The effect of collective phonon excitations on the properties of cold atoms in optical lattices is investigated. These phonon excitations are collective excitations, whose appearance is caused by intersite atomic interactions correlating the atoms, and they do not arise without such interactions. These collective excitations should not be confused with lattice vibrations produced by an external force. No such force is assumed. But the considered phonons are purely self-organized collective excitations, characterizing atomic oscillations around lattice sites, due to intersite atomic interactions. It is shown that these excitations can essentially influence the possibility of atoms' being localized. The states that would be insulating in the absence of phonon excitations can become delocalized when these excitations are taken into account. This concerns long-range as well as local atomic interactions. To characterize the region of stability, the Lindemann criterion is used.

We demonstrate on-chip pulse compression based on the dynamic tuning in a copropagating slow-light system. Low-dispersion slow-light control pulse in lattice-shifted photonic-crystal waveguides enhances two-photon absorption. Generating free carriers dynamically through the two-photon absorption so as to sweep a signal slow-light pulse, the spectral broadening occurs with a monotonic chirp. The signal pulse is then compressed through the compensation of the chirp by means of integrated heaters. An input pulse of 13.9 ps length was compressed to 1.4 ps, corresponding to a compression factor of 9.9.

We employ the quench action method for a recently considered geometrical quantum quench: two free fermionic chains initially at different temperatures are joined together in the middle and let evolve unitarily with a translation invariant Hamiltonian. We show that two different stationary regimes are reached at long times, depending on the interplay between the observation time scale $T$ and the total length $L$ of the system. We show the emergence of a nonequilibrium steady state supporting an energy current for observation time $T$ much smaller than the system size $L$. We then identify a longer time scale for which thermalization occurs in a generalized Gibbs ensemble.

The dynamics of the quantum Fisher information of the parameters of the initial atomic state and atomic transition frequency is studied, in the framework of open quantum systems, for a static polarizable two-level atom coupled in the multipolar scheme to a bath of fluctuating vacuum electromagnetic fields without and with the presence of a reflecting boundary. Our results show that in the case without a boundary, the electromagnetic vacuum fluctuations always cause the quantum Fisher information of the initial parameters and thus the precision limit of parameter estimation to decrease. Remarkably, however, with the presence of a boundary, the quantum Fisher information becomes position and atomic polarization dependent and, as a result, it may be enhanced as compared to that in the case without a boundary and may even be shielded from the influence of the vacuum fluctuations in certain circumstances as if it were a closed system.

We propose an approach for finding an optimal measurement for quantum state discrimination that maximizes the probability of correct detection with a fixed rate of inconclusive results. In our approach, we obtain the optimal measurement by solving the problem of finding a measurement that maximizes the weighted sum of the probability of correct detection and that of inconclusive results. We show that this problem can be reduced to the widely studied problem of finding a minimum error measurement for a certain state set, which maximizes the probability of correct detection without inconclusive results. As an application of our approach, we show how to solve the problem of finding an optimal measurement for qubit states with a fixed rate of inconclusive results.

We study the interaction energy between two surfaces, one of them flat, the other describable as the composition of a small-amplitude roughness and a slightly curved, smooth surface. The roughness, represented by a spatially random variable, involves Fourier wavelengths shorter than the (local) curvature radii of the smooth component of the surface. After averaging the interaction energy over the roughness distribution, we obtain an expression which only depends on the smooth component. We then approximate that functional by means of a derivative expansion, calculating explicitly the leading- and next-to-leading-order terms in that approximation scheme. We analyze the resulting interplay between shape and roughness corrections for some specific roughness models in the cases of electrostatic and Casimir interactions.

We present an extension of the classical overbarrier model [F. Sattin, Phys. Rev. A 62, 042711 (2000)] to include the effect of electron backcapture. Backcapture is the process by which an electron that has already been captured by the projectile ion is recaptured by the target atom. Backcapture reduces the electron capture cross section at low impact velocities when the projectile ionization energy is less than that of the target. This creates a cross-section peak. We alter the location of this peak to correspond to that predicted by an adiabatic criterion by using a free parameter of the model. These extensions bring the overbarrier model more in line with experimental data, especially at low impact velocity.

