In a recent paper [L. Feng et al., Phys. Rev. Lett. 109, 233006 (2012)], we demonstrated a sub-transit-linewidth resonance based on cross correlation between optical fields in an electromagnetically induced transparency (EIT) system. Here, we investigate the ability of such resonance to resolve multiple resonance peaks. We first report the implementation of the cross-correlation resonance in a hyperfine EIT system, with a linewidth of about $1/16$ of the transit EIT width. Then, a magnetic field is applied to create multipeak EIT resonance with tunable spacing. Cross-correlation resonance with multipeaks is observed and the line shape agrees qualitatively with our numerical simulations. We find that the multipeak correlation resonance has better contrast than the corresponding EIT resonance, but with a lower signal to noise. Furthermore, this correlation resonance cannot resolve peaks with spacing close to or less than the intrinsic width, like any other resonance techniques. We analyze the origin of this limitation and make connections to the Ramsey-type subnatural spectroscopy technique. This work shows potential applications of the correlation resonance in atomic frequency standard and laser spectroscopy.

We present a rigorous reformulation of the quantum mechanical equations of motion for the coupled system of electrons and nuclei that focuses on the dynamics of the electronic subsystem. Usually the description of electron dynamics involves an electronic Schrödinger equation where the nuclear degrees of freedom appear as parameters or as classical trajectories. Here we derive the exact Schrödinger equation for the subsystem of electrons, staying within a full quantum treatment of the nuclei. This exact Schrödinger equation features a time-dependent potential energy surface for electrons (e-TDPES). We demonstrate that this exact e-TDPES differs significantly from the electrostatic potential produced by classical or quantum nuclei.

Localization of relativistic particles and their position-momentum uncertainty relations are not yet fully understood. We discuss two schemes of photon localization that are based on the energy density. One scheme produces a positive operator-valued measure for localization. It coincides with the number density operator and reproduces an effective $3\times 3$ polarization density matrix. Another scheme results in a probability distribution that is conditioned on the detection. In both schemes the uncertainty relations for transversal position and momentum approach the Heisenberg bound $\Delta p\Delta z=\frac{1}{2}\hslash$.

We observe the localization effect of one-dimensional quantum walks with single-point phase defects. The walker's spread velocity is dramatically suppressed by interference effects due to the phase defect. We show that the localization effect depends on four factors: the value and the position of the phase defect, the parameter of coin flipping, and the initial state of the walker + coin system.

Molecular rotational spectroscopy based on a strong-field-ionization-induced nitrogen laser is employed to investigate the time evolution of the rotational wave packet composed by a coherent superposition of quantum rotational states created in a field-free molecular alignment. We show that this technique uniquely allows real-time observation of the ultrafast dynamics of the molecular rotational wave packet. Our analysis also shows that there exist two channels of generation of the nitrogen laser, shedding light on the population inversion mechanism behind the air laser generated by intense femtosecond laser pulses.

We present calculations of nuclear-spin-dependent and nuclear-spin-independent parity-violating amplitudes in Ba, Ra, ${\mathrm{Ac}}^{+}$, Th, and Pa. Parity nonconservation in these systems is greatly enhanced due to the presence of very close electronic energy levels of opposite parity, large nuclear charge, and strong nuclear enhancement of parity-violating effects. The presented amplitudes constitute several of the largest atomic parity-violating signals predicted so far. Experiments using these systems may be performed to determine values for the nuclear anapole moment, a $P$-odd $T$-even nuclear moment given rise to by parity-violating nuclear forces. Such measurements may prove to be valuable tools in the study of parity violation in the hadron sector. The considered spin-independent transitions could also be used to measure the ratio of weak charges for different isotopes of the same atom, the results of which would serve as a test of the standard model and also of neutron distributions. Barium, with seven stable isotopes, is particularly promising in this regard.

We report on the use of multistage Zeeman deceleration to generate beams of ${\mathrm{He}}_2$ molecules in the metastable $a{}^3{\Sigma }_u^{+}$ state with velocities tunable down to 100 m/s. The metastable molecules are generated by striking a discharge in a supersonic expansion of pure helium gas from a pulsed valve held at cryogenic temperature. The velocity and internal-state distributions of the metastable ${\mathrm{He}}_2$ molecules are measured for nozzle temperatures of 300, 77, and 10 K by high-resolution photoelectron and photoionization spectroscopy. The deceleration process does not exhibit any rotational state selectivity with rotational levels up to $N^{\prime \prime }=21$ being populated, but eliminates molecules in spin-rotational sublevels with $J^{\prime \prime }=N^{\prime \prime }$ from the beam, where $J^{\prime \prime }$ and $N^{\prime \prime }$ are the total and the rotational angular momentum quantum number, respectively. The lack of rotational state selectivity is attributed to the fact that the Paschen-Back regime of the Zeeman effect in the rotational levels of ${\mathrm{He}}_2$ is already reached at fields of only 0.1 T.

