Using the negativity as an entanglement measure, we investigate the possible amount of remotely prepared entanglement. For two identical isotropic states on two-qudit systems 12 and 34, we calculate the average amount of entanglement remotely distributed on the system 13 by joint measurement on the system 24, and show that the remote preparation of entanglement by the generalized Bell measurement is optimal among rank-one measurements if the isotropic states have a certain fidelity with a maximally entangled state in higher dimensional quantum systems, or if the fidelity of the isotropic states is greater than a certain value depending on the dimension. In addition, we construct a measurement better than the generalized Bell measurement with respect to the remote preparation of entanglement when the isotropic states have small fidelity.

We analyze the dynamical generation of entanglement in systems of two interacting spins initially prepared in a product of spin coherent states. For arbitrary time-independent Hamiltonians, we derive a semiclassical expression for the purity of the reduced density matrix as a function of time. The final formula, subsidiary to the linear entropy, shows that the short-time dynamics of entanglement depends exclusively on the stability of trajectories governed by the underlying classical Hamiltonian. In addition, this semiclassical measure is shown to reproduce the general properties of its quantum counterpart and give the expected result in the large spin limit. The accuracy of the semiclassical formula is further illustrated in a problem of phase exchange for two particles of spin $j$.

Stereodynamics of dissociative electron attachment to CO${}_2$ is investigated using O${}^{-}$ anion velocity imaging experiments. Here ${}^2{\Pi }_g$ as a Feshbach resonant state of CO${}_2$${}^{-}$ is confirmed to play a role in the dissociation CO${}_2$${}^{-}$ → CO(${}^1{\Sigma }^{+}$) + O${}^{-}$(${}^{2}P$) around 8.0 eV. At the lower energy, 7.7 eV, two split states of ${}^2{\Pi }_g$ due to the Renner-Teller effect are found to couple with the dissociation path; while above this energy, at 8.2 and 8.7 eV, the coupling between the dissociation and the C-O bond stretching becomes predominant. The evolutions of the potential energy surface around this Feshbach resonant state result in the dramatically different angular distributions of the O${}^{-}$ momentum.

We study the tunneling dynamics in a time-periodically modulated two-mode Bose-Hubbard model using Floquet theory. We consider situations where the system is in the self-trapping regime and either the tunneling amplitude, the interaction strength, or the energy difference between the modes is modulated. In the former two cases, the tunneling is enhanced in a wide range of modulation frequencies, while in the latter case the resonance is narrow. We explain this difference with the help of Floquet analysis. If the modulation amplitude is weak, the locations of the resonances can be found using the spectrum of the nonmodulated Hamiltonian. Furthermore, we use Floquet analysis to explain the coherent destruction of tunneling (CDT) occurring in a large-amplitude modulated system. Finally, we present two ways to create a NOON state (a superposition of $N$ particles in mode 1 with zero particles in mode 2 and vice versa). One is based on a coherent oscillation caused by detuning from a partial CDT. The other makes use of an adiabatic variation of the modulation frequency. This results in a Landau-Zener type of transition between the ground state and a NOON-like state.

We analyze the quench dynamics of a one-dimensional bosonic Mott insulator and focus on the time evolution of density correlations. For these we identify a pronounced propagation front, the velocity of which, once correctly extrapolated at large distances, can serve as a quantitative characteristic of the many-body Hamiltonian. In particular, the velocity allows the weakly interacting regime, which is qualitatively well described by free bosons, to be distinguished from the strongly interacting one, in which pairs of distinct quasiparticles dominate the dynamics. In order to describe the latter case analytically, we introduce a general approximation to solve the Bose-Hubbard Hamiltonian based on the Jordan-Wigner fermionization of auxiliary particles. This approach can also be used to determine the ground-state properties. As a complement to the fermionization approach, we derive explicitly the time-dependent many-body state in the noninteracting limit and compare our results to numerical simulations in the whole range of interactions of the Bose-Hubbard model.

We report on theoretical and experimental observations of self-sustained fast [gigahertz (GHz)] electronic oscillations resulting from coupled electron-photon dynamics in ultrahigh-$Q$ Si microdisk resonators with cw pumping. Our theoretical analysis identifies conditions for generating steady-state GHz oscillations while suppressing thermal oscillations [megahertz (MHz)] with submilliwatt input laser power. Such fast oscillations are tunable via changing the free-carrier (FC) lifetime of the resonator. Integrating a $p$-$i$-$n$ diode with these high-$Q$ resonators for controlling the FC lifetime promises the realization of an integrated voltage-controlled oscillator (VCO) in a silicon photonics chip.