In this paper, we report a version of the coupled-coherent-states method which is able to accurately compute the high-order harmonic generation (HHG) spectrum of an electron in a laser field in one dimension by the use of trajectory-guided grids of Gaussian wave packets. It is shown that by periodic reprojection of the wave function and dynamically altering the basis set size, the method can account for a wave function which spreads out to cover a large area in phase space while still keeping computational expense low and ensuring the preservation of coherence of the wave function. The HHG spectra obtained show good agreement with those from a time-dependent Schrödinger equation solver. We show also that the part of the wave function which is responsible for HHG moves along a periodic orbit which is far from that of classical motion. Although this paper is a proof of principle and therefore focused on a simple one-dimensional system, future generalizations for the multielectron case are discussed.

We have theoretically investigated the wavelength-dependent below-threshold harmonic (BTH) generation in different laser intensity regimes. Based on the simulations of the time-dependent Schrödinger equation, we find a periodic fluctuation in the wavelength-dependent BTH yields with laser intensities in the magnitude of ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. This result shows significant differences from the resonance peak structures in the recent study with intensities about ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ [Xiong et al., Phys. Rev. Lett. 112, 233001 (2014)]. By performing both the quantum path analysis and time-frequency transform of the harmonic spectra, we demonstrate that the difference in the wavelength-dependent BTH yields is due to the complicated quantum path distributions of BTHs with the intensities in the magnitude of ${10}^{13}\mathrm{W}/{\mathrm{cm}}^2$. And the periodic fluctuation is further demonstrated to originate from the interference between the two quantum paths that dominate the harmonic generation.

Motivated by recent experiments, we analyze the stability of a three-dimensional Bose-Einstein condensate loaded in a periodically driven one-dimensional optical lattice. Such periodically driven systems do not have a thermodynamic ground state but may have a long-lived steady state which is an eigenstate of a “Floquet Hamiltonian.” We explore collisional instabilities of the Floquet ground state which transfer energy into the transverse modes. We calculate decay rates, finding that the lifetime scales as the inverse square of the scattering length and inverse of the peak three-dimensional density. These rates can be controlled by adding additional transverse potentials.

We describe Doppler spectroscopy of Bose-Einstein condensates of ytterbium atoms using a narrow optical transition. We address the optical clock transition around 578 nm between the ${}^1{S}_0$ and the ${}^3{P}_0$ states with a laser system locked on a high-finesse cavity. We show how the absolute frequency of the cavity modes can be determined within a few tens of kilohertz using high-resolution spectroscopy on molecular iodine. We show that optical spectra reflect the velocity distribution of expanding condensates in free fall or after release inside an optical waveguide. We demonstrate subkilohertz spectral linewidths, with long-term drifts of the resonance frequency well below 1 kHz/h. These results open the way to high-resolution spectroscopy of many-body systems.

We investigate a two-way cascaded cavity QED system consisting of microtoroidal resonators coupled through an optical fiber. Each microtoroidal cavity supports two counterpropagating whispering-gallery modes coupled to single atoms through their evanescent fields. We focus on a pair of atom-microtoroid systems and compute the spectrum of spontaneous emission into the fiber with one atom initially excited. Explicit results are presented for strong-coupling and bad-cavity regimes, where the latter allows the effective atom-atom interaction to be controlled through the atom-cavity coupling and detuning: the atoms exhibit either collective spontaneous emission with no dipole-dipole interaction or a (coherent) dipole-dipole interaction and independent (single-atom) emission. This capacity for switching the character of the interaction is a feature of bidirectional coupling and connects our two-way cascaded system to work on one-dimensional waveguides. Building upon our bad-cavity results, we generalize to many atom-microtoroid systems coupled through an optical fiber.

We derive an analytical solution uncovering the origin of few-cycle attosecond pulse formation from vacuum-ultraviolet (VUV) radiation in an atomic gas simultaneously irradiated by a moderately strong infrared (IR) laser field, which does not perturb atoms in the ground state, but induces rapid quasistatic ionization from the excited states [Polovinkin et al., Opt. Lett. 36, 2296 (2011)]. The derived solution shows that the pulses are produced due to periodic switching of the resonant interaction between the incident VUV radiation and the atoms: turning it off near the crests of the IR-field strength and switching it back on near the IR-field zero crossings. We extend the method originally proposed by Polovinkin  et al. [Opt. Lett. 36, 2296 (2011)] to non-hydrogen-like media and show that the pulses can be produced from resonant VUV radiation in a variety of atomic gases. The pulses are nearly bandwidth limited without external adjustment of phases of the generated sidebands. Proximity of the carrier frequency of the produced pulses to intra-atomic resonances may allow their efficient utilization for nondestructive steering of ultrafast dynamics of the bound electrons. The experimental possibilities for attosecond pulse formation from 58.4 nm VUV radiation in helium and from 73.6 nm VUV radiation in neon dressed by the 3.9 μm laser field, as well as from 122 nm VUV radiation in atomic hydrogen dressed by $\mathrm{C}{\mathrm{O}}_2$-laser field are discussed.

