We use the mean-field approximation to numerically study the dynamics of a split condensate in Mach-Zehnder-type atom interferometers that employ Bragg diffraction. The Husimi representation is used as a direct tool for diagnosing the dynamics in phase space, thus providing information on the interference fringe contrast. We analyze how optimization of the contrast of interference signals can result by means of compensating for the nonlinear interaction and the trapping potential for three alternative experimental schemes. The phase-space overlap in Husimi distribution as well as dynamics in phase space is shown to be important in determining the interference fringe contrast.

Electromagnetically induced transparency (EIT) experiments in $\Lambda$-type systems benefit from the use of hot vapor where the thermal averaging results in reducing the width of the EIT resonance well below the natural linewidth. Here, we demonstrate a technique for further reducing the EIT width in room-temperature vapor by the application of a small longitudinal magnetic field. The Zeeman shift of the energy levels results in the formation of several shifted subsystems; the net effect is to create multiple EIT dips each of which is significantly narrower than the original resonance. We observe a reduction by a factor of 3 in the $D_2$ line of ${}^{87}\mathrm{Rb}$ with a field of $3.2\mathrm{G}$.

We employ an effective discrete model for the study of wave propagation in a photonic crystal waveguide side-coupled to a cavity with Kerr-type nonlinearity. Taking into account the linear coupling between guided and localized states and applying the time-dependent version of a Green’s function formalism, we study and characterize analytically the scattering of continuous waves. The resonant reflectivity, which is tunable via the nonlinearity, takes the form of a nonlinear Fano resonance because the output field is composed of a linearly transmitted wave and a resonantly reflected contribution from the localized cavity. By studying the stability of the nonlinear Fano resonance, we reveal that the continuous-wave scattering may exhibit modulational instability near the resonance when the light intensity in the cavity starts growing in time. However, we demonstrate that this instability may be suppressed for Gaussian pulses, such that the bistable transmission curve can still be recovered in accordance with the analysis of the steady-state transmission. We demonstrate that our analytical results based on an effective discrete model are in excellent agreement with numerical results obtained by direct finite-difference time-domain simulations.

The evolution of the number of vortices in an artificial vortex field is investigated using numerical simulations. These vortex fields are artificial in the sense that vortices with the same topological charge are separated from vortices with the opposite topological charge and placed in the same region or cell. It is found that the number of vortices in such a field increases during propagation until equilibrium is reached where the vortex field behaves like a random wave field: the topological charges of neighboring vortices are anticorrelated and the vortex density is determined by the coherence area. The restoration scale—i.e., the propagation distance necessary to achieve equilibrium—is found to be inversely proportional to both the wavelength and initial vortex density and a logarithmic function of the topological charge of a cell.

Recently, we proposed a multiphase-matching method for the Grover algorithm with a matching rule for multiple phases ${\alpha }_j$ and ${\beta }_j$, $j=1,\dots ,k$, where $k$ is the number of Grover operations. The phases are matched such that ${\alpha }_j=-{\beta }_{k-j+1}$ globally over a sequence of $k$ Grover operations. The success probability $P_6\left(\lambda \right)$ for $k=6$ was found to be almost constant and unity over a wide range of $\lambda$, i.e., $0.10777⩽\lambda ⩽1$, where $\lambda$ is the fraction of marked items in a database state. For $\lambda <0.10777$, however, $P_6\left(\lambda \right)$ decreases rapidly to zero as $\lambda$ decreases and the efficiency of the method deteriorates. In this Brief Report we show that the difficulty with small values of $\lambda$ mentioned above can be alleviated by increasing the number of operations $k$. With $k=20$, for example, we find a value of $P_{20}\left(\lambda \right)$ that is almost constant and unity in the region of small $\lambda$ where the matching with six Grover operations is not effective. The matching with $k=6$ and the one with $k=20$ complement each other so that the entire range of $0\lesssim \lambda <1$ can be well covered.

