We propose an approach to steer the launch of attosecond (as) pulses with a high precision. We numerically demonstrate that by adding a weak second-harmonic (SH) field to the fundamental beam the ionization and recollision process of the electron will be perturbed, which can induce a variation of the emission time of high harmonics. Through modifying the relative intensity of the SH and fundamental fields, the launch of as pulses can be manipulated with a resolution less than $40\mathrm{as}$. This will show significant potential for ultrafast optics.

We study the quantum version of a tilting and flashing Hamiltonian ratchet, consisting of a periodic potential and a time-periodic driving field. The system dynamics is governed by a Floquet evolution matrix bearing the symmetry of the corresponding Hamiltonian. The dc-current appears due to the desymmetrization of Floquet eigenstates, which become transporting when all the relevant symmetries are violated. Those eigenstates that mostly contribute to a directed transport reside in phase space regions corresponding to classical resonances. Quantum dynamics leads to the dependence of the average velocity on the initial phase of the ac-field. A resonant enhancement (or suppression) of the dc-current, due to avoided crossings between different Floquet states, takes place upon tuning some control parameters. Our studies are predominantly aimed at experimental realizations of ac-driven quantum ratchets with cold atoms.

We investigate the photonic bands of an atomic Bose-Einstein condensate with a triangular vortex lattice. Index contrast between the vortex cores and the bulk of the condensate is achieved through the enhancement of the index via atomic coherence. The frequency-dependent dielectric function is used in the calculations of the bands, resulting in photonic band gap widths of a few megahertz.

Quantum theory of the intraband resonant tunneling of a Bose-Einstein condensate loaded in a two-dimensional optical lattice is considered. It is shown that the phenomena of quantum collapse and revival can be observed in the fully quantum problem. The mean-field limit of the theory is analyzed using the WKB approximation for discrete equations, establishing in this way a direct connection between the two approaches conventionally used in very different physical contexts. More specifically we show that there exist two different regimes of tunneling and study dependence of quantum collapse and revival on the number of condensed atoms.

We study the Bogoliubov collective excitations of harmonically trapped superfluid Fermi gases in the crossover from Bardeen-Cooper-Schrieffer (BCS) superfluid to Bose-Einstein condensate (BEC) beyond Thomas-Fermi (TF) limit. Starting from a generalized Gross-Pitaevskii equation and an equation of state valid for the whole crossover, we derive Bogoliubov–de Gennes (BdG) equations for low-lying collective modes at zero temperature. We use a Fetter-like variational ground state wave function to remove the noncontinuity of slope at the boundary of condensate, which appears in the TF limit. We solve the BdG equations analytically and obtain explicit expressions for all eigenvalues and eigenfunctions, valid for various crossover regimes and for traps with spherical and axial symmetries. We discuss the feature of these collective excitations in the BCS-BEC crossover and show that the theoretical result obtained agrees with available experimental data near and beyond the TF limit.

We report the measurement of the atomic spin noise of the diamagnetic atom ytterbium (Yb). Yb has various merits for utilizing the quantum nature of the atomic spin ensemble compared with the paramagnetic atoms used in all previous experiments. From the magnitude of the noise level and dependence on the detuning, we concluded that we succeeded in the measurement of ${}^{171}\mathrm{Yb}$ atomic spin noise in an atomic beam.

We develop an asymptotic analysis of nonlinear energy propagation in lattices subject to slowly varying perturbations, investigating symmetry breaking and its effects. We derive a general set of evolution equations and study them by using catastrophe theory, revealing a wealth of system dynamics. Below a power threshold, symmetry breaking drives nonreciprocal oscillations; beyond that, symmetry breaking yields an effect of “macroscopic” self-trapping, which supports a self-maintained energy imbalance between Bloch bands. We numerically verify the theoretical results and discuss their possible implementation in waveguide arrays.