We study the ${\mathrm{Ho}}^{14+}$ ion as a candidate for an extremely accurate and stable optical atomic clock which is sensitive to the time variation of the fine-structure constant. We demonstrate that the proposed system has all the desired features including relatively strong optical electric-dipole and magnetic-dipole transitions which can be used for cooling and detection. Zero quadrupole moments for the relevant states in the clock transition allow interrogation of multiple ions to improve the clock stability.

The article deals with the general implementation of the four-component spinor relativistic extended-coupled-cluster (ECC) method to calculate first-order property of atoms and molecules in their open-shell ground-state configuration. The implemented relativistic ECC is employed to calculate hyperfine structure constants of alkali metals (Li, Na, K, Rb, and Cs), singly charged alkaline-earth-metal atoms $({\mathrm{Be}}^{+},{\mathrm{Mg}}^{+},{\mathrm{Ca}}^{+}$, and ${\mathrm{Sr}}^{+})$, and molecules (BeH, MgF, and CaH). We have compared our ECC results with the calculations based on the restricted active space configuration interaction (RAS-CI) method. Our results are in better agreement with the available experimental values than those of the RAS-CI values.

We report on simultaneous sub-Doppler laser cooling of fermionic ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$ using the $D_1$ optical transitions. We compare experimental results to a numerical simulation of the cooling process applying a semiclassical Monte Carlo wave-function method. The simulation takes into account the three-dimensional optical molasses setup and the dipole interaction between atoms and the bichromatic light field driving the $D_1$ transitions. We discuss the physical mechanisms at play, identify the important role of coherences between the ground-state hyperfine levels, and compare $D_1$ and $D_2$ sub-Doppler cooling. In 5 ms, the $D_1$ molasses phase greatly reduces the temperature for both ${}^6\mathrm{Li}$ and ${}^{40}\mathrm{K}$ at the same time, with final temperatures of 44 and $11\mu \mathrm{K}$, respectively. For both species this leads to a phase-space density close to ${10}^{-4}$. These conditions are well suited to direct loading of an optical or magnetic trap for efficient evaporative cooling to quantum degeneracy.

We present an application of dark-field imaging that enables in situ detection of two-dimensional vortex distributions in single-component Bose-Einstein condensates (BECs). By rotating a ${}^{87}\mathrm{Rb}$ BEC in a magnetic trap, we generate a triangular lattice of vortex cores in the BEC, with core diameters on the order of 400 nm and cores separated by approximately $9\mu \mathrm{m}$. We have experimentally confirmed that the positions of the vortex cores near the BEC center can be determined without the need for ballistic expansion of the BEC. Our imaging method should allow for the determination of arbitrary distributions of vortices and other superfluid density defects in cases where expansion of the BEC is either impractical or would significantly alter the physical characteristics and appearance of vortices or defects. Our method is also a step toward real-time measurements of complex two-dimensional vortex dynamics within a single BEC.

Fermionic superfluidity in atomic Fermi gases across a Feshbach resonance is normally described by the atom-molecule theory, which treats the closed channel as a noninteracting point boson. In this work we present a theoretical description of the resonant superfluidity in analogy to the two-band superconductors. We employ the underlying two-channel scattering model of Feshbach resonance where the closed channel is treated as a composite boson with binding energy ${\varepsilon }_0$ and the resonance is triggered by the microscopic interchannel coupling $U_{12}$. The binding energy ${\varepsilon }_0$ naturally serves as an energy scale of the system, which has been sent to infinity in the atom-molecule theory. We show that the atom-molecule theory can be viewed as a leading-order low-energy effective theory of the underlying fermionic theory in the limit ${\varepsilon }_0\to \infty$ and $U_{12}\to 0$, while keeping the phenomenological atom-molecule coupling finite. The resulting two-band description of the superfluid state is in analogy to the BCS theory of two-band superconductors. In the dilute limit ${\varepsilon }_0\to \infty$, the two-band description recovers precisely the atom-molecule theory. The two-band theory provides a natural approach to study the corrections because of a finite binding energy ${\varepsilon }_0$ in realistic experimental systems. For broad and moderate resonances, the correction is not important for current experimental densities. However, for extremely narrow resonance, we find that the correction becomes significant. The finite binding energy correction could be important for the stability of homogeneous polarized superfluid against phase separation in imbalanced Fermi gases across a narrow Feshbach resonance.