We consider an ensemble of closed finite-level systems described by a density matrix where the goal is to find an optimal control field to maximize the expectation value of an observable. The eigenvalues of the initial density matrix are assumed to depend on an experimental parameter (e.g., the temperature), whereas the eigenvalues of the observable may depend on an additional application-specific experimental parameter. We show that an optimal control will remain optimal for all such experimental parameters, if the relative ordering, by magnitude, of the eigenvalues of the initial density matrix as well as of the observable is unaltered regardless of the parameter values. More generally, we show like invariance of a control associated with any particular critical point on the corresponding control landscape. The invariance of control laser fields with respect to temperature is illustrated for vibrational excitation of diatomic molecules and photoassociation of atoms.

We provide an efficient and accurate numerical method to deduce the recoil-ion-momentum spectrum of He from the two-electron momentum distribution, which is obtained by solving the full-dimensional time-dependent Schrödinger equation. We apply this method to study the ion spectra of one-photon double ionization and two-photon sequential and nonsequential double ionization of He. The present calculations agree rather well with the absolute magnitude of the recoil-ion triply differential cross sections published recently [S. A. Abdel-Naby, M. S. Pindzola, and J. Colgan, Phys. Rev. A 86, 013424 (2012); S. A. Abdel-Naby et al., Phys. Rev. A 87, 063425 (2013)]. Nevertheless, significant differences are also found in several detailed features of the spectra and straightforward physical analysis indicates that the present results appear more reasonable, which should be confirmed by future experiments or additional independent calculations.

We have studied the nondissociative and dissociative photo-double-ionization of 1,1-difluoroethylene using single photons of energies ranging from 40 to 70 eV. Applying a coincident electron-ion three-dimensional momentum imaging technique, kinematically complete measurements have been achieved. We present the branching ratios of the six reaction channels identified in the experiment. Electron-ion energy maps and relative electron emission angles are used to distinguish between direct and indirect photo-double-ionization mechanisms at a few different photon energies. The influence of selection and propensity rules is discussed. Threshold energies of double ionization are extracted from the sum of the kinetic energies of the electrons, which hint to the involvement of different manifolds of states. The dissociative ionization channels with two ionic fragments are explored in detail by measuring the kinetic energy release of the fragment ions, sum of the kinetic energies, as well as the energy sharing of the two emitted electrons. We investigate the migration of hydrogen and fluorine atoms and compare the experimental results to the photo-double-ionization of centrosymmetric linear and planar hydrocarbons (${\mathrm{C}}_2$${\mathrm{H}}_2$ and ${\mathrm{C}}_2$${\mathrm{H}}_4$) whenever possible.

We investigate the fidelity susceptibility (FS) of a two-dimensional spin-orbit-coupled (SOC) Fermi superfluid and the topological phase transition driven by a Zeeman field in the perspective of its ground state wave function. Without Zeeman coupling, FS shows additional features characterizing the BCS-BEC crossover induced by SOC. In the presence of a Zeeman field, the topological phase transition is explored using both FS and the topological invariant. In particular, we obtain the analytical result of the topological invariant which explicitly demonstrates that the topological phase transition corresponds to a sudden change of the ground state wave function. Consequently, FS diverges at the phase transition point with its critical behavior being $\chi \propto \text{ln}|h-h_c|$ . Based on this observation, we conclude that the topological phase transition can be detected by measuring the momentum distribution in cold atom experiments.

In this paper, we examine the dynamical properties of vortices in atomic Bose-Einstein condensates in the presence of phenomenological dissipation, used as a basic model for the effect of finite temperatures. In the context of this so-called dissipative Gross-Pitaevskii model, we derive analytical results for the motion of single vortices and, importantly, for vortex dipoles, which have become very relevant experimentally. Our analytical results are shown to compare favorably to the full numerical solution of the dissipative Gross-Pitaevskii equation where appropriate. We also present results on the stability of vortices and vortex dipoles, revealing good agreement between numerical and analytical results for the internal excitation eigenfrequencies, which extends even beyond the regime of validity of this equation for cold atoms.