We utilize electromagnetically induced absorption (EIA) in a one-dimensional functional (active) photonic band gap structure to propose a coherently generated purely loss-coupled distributed feedback (DFB) laser. Such a laser happens when interaction of the photonic band structure with a control laser field totally removes its refractive index perturbation and replaces it with periodic regions of EIA, forming an optical dark lattice. Such a lattice does not have any stop band, but scatters light coherently at the Bragg wavelength. We also show that before the optical dark lattice happens, the photonic band gap structure can act as a coherently generated partly loss-coupled DFB laser with dominant lasing mode at the longer wavelength side of the stop band. We expect such a distinct feature makes this laser quite stable.

The effect of a weak uniform magnetic field on the electronic structure of slightly deformed fullerene molecules is studied within the continuum field-theory model. It is shown that fine structure of the electronic energy spectrum is very sensitive to the orientation of the magnetic field. In particular, we found that the magnetic field pointed in the $x$ direction does not influence the first electronic level whereas it causes a splitting of the second energy level. This behavior differs markedly from the case of the magnetic field pointed in the $z$ direction, where $z$ is chosen to be the symmetry axis of the fullerene.

Based on our previous scheme [Hemmer et al., Phys. Rev. Lett. 96, 163603 (2006)] we show the procedures to obtain any one- or two-dimensional pattern by multiple exposures. A key modification to the resonance condition is made to achieve subwavelength pattern resolution. The level separation of the substrate does not pose an upper limit to the frequency summation. So the fundamental frequency can be high and the number of Fourier terms can be large, making our scheme very suitable for subwavelength arbitrary patterns.

An environment interacting with portions of a system leads to multiexponential interaction rates. Within the Keldysh formalism, we fictitiously homogenize the system-environment interaction yielding a uniform decay rate facilitating the evaluation of the propagators. Through an injection procedure we neutralize the fictitious interactions. This technique justifies a stroboscopic representation of the system-environment interaction which is useful for numerical implementation and converges to the natural continuous process. We apply this procedure to a fermionic two-level system and use the Jordan-Wigner transformation to solve a two-spin swapping gate in the presence of a spin environment.

We present a measure of quantum correlation of an ensemble of multiparty states. It is based on the idea of minimal entropy production in a measurement whose elements are locally distinguishable. It is shown, for pure ensembles, to be a relative entropy distance from a set of ensembles. For pure bipartite ensembles, which span the whole bipartite Hilbert space, the measure is bounded below by the average entanglement. We naturally obtain a monotonicity axiom for any measure of quantum correlation of ensembles for single systems. We evaluate the measure for certain cases. Subsequently we use this measure to propose a complementarity relation between our measure and the accessible information obtainable about the ensemble under local operations. The measure is well defined even for the case of a single system, where the complementarity relation is seen to be yet another face of the “Heisenberg uncertainty relation.”

This paper deals with the radius of the largest ball of separable mixed states around the maximally mixed state for multiqubit systems. This radius determines how close entangled states can be to the maximally mixed state. In Aubrun and Szarek (e-print arXiv:quant-ph/0503221) an upper bound on the radius was given, while in Gurvits and Barnum (e-print arXiv:quant-ph/0409095) a lower bound was provided. In this paper we improve both the upper and the lower bound, bringing the ratio of these bounds down to a constant $c=\sqrt{34∕27}\approx 1.122$, as opposed to a term of order $\sqrt{m\log m}$ for the best bounds known previously, where $m$ is the number of qubits in the system. We construct concrete examples of separable states on the boundary to entanglement which realize the upper bounds. As a by-product, we compute the radii of the largest balls that fit into the projective tensor product of three and four unit balls in ${\mathbb{R}}^3$ and in the projective tensor product of an arbitrary number of unit balls in ${\mathbb{R}}^n$ for $n=2$, 4, and 8.