A silica microsphere on a low-spring-constant cantilever (pendulum) is fabricated and evanescently coupled to a tapered optical fiber. The motion of the pendulum is detected as variations in the transmitted laser power through the tapered fiber. The optical coupling noise created by the pendulum motion is recorded by taking a fast Fourier transform of the transmitted laser power and the fundamental mechanical mode of the pendulum at 1.16 kHz is observed. The thermal damping and amplification of the coupling noise is investigated and the effect of the thermal feedback on the noise spectrum is examined. The response of the thermo-optical feedback to small transient and driven variations in the taper-pendulum separation for different values of laser detuning is demonstrated. Preliminary results on the optical force between the pendulum and the tapered fiber are also presented. Microspherical pendulums, with low mechanical spring constant, could be used for studying nanoscopic optical and mechanical forces, or optical cooling.

Recently, Chen et al. [Phys. Rev. A 84, 033835 (2011)] reported observation of anticorrelated photon coincidences in a Mach-Zehnder interferometer whose input light came from a mode-locked Ti:sapphire laser that had been rendered spatially incoherent by passage through a rotating ground-glass diffuser. They provided a quantum-mechanical explanation of their results, which ascribes the anticorrelation to two-photon interference. They also developed a classical-light treatment of the experiment and showed that it was incapable of explaining the anticorrelation behavior. Here we show that semiclassical photodetection theory, i.e., classical electromagnetic fields plus photodetector shot noise, does indeed explain the anticorrelation found by Chen et al. The key to our analysis is properly accounting for the disparate time scales associated with the laser's pulse duration, the speckle-correlation time, the interferometer's differential delay, and the duration of the photon-coincidence gate. Our result is consistent with the long-accepted dictum that laser light which has undergone linear-optical transformations is classical-state light, so that the quantum and semiclassical theories of photodetection yield quantitatively identical results for its measurement statistics. The interpretation provided by Chen et al. for their observations implicitly contradicts that dictum.

We propose a scheme for quantum computation in optical lattices. The qubits are encoded in the spatial wave function of the atoms such that spin decoherence does not influence the computation. Quantum operations are steered by shaking the lattice, while qubit addressability can be provided with experimentally available techniques of changing the lattice with single-site resolution. Numerical calculations show possible fidelities above 99$\%$ with gate times on the order of milliseconds.

We investigate the connection between the optimal collective eavesdropping attack and the optimal cloning attack where the eavesdropper employs an optimal cloner to attack the quantum key distribution (QKD) protocol. The analysis is done in the context of the security proof in Refs. [Proc. R. Soc. London A 461, 207 (2005); Phys. Rev. Lett. 95, 080501 (2005)] for discrete variable protocols in $d$-dimensional Hilbert spaces. We consider a scenario in which the protocols and cloners are equipped with symmetries. These symmetries are used to define a quantum cloning scenario. We find that, in general, it does not hold that the optimal attack is an optimal cloner. However, there are classes of protocols, where we can identify an optimal attack by an optimal cloner. We analyze protocols with 2, $d$ and $d+1$ mutually unbiased bases where $d$ is a prime, and show that for the protocols with 2 and $d+1$ mutually unbiased bases (MUBs) the optimal attack is an optimal cloner but, for the protocols with $d$ MUBs, it is not. Finally, we give criteria to identify protocols which have different signal states, but the same optimal attack. Using these criteria, we present qubit protocols which have the same optimal attack as the Bennett-Brassard 1984 (BB84) protocol or the 6-state protocol.

A method for highly selective remote sensing of atmospheric trace polar molecular gases is described. Based on infrared-terahertz double-resonance spectroscopic techniques, the molecule-specific coincidence between the lines of a CO${}_2$ laser and rotational-vibrational molecular absorption transitions provide two dimensions of recognition specificity: infrared coincidence frequency and the corresponding terahertz frequency whose absorption strength is modulated by the laser. Atmospheric pressure broadening expands the molecular recognition “specificity matrix” by simultaneously relaxing the infrared coincidence requirement and strengthening the corresponding terahertz signature. Representative double-resonance spectra are calculated for prototypical molecules CH${}_3$F and CH${}_3$Cl and their principal isotopomers from which a heuristic model is developed to estimate the specificity matrix and double-resonance signature strength for any polar molecule.

The many-body system comprising a He nucleus, three electrons, and a positron has been studied using an explicitly correlated Gaussians basis and a stochastic variational method for the optimization of the basis. The purpose has been to clarify to which extent the system can be considered as a distinguishable positronium (Ps) atom interacting with a He atom and, thereby, to pave the way to a practical atomistic modeling of Ps states and annihilation in matter. The maximum value of the distance between the positron and the nucleus is constrained and the Ps atom at different distances from the nucleus is identified from the electron and positron densities, as well as from the electron-positron distance and center-of-mass distributions. The polarization of the Ps atom increases as its distance from the nucleus decreases. The contact density of the electrons of He with the positron is depleted, particularly when the overlap is small. The ortho-Ps pick-off annihilation rate calculated as the overlap of the positron and the free He electron densities has to be corrected for the observed depletion, especially at large pores or voids.