We show that by switching on a spin-orbit interaction in a cold-atom system, experiencing a Zeeman-like coupling to an external field, e.g., in a Bose-Einstein condensate, one can simulate a quantum measurement on a precessing spin. Depending on the realization, the measurement can access both the ergodic and the Zeno regimes, while the time dependence of the spin's decoherence may vary from a Gaussian to an inverse fractional power law. Back action of the measurement forms time- and coordinate-dependent profiles of the atoms' density, resulting in its translation, spin-dependent fragmentation, and appearance of interference patterns.

We theoretically investigate the photon statistics in a cavity quantum electrodynamics system of a single quantum dot coupled to a bimodal nanocavity. It is shown in a recent work [A. Majumdar, M. Bajcsy, A. Rundquist, and J. Vučković, Phys. Rev. Lett. 108, 183601 (2012)] that the system can generate strongly sub-Poissonian light when one of the cavity modes is driven coherently and resonantly. We study the two-mode coherent driving regime of the coupled system. The effect of additional cavity mode driving on statistical characteristics of photon emission is presented by evaluating the zero-delay second-order correlation function $g^2$(0). The antibunching character can be optimized by regulating the ratio between driving strengths of two cavity modes to achieve optimal combination of superbunched and coherent light. As a result $g^2$(0) can be reduced up to several orders of magnitude [$g^2$(0)$\sim$10${}^{-7}$] with proper system parameters, compared with the one-mode driving system [$g^2$(0)$\sim$0.1].

We compare two different models of transport of light in a disordered system with a spherical Gaussian distribution of scatterers. A coupled dipole model, taking into account all interference effects, is compared to an incoherent model, using a random walk of particles. Besides the well-known coherent backscattering effect and a well pronounced forward lobe, the incoherent model reproduces extremely well all scattering features. In an experiment with cold atoms, we use the momentum recoil imparted on the center of mass of the sample as a partial probe of the light-scattering properties. We find that the force acting on the center of mass of the atoms is not well suited to exhibit the coherence effects in light propagation under multiple-scattering conditions.

We report efficient generation of a broadband supercontinuum (SC) in water upon irradiation by intense, near-infrared, femtosecond laser pulses. The SC spectra generated when the pump wavelength lies in the anomalous chromatic dispersion region are characterized by largely blueshifted peaks in the visible range. These SC spectral features are rationalized within the framework of a phenomenological self-phase-modulation model, taking into account higher-order group velocity dispersion. Our model provides an intuitive explanation for the relation between the observed pump-wavelength dependence of the blueshifted SC peaks and the anomalous chromatic dispersion.

Blind quantum computation allows a client who does not have enough quantum resources or technologies to achieve quantum computation on a remote quantum server such that the client's input, output, and algorithm remain unknown to the server. Up to now, single- and double-server blind quantum computation have been considered. In this work, we propose a triple-server blind computation protocol where the client can delegate quantum computation to three quantum servers by the use of entanglement swapping. Furthermore, the three quantum servers can communicate with each other and the client is almost classical since one does not require any quantum computational power, quantum memory, and the ability to prepare any quantum states and only needs to be capable of getting access to quantum channels.

We highlight the existence of a joint probability distribution as the common underpinning assumption behind Bell-type, contextuality, and Leggett-Garg-type tests. We then present a procedure to translate contextual scenarios into temporal Leggett-Garg-type and spatial Bell-type ones. To demonstrate the generality of this approach we construct a family of spatial Bell-type inequalities. We show that in the Leggett-Garg scenario a necessary condition for contextuality in time is given by a violation of consistency conditions in the consistent histories approach to quantum mechanics.

We revisit the issue of extracting information from parallel and antiparallel spins, as initiated by Gisin and Popescu [Phys. Rev. Lett. 83, 432 (1999)], from the viewpoint of parameter estimation. By comparing two fundamental figures of merit based on fidelity and quantum Fisher information for assessing information quality, we demonstrate that these criteria yield different strategies for optimally extracting spin information. A surprising observation is that while for a single spin, as well as for parallel spins, quantum Fisher information cannot be fully extracted by any uniform measurement, this is not the case for antiparallel spins. A simple uniform measurement fully extracting quantum Fisher information in antiparallel spins is identified. This reveals a significant feature of antiparallel spins from the perspective of Fisher information and provides an alternative illustration of the idea that antiparallel spins carry more information than parallel spins.