object, instead of the transmitted ones. This new feature is not only useful for practical applications, but is also important fundamentally. It further explores the nonclassical interference nature of thermal light ghost imaging.

The variational method in a reformulated Hamiltonian formalism of quantum electrodynamics is used to derive relativistic wave equations for a system consisting of $n$ fermions and antifermions. Simple Fock-space variational trial states are used to obtain the relativistic $n$-body equations. The derived kernels of these equations (i.e., momentum-space relativistic potentials) include one-photon exchange and virtual annihilation interactions. The equations are shown to have the Schrödinger nonrelativistic limit. Application to the particular cases of positronium (Ps), positronium negative ion $\left({\text{Ps}}^{-}\right)$, and positronium molecule (${\text{Ps}}_2$, $e^{-}{e}^{+}{e}^{-}{e}^{+}$) are discussed.

We introduce a graphical representation of stabilizer states and translate the action of Clifford operators on stabilizer states into graph operations on the corresponding stabilizer-state graphs. Our stabilizer graphs are constructed of solid and hollow nodes, with (undirected) edges between nodes and with loops and signs attached to individual nodes. We find that local Clifford transformations are completely described in terms of local complementation on nodes and along edges, loop complementation, and change of node type or sign. Additionally, we show that a small set of equivalence rules generates all graphs corresponding to a given stabilizer state; we do this by constructing an efficient procedure for testing the equality of any two stabilizer graphs.

We study the macroscopic quantum tunneling of two weakly linked superfluids made of interacting fermionic atoms. We derive atomic Josephson junction equations and find that zero-mode and $\pi$-mode frequencies of coherent atomic oscillations depend on the tunneling coefficient and the sound velocity of the superfluid. By considering a superfluid of ${}^{40}\text{K}$ atoms, we calculate these oscillation frequencies in the crossover from the Bardeen-Cooper-Schrieffer state of weakly bound Cooper pairs to the Bose-Einstein condensate of strongly bound molecular dimers.

We study dynamics of a nonlinear three-level $\Lambda$ system describing Bose-Einstein condensates of atoms and diatomic molecules coupled by a two-color laser field. The system has a nonlinear dark state which is a generalization of the usual atomic dark state. In the recent paper [A. P. Itin and S. Watanabe, Phys. Rev. Lett. 99, 223903 (2007)], nonlinear instabilities of the dark state due to 1:1 and 1:2 resonances were discussed in the model without mean-field collisional interactions. Here we investigate the dark state of a model with collisional interactions. We show that nonlinear instabilities can be used, in particular, for precise determination of the scattering lengths.

We propose a cavity QED setup which implements a dissipative Lipkin-Meshkov-Glick model—an interacting collective spin system. By varying the external model parameters the system can be made to undergo both first- and second-order quantum phase transitions, which are signified by dramatic changes in cavity output field properties, such as the probe laser transmission spectrum. The steady-state entanglement between pairs of atoms is shown to peak at the critical points and can be experimentally determined by suitable measurements on the cavity output field. The entanglement dynamics also exhibits pronounced variations in the vicinities of the phase transitions.

The two-photon temporal coincidence detection amplitude obeys a pair of equations identical to those of classical partially coherent plane-wave pulses propagating in linearly dispersive media. These equations are also the same as the paraxial Wolf equations, for both the two-photon spatial probability amplitude and the cross-spectral density function. Therefore, a fourfold analogy between space and time, as well as between quantum entanglement and partial coherence, arises. In accordance to this, we predict nonlocal interference structures in a fourth-order interferometric configuration with classical partially coherent pulses under the assumption of Gaussian statistics. As an example, we present the classical temporal counterpart of the ghost diffraction phenomenon. Our work suggests that some time-domain entanglement phenomena that hitherto were considered as uniquely quantum can be mimicked by conventional partially coherent light pulses.

The complex scaling method combined with the stochastic variational approach has been used to calculate the energy and widths of a number of $L=0$ resonant states of the positron-hydrogen system. The interparticle correlation functions reveal evidence of cluster formation in the resonance state. The lowest $L=0$ resonance associated with the $\text{H}\left(n=2\right)$ threshold shows a bulge in the $e^{-}\text{−}{e}^{+}$ correlation function located at the same distance as the probability maximum of the $\text{Ps}\left(1s\right)$ state.