The characterization of the dynamics of quantum systems is a task of both fundamental and practical importance. A general class of methods which have been developed in quantum information theory to accomplish this task is known as quantum process tomography (QPT). In an earlier paper [M. Mohseni and D. A. Lidar Phys. Rev. Lett. 97, 170501 (2006)] we presented an algorithm for direct characterization of quantum dynamics (DCQD) of two-level quantum systems. Here we provide a generalization by developing a theory for direct and complete characterization of the dynamics of arbitrary quantum systems. In contrast to other QPT schemes, DCQD relies on quantum error-detection techniques and does not require any quantum state tomography. We demonstrate that for the full characterization of the dynamics of $n$ $d$-level quantum systems (with $d$ prime), the minimal number of required experimental configurations is reduced quadratically from $d^{4n}$ in separable QPT schemes to $d^{2n}$ in DCQD.

The ground-state electronic and magnetic properties of small silver clusters ${\mathrm{Ag}}_n$ $(2⩽n⩽22)$ have been studied using a linear combination of atomic Gaussian-type orbitals within the density functional theory. The results show that the silver atoms, which are diamagnetic in a bulk environment, can be magnetic when they are grouped together in clusters. The ${\mathrm{Ag}}_{13}$ cluster with icosahedral symmetry has the highest magnetic moment per atom among the silver clusters studied. The cluster symmetry and the reduced coordination number specific to small clusters are revealed as a fundamental factor for the onset of magnetism.

To apply the adiabatic rapid passage process repetitively [T. Lu, X. Miao, and H. Metcalf, Phys. Rev. A 71, 061405(R) (2005)], the nonadiabatic transition probability of a two-level atom subject to chirped light pulses over a finite period of time needs to be calculated. Using a unitary first-order perturbation method in the rotating adiabatic frame, an approximate formula has been derived for such transition probabilities in the entire parameter space of the pulses.

We theoretically study quadrature and polarization squeezing in dispersive optical bistability through a vectorial Kerr cavity model describing a nonlinear cavity filled with an isotropic ${\chi }^{\left(3\right)}$ medium in which self-phase- and cross-phase-modulation, as well as four-wave mixing, occur. We derive expressions for the quantum fluctuations of the output field quadratures as a function of which we express the spectrum of fluctuations of the output-field Stokes parameters. We pay particular attention to study how the bifurcations affecting the non-null linearly polarized output mode squeezes the orthogonally polarized vacuum mode, and show how this produces polarization squeezing.

We investigate counterpropagating dark (black) solitons with different envelope profiles in defocusing Kerr media in the presence of a shallow reflection grating. In the quasi-Bragg-matching regime, we are able to predict the existence of a family of counterpropagating reflection dark solitons making use of their underlying connection with the counterpropagating nonlinear plane-wave pairs with intensities that coincide with the asymptotic soliton ones. In agreement, we find through modulation instability analysis that the nonlinear plane waves are stable in that very parameter region where these dark solitons exist.

We construct solitons for two-dimensional (2D) spatiotemporal solitons [light bullets (LBs)] in models of a planar optical layer with an intrinsic guiding channel. We demonstrate that LBs may be stabilized, instead of an ordinary index-guiding channel, by a structure generated through localized transverse modulation of the group velocity dispersion (GVD) and ${\chi }^{\left(3\right)}$ (nonlinearity) coefficients. Such GVD- and ${\chi }^{\left(3\right)}$-guiding channels can be induced by a change of the layer’s thickness. In addition, the model with only the nonlinearity coefficient subject to the transverse modulation may be realized in terms of Bose-Einstein condensates. Stability regions for 2D solitons in these settings are identified in a numerical form, and explained by means of a variational approximation (VA). Since the transverse GVD nonuniformity destroys the Galilean invariance, boosted (moving) solitons are constructed too, in both numerical and variational forms, and a maximum boost, past which the solitons do not exist, is found. In the model based on the usual index-guiding channel, head-on collisions between boosted LBs are studied. Simulations demonstrate that fast solitons collide elastically, while slow ones merge into a breather. The transition between quasielastic collisions and the merger is explained by means of a dynamical variant of the VA.