Some results from a recent work of Reiss [Phys. Rev. Lett. 101, 043002 (2008); 101, 159901(E) (2008)] are generalized for the case of elliptical polarization of a laser field. We also discuss the tunneling limit of strong-field photoionization.

A method to create paired-atom laser beams from a metastable helium atom laser via four-wave mixing is demonstrated. Radio-frequency outcoupling is used to extract atoms from a Bose-Einstein condensate near the center of the condensate and initiate scattering between trapped and untrapped atoms. The unequal strengths of the interactions for different internal states allows an energy-momentum resonance which leads to the creation of pairs of atoms scattered from the zero-velocity condensate. The resulting scattered beams are well separated from the main atom laser in the two-dimensional transverse atom laser profile. Numerical simulations of the system are in good agreement with the observed atom laser spatial profiles and indicate that the scattered beams are generated by a four-wave mixing process, suggesting that the beams are correlated.

We introduce an operator fidelity and propose to use its susceptibility for characterizing the sensitivity of quantum systems to perturbations. Two typical models are addressed: One is the transverse Ising model exhibiting a quantum phase transition, and the other is the one-dimensional Heisenberg spin chain with next-nearest-neighbor interactions, which has the degeneracy. It is revealed that the operator fidelity susceptibility is a good indicator of quantum criticality regardless of the system degeneracy.

We estimate a lower bound to the secret key rate of a binary modulated continuous variable quantum key distribution scheme. We consider the collective attack scenario with quantum channels that impose arbitrary noise on the exchanged signals. The analysis is done in the infinite key limit. Bob performs ideal homodyne measurements on the received states and the two honest parties employ a reverse reconciliation procedure in the classical postprocessing step of the protocol.

The dynamical aspects of quantum Brownian motion at low temperatures are investigated. Exact calculations of quantum entanglement among two Brownian oscillators are given without invoking the Born-Markov or Born approximation widely used for the study of open systems. Nonlocality due to finite oscillator separation is studied. Our approach is applicable for arbitrary time scale, in particular suitable to probe the short-time regime at cold temperatures where many experiments on quantum information processing are performed. We study separability criteria based on the uncertainty relation, negativity, and entanglement of formation. We found a crossover behavior in a disentanglement process between quantum- and thermal-fluctuation-dominated regimes. The fluctuation-dissipation theorem is violated for two Brownian particles interacting with a common environment. The deviation from the relation originates in the interaction mediated by the environment, which drives two particles into a steady oscillatory state, preventing the system from thermalizing. Consequently there is a residual entanglement at low temperatures for arbitrary coupling strength. This entanglement is generated from the environment nonperturbatively even when two modes are not entangled initially. When the distance between two oscillators is varied, competition between entanglement induced from the environment and modified decoherence due to finite separation leads to the characteristic distance where entanglement is minimized.

We report on multiconfiguration Breit-Pauli calculations for the photorecombination of ${\mathrm{Ti}}^{4+}$ ions. Through a detailed comparison with the Test Storage Ring measurements of Schippers et al., we quantify the interference effects for the broad, asymmetric, near-threshold, highly correlated $3p^{5}3d^2\left({}^{2}F\right)$ resonances. We also discuss the enhanced field ionization effects on recombined $3p^{6}n\mathcal{l}$ Rydberg states. This is due to the perturbation of the below-threshold tails of two of the broad $3p^{5}3d^2\left({}^{2}F\right)$ resonances, giving rise to a “forced autoionization” phenomenon, increasing the field ionization effects. By accounting for interference for the lowest $n=3,4$ resonances, and field effects as $n\to \infty$, excellent agreement between our computed results and the experimental photorecombination spectrum is obtained.

