We study the capacitance-coupled Josephson-junction array beyond the small-coupling limit. We find that, when the scale of the system is large, its Hamiltonian can be obtained without the small-coupling approximation and the system can be used to simulate strongly frustrated one-dimensional Ising spin problems. To engineer the system Hamiltonian for an ideal theoretical model, we apply a dynamical-decoupling technique to eliminate undesirable couplings in the system. Using a six-site junction array as an example, we numerically evaluate the system to show that it exhibits important characteristics of the frustrated spin model.

In the present work we investigate the sequential radiative recombination (RR) of initially bare ions colliding with two spatially separated electron targets. It is shown that magnetic sublevel population of the hydrogenlike ions, produced by the electron capture from the first target, depends on the emission direction of the (first) RR photon. This population, which can be expressed in terms of the polarization parameters, affects then the angular and polarization properties of the radiation emitted in the collision with the second target. The coincidence measurements of two subsequent RR photons may allow one to understand, therefore, the production and diagnostics of the ion spin polarization. In order to describe this polarization production and diagnostic scheme we derive the general expression for the $\gamma -\gamma$ correlation function. Detailed calculations for the dependence of this function on the geometry of photon emission and collision energy are performed for the radiative recombination of bare uranium ions.

Differential elastic scattering cross sections of tetrahydrofuran for electrons were measured absolutely in the energy range from 20 eV to 1 keV at scattering angles between 5° and 135°. The measurements were carried out using a crossed-beam arrangement without the application of the widely used relative flow technique. The experimental differential scattering cross sections could be put on an absolute scale by means of the total electron scattering cross sections of tetrahydrofuran and of the current loss of the primary electron beam in the forward direction arising due to the scattering by the molecular beam. The total scattering cross sections were determined for electron energies between 6 eV and 1 keV using a separate linear transmission experiment. The differential cross sections of tetrahydrofuran for the elastic scattering of electrons were also calculated in the energy range between 60 eV and 1 keV by applying the modified independent-atom model. A comparison with the experimental results showed a satisfactory agreement, indicating that the selected theoretical model is adequate for these calculations.

X-ray lasers offer new possibilities for creating and probing extreme states of matter. We used intense and short x-ray pulses from the FLASH soft x-ray laser to trigger the explosions of CH${}_4$ and CD${}_4$ molecules and their clusters. The results show that the explosion dynamics depends on cluster size and indicate a transition from Coulomb explosion to hydrodynamic expansion in larger clusters. The explosion of CH${}_4$ and CD${}_4$ clusters shows a strong isotope effect: The heavier deuterons acquire higher kinetic energies than the lighter protons. This may be due to an extended inertial confinement of deuterons vs. protons near a rapidly charging cluster core during exposure.

Charge production, charge redistribution, and ion fragmentation are explored in the decay of a Xe $K$-shell vacancy in XeF${}_2$. Coincidence measurements of all ionic fragments in XeF${}_2$ provide evidence that an interatomic-Coulombic-decay-like (ICD-like) process plays a role in the cascade decay. The signature of the ICD-like process is an enhancement of the total number of electrons ejected as compared to the case of atomic Xe. The results indicate that the F atoms participate in the decay cascade within the first few femtoseconds after core-hole formation and that fragmentation begins during the decay process.

The spectrum of resonance fluorescence is calculated for a two-level system excited by an intense, ultrashort x-ray pulse made available, for instance, by free-electron lasers such as the Linac Coherent Light Source. We allow for inner-shell hole decay widths and destruction of the system by further photoionization. This two-level description is employed to model neon cations strongly driven by x rays tuned to the $1s2p^{-1}\to 1s^{-1}2p$ transition at $848\mathrm{eV}$; the x rays induce Rabi oscillations which are so fast that they compete with Ne $1s$-hole decay. We predict resonance fluorescence spectra for two different scenarios: first, chaotic pulses based on the self-amplified spontaneous emission principle, like those presently generated at x-ray free-electron-laser facilities and, second, Gaussian pulses which will become available in the foreseeable future with self-seeding techniques. As an example of the exciting opportunities derived from the use of seeding methods, we predict, in spite of the above obstacles, the possibility to distinguish at x-ray frequencies a clear signature of Rabi flopping in the spectrum of resonance fluorescence.