We report on a study of electron-hydrogen scattering, using a combination of a modified method of polarized orbitals and the optical potential formalism. The calculation is restricted to $P$ waves in the elastic region, where the correlation functions are of Hylleraas type. It is found that the phase shifts are not significantly affected by the modification of the target function by a method similar to the method of polarized orbitals and, except at the very lowest energies, they are close to the phase shifts calculated earlier by Bhatia [A. K. Bhatia, Phys. Rev. A 69, 032714 (2004)]. This indicates that the correlation function is general enough to include the target distortion (polarization) in the presence of the incident electron. The important fact is that in the present calculation, to obtain similar results only a 35-term correlation function is needed in the wave function compared to the 220-term wave function required in the above-mentioned previous calculation. Results for the phase shifts, obtained in the present hybrid formalism, are rigorous lower bounds to the exact phase shifts. Accurate values for the effective range parameters are also calculated.

We develop an asymptotic theory describing the dynamics of two-dimensional dark quasisolitons in a smoothly inhomogeneous Bose-Einstein condensate (BEC) with repulsive interaction between atoms. It is shown that the trajectories of such quasisolitons coincide with the geometric-optical rays in an equivalent isotropic medium characterized by the effective refractive index dependent on both the density of the undisturbed condensate and the energy of the structures. An expression for this effective refractive index is found and specific features of the refraction of quasisolitons with different initial energies by the condensate inhomogeneities are studied. The obtained results are confirmed by direct numerical simulation using the Gross-Pitaevskii (GP) equation.

We study fast-moving bright solitons in the focusing nonlinear Schrödinger equation perturbed by a narrow Gaussian potential barrier. In particular, we present a general and comprehensive analysis of the case where two fast-moving bright solitons collide at the location of the barrier. In the limiting case of a $\delta$-function barrier, we use an analytic method to show that the relative norms of the outgoing waves depends sinusoidally on the relative phase of the incoming waves, and to determine whether one, or both, of the outgoing waves are bright solitons. We show using numerical simulations that this analytic result is valid in the high velocity limit: outside this limit nonlinear effects introduce a skew to the phase dependence, which we quantify. Finally, we numerically explore the effects of introducing a finite-width Gaussian barrier. Our results are particularly relevant, as they can be used to describe a range of interferometry experiments using bright solitary matter-waves.

The dynamical stability of nonstationary states of homogeneous spin-2 rubidium Bose-Einstein condensates is studied. The states considered are such that the spin vector remains parallel to the magnetic field throughout the time evolution, making it possible to study the stability analytically. These states are shown to be stable in the absence of an external magnetic field, but they become unstable when a finite magnetic field is introduced. It is found that the growth rate and wavelength of the instabilities can be controlled by tuning the strength of the magnetic field and the size of the condensate.

The use of coherent optical dressing of atomic levels allows the coupling of ultracold atoms to effective nondynamical gauge fields. These can be used to generate effective magnetic fields and have the potential to generate non-Abelian gauge fields. We consider a model of a gas of bosonic atoms coupled to a gauge field with $\text{U}\left(2\right)$ symmetry and with constant effective magnetic field. We include the effects of weak contact interactions by applying Gross-Pitaevskii mean-field theory. We study the effects of a $\text{U}\left(2\right)$ non-Abelian gauge field on the vortex lattice phase induced by a uniform effective magnetic field, generated by an Abelian gauge field or, equivalently, by rotation of the gas. We show that, with increasing non-Abelian gauge field, the nature of the ground state changes dramatically, with structural changes of the vortex lattice. We show that the effect of the non-Abelian gauge field is equivalent to the introduction of effective interactions with nonzero range. We also comment on the consequences of the non-Abelian gauge field for strongly correlated fractional quantum Hall states.

We propose a scheme for creating a maximally entangled state comprising two field quanta. In our scheme, two weak light fields, which are initially prepared in either coherent or polarization states, interact with a composite medium near an interface between a dielectric and a negative index metamaterial. This interaction leads to a large Kerr nonlinearity, reduction of the group velocity of the light, and significant confinement of the light fields, while simultaneously avoiding amplitude losses of the incoming radiation. All these considerations make our scheme efficient.

In the first days of quantum mechanics Dirac pointed out an analogy between the time-dependent coefficients of an expansion of the Schrödinger equation and the classical position and momentum variables solving Hamilton's equations. Here it is shown that the analogy can be made an equivalence in that, in principle, systems of classical oscillators can be constructed whose position and momenta variables form time-dependent amplitudes which are identical to the complex quantum amplitudes of the coupled wave function of an $N$-level quantum system with real coupling matrix elements. Hence classical motion can reproduce quantum coherence.