We report the realization of a quantum device for force sensing at the micrometric scale. We trap an ultracold ${}^{88}\mathrm{Sr}$ atomic cloud with a one-dimensional (1D) optical lattice; then we place the atomic sample close to a test surface using the same optical lattice as an elevator. We demonstrate precise positioning of the sample at the micrometer scale. By observing the Bloch oscillations of atoms into the 1D optical standing wave, we are able to measure the total force on the atoms along the lattice axis, with a spatial resolution of few micrometers. We also demonstrate a technique for transverse displacement of the atoms, allowing us to perform measurements near either transparent or reflective test surfaces. In order to reduce the minimum distance from the surface, we compress the longitudinal size of the atomic sample by means of an optical tweezer. This system is suited for studies of atom-surface interaction at short distance, such as measurement of the Casimir force and the search for possible non-Newtonian gravity effects.

The elastic scattering amplitudes of indistinguishable, bosonic, strongly polar molecules possess universal properties at the coldest temperatures due to wave propagation in the long-range dipole-dipole field. Universal scattering cross sections and anisotropic threshold angular distributions, independent of molecular species, result from careful tuning of the dipole moment with an applied electric field. Three distinct families of threshold resonances also occur for specific field strengths, and can be both qualitatively and quantitatively predicted using elementary adiabatic and semiclassical techniques. The temperatures and densities of heteronuclear molecular gases required to observe these universal characteristics are predicted.

We theoretically study classical step-modulated pulse propagation in an optically thick medium with electromagnetically induced transparency (EIT) using a fast Fourier transform and a proposed hybrid-asymptotic analysis. The 100% transmitted leading edge traveling at $c$ consists of Sommerfeld-Brillouin precursors. The main signal, falling inside of the narrow EIT window, experiences a steep dispersion and slow group velocity. We suggest active control of the precursors and main signal separately by adjusting the parameters of the medium and incident pulse.

We present an analytic perturbation theory which extends the paraxial approximation for a common cylindrically symmetric stable optical resonator and incorporates the differential, polarization-dependent reflectivity of a Bragg mirror. The degeneracy of Laguerre-Gauss modes with distinct orbital angular momentum (OAM) and polarization, but identical transverse order $N$, will become observably lifted at sufficiently small size and high finesse. The resulting paraxial eigenmodes possess two distinct OAM components, the fractional composition subtly depending on mirror structure.

We present a strategy to control the evolution of a quantum system using a single auxiliary subsystem. Two applications are given, one which allows for state preservation and another which controls the degree of entanglement of a given initial state.

We study the properties of the fidelity of one-qubit operations in a noisy channel and reveal its properties in dependence on coupling to the outer environment. We show that for an asymmetric qubit-environment coupling, the fidelity can be improved by a tuning of the external parameters acting on the qubit energy splitting. In particular, for the case of a spin qubit, the fidelity can be improved by an appropriate tuning of the external magnetic field. We observe that within tailored parameter regimes, the fidelity (typically being oscillatory) evolves monotonically and remains significant in the long-time regime, for both an environment prepared in vacuum and coherent states. This result holds true also for the entanglement fidelity of the two-qubit system.

We analyze whether a pair of neutral two level atoms can become entangled in a finite time while they remain causally disconnected. The interaction with the electromagnetic field is treated perturbatively in the electric dipole approximation. We start from an initial vacuum state and obtain the final atomic correlations for the cases where $n=0$, 1, or 2 photons are produced in a time $t$, and also when the final field state is unknown. Our results show that correlations are sizable inside and outside the mutual light cone for $n=1$ and 2, and also that quantum correlations become classical by tracing over the field state. For $n=0$ we obtain entanglement generation by photon propagation between the atoms, the correlations come from the indistinguishability of the source for $n=1$, and may give rise to entanglement swapping for $n=2$.

We use NMR quantum simulators to study antiferromagnetic Ising spin chains undergoing quantum phase transitions. Taking advantage of the sensitivity of the systems near criticality, we detect the critical points of the transitions using a direct measurement of the Loschmidt echo. We test our simulators for spin chains of even and odd numbers of spins, and compare the experimental results to theoretical predictions.