We study the formation and destabilization of dark states in a single trapped ${}^{88}{\mathrm{Sr}}^{+}$ ion caused by the cooling and repumping laser fields required for Doppler cooling and fluorescence detection of the ion. By numerically solving the time-dependent density matrix equations for the eight-level system consisting of the sublevels of the $5s{}^2{S}_{1/2}$, $5p{}^2{P}_{1/2}$, and $4d{}^2{D}_{3/2}$ states, we analyze the different types of dark states and how to prevent them in order to maximize the scattering rate, which is crucial for both the cooling and the detection of the ion. The influence of the laser linewidths and ion motion on the scattering rate and the dark resonances is studied. The calculations are then compared with experimental results obtained with an endcap ion trap system located at the National Research Council of Canada and found to be in good agreement. The results are applicable also to other alkaline-earth-metal ions and isotopes without hyperfine structure.

Using a two-channel model, we investigate theoretically the binding energy of confinement-induced Feshbach molecules in two- and one-dimensional ultracold atomic systems near a Feshbach resonance. We show that the two-channel prediction will evidently deviate from the simple single-channel theory as the width of Feshbach resonances decreases. For the one-dimensional system, we perform a full two-channel calculation, with the inclusion of bare interatomic interactions in the open channel. Away from the resonance, we find a sizable correction to the binding energy, if we neglect the bare interatomic interactions as in the previous work [D. B. M. Dickerscheid and H. T. C. Stoof, Phys. Rev. A 72, 053625 (2005)]. We compare our theoretical results with the existing experimental data and present predictions for narrow Feshbach resonances that could be tested in future experiments.

We investigate the half-skyrmion excitations induced by spin-orbit coupling in the rotating and rapidly quenched spin-1 Bose-Einstein condensates. We give three expressions of the corresponding spin vectors to describe the half-skyrmion. Our results show that the half-skyrmion excitation depends on the combination of spin-orbit coupling and rotation, and it originates from a dipole structure of spin which is always embedded in three vortices constructed by each condensate component respectively. When both the strength of spin-orbit coupling and the rotation frequency are larger than some critical values, the half-skyrmions encircle the center with one or several circles to form a radial lattice, which occurs even in the strong ferromagnetic/antiferromagnetic condensates. We can use both the spin-orbit coupling and the rotation to adjust the radial lattice. The realization and the detection of the half-skyrmions are compatible with current experimental technology.

We investigate the superfluid transition temperature of quasi-two-dimensional imbalanced Fermi gases beyond the mean-field approximation, through the second-order (or induced) interaction effects. For a balanced Fermi system the transition temperature is suppressed by a factor $\approx$$2.72$. For imbalanced Fermi systems, the polarization and transition temperature of the tricritical point are significantly reduced as the two-body binding energy $|{\epsilon }_B|$ increases.

Motivated by recent experiments on atomic Dirac fermions in a tunable honeycomb optical lattice, we study the attractive Hubbard model of superfluidity in the anisotropic honeycomb lattice. At weak coupling, we find that the maximum mean-field pairing transition temperature, as a function of density and interaction strength, occurs for the case with isotropic hopping amplitudes. In this isotropic case, we go beyond mean-field theory and study collective fluctuations, treating both pairing and density fluctuations for interaction strengths ranging from weak to strong coupling. We find evidence for a sharp sound mode, together with a well-defined Leggett mode over a wide region of the phase diagram. We also calculate the superfluid order parameter and collective modes in the presence of nonzero superfluid flow. The flow-induced softening of these collective modes leads to dynamical instabilities involving stripelike density modulations as well as a Leggett-mode instability associated with the natural sublattice symmetry-breaking charge-ordered state on the honeycomb lattice. The latter provides a nontrivial test for the experimental realization of the one-band Hubbard model. We delineate regimes of the phase diagram where the critical current is limited by depairing or by such collective instabilities, and discuss experimental implications of our results.

It is shown that slow Bragg soliton solutions are possible in nonlinear complex parity-time ($\mathcal{PT}$) symmetric periodic structures. Analysis indicates that the $\mathcal{PT}$-symmetric component of the periodic optical refractive index can modify the grating band structure and hence the effective coupling between the forward and backward waves. Starting from a classical modified massive Thirring model, solitary wave solutions are obtained in closed form. The basic properties of these slow solitary waves and their dependence on their respective $\mathcal{PT}$-symmetric gain-loss profile are then explored via numerical simulations.

Propagation of a modulationally amplified Gaussian pulse is considered inside a birefringent optical fiber. We show analytically that, in addition to a reduction in the parametric gain and a change in the pulse's group velocity, the third-order dispersion allows the generation of a temporal Airy-shaped pulse. Furthermore, pulse spreading in the temporal domain and chirping in the spectral domain both can be controlled through an interplay between third-order dispersion and modulational instability.

We experimentally demonstrate the up-conversion of light carrying an image from the infrared spectrum to the visible spectrum using four-wave mixing via a ladder-type configuration in an atomic vapor. The results we obtained show the high correlation between the input image and the up-converted image. We also discuss the possible influences of experimental parameters on the resolution. Our work might be useful for research in astrophysics, night-vision technology, and chemical sensing.

Using computational methods, it is found that the two-dimensional nonlinear Schrödinger (NLS) equation with a quasicrystal lattice potential admits multiple dipole and vortex solitons. The linear and the nonlinear stability of these solitons is investigated using direct simulations of the NLS equation and its linearized equation. It is shown that certain multiple vortex structures on quasicrystal lattices can be linearly unstable but nonlinearly stable. These results have application to investigations of localized structures in nonlinear optics and Bose-Einstein condensates.

We present a simple method of stabilizing the laser phase and frequency by polarization spectroscopy of an atomic vapor. In analogy to the Pound-Drever-Hall method, which uses a cavity as a memory of the laser phase, this method uses atomic coherence (dipole oscillations) as a phase memory of the transmitting laser field. A preliminary experiment using a distributed feedback laser diode and a rubidium vapor cell demonstrates a shot-noise-limited laser linewidth reduction (from 2 MHz to 20 kHz). This method would improve the performance of gas-cell-based optical atomic clocks and magnetometers and facilitate laser-cooling experiments using narrow transitions.

Ultrashort pulses can exhibit two distinctive types of singularity: self-focusing collapse and self-steepening shock. We examine various ultrashort pulse propagation models and their relative effectiveness in explaining these phenomena. In particular, the modified Kadomtsev-Petviashvilli equation of type 1 (MKP1) is examined in some detail. We show that MKP1 is not simply a few-cycle pulse model but is valid in a more general broad spectrum setting. Furthermore, we emphasize that the dispersion of the MKP1 model can result in poor estimation of frequency-dependent phenomena, such as harmonic generation, which occur far away from the carrier frequency. Some of this loss of accuracy can be removed by using a more general MKP1 dispersion relation.

We present a Lagrangian-Hamiltonian formalism of a moving dielectric sphere interacting with radiation fields. By including the interaction up to the first order in the speed of the sphere, we derive the Hamiltonian and perform quantization of both the field and the mechanical motion of the sphere. In particular, we show how independent degrees of freedom can be consistently identified under the generalized radiation gauge via instantaneous mode projection. Our Hamiltonian indicates the form of coupling due to velocity-dependent interactions beyond adiabatic approximation. In addition, the Hamiltonian predicts that a geometrical quantum phase can be gained by the sphere moving in a light field.

We examine various notions related to the optimality for entanglement witnesses arising from Choi-type positive linear maps. We found examples of optimal entanglement witnesses which are nondecomposable but are not “nondecomposable optimal entanglement witnesses” in the sense of Lewenstein, Kraus, Cirac, and Horodecki [Phys. Rev. A 62, 052310 (2000)]. We suggest using the terms PPTES witness and optimal PPTES witness in place of “nondecomposable entanglement witness” and “nondecomposable optimal entanglement witnesses” in order to avoid possible confusion. Here, PPTES refers entangled states with positive partial transposes. We also found examples of nonextremal optimal entanglement witnesses which are indecomposable.

Single-photon entanglement provides a valuable resource for quantum communication and quantum repeaters. However, single-photon entanglement is sensitive to photon loss. In this Brief Report, we show that a noiseless linear amplifier is an efficient tool for protecting single-photon entanglement. The performance of ideal noiseless linear amplification as well as the nonideal amplification with on-off detectors and heralded single photons are also investigated.