We discuss the feasibility of negative refraction with reduced absorption in coherently driven atomic media. Coherent coupling of an electric and a magnetic dipole transition by laser fields induces magnetoelectric cross coupling and negative refraction at dipole densities which are considerably smaller than necessary to achieve a negative permeability. At the same time the absorption gets minimized due to destructive quantum interference and the ratio of negative refraction index to absorption becomes orders of magnitude larger than in systems without coherent cross coupling. The proposed scheme allows for a fine tuning of the refractive index. We derive a generalized expression for the impedance of a medium with magnetoelectric cross coupling and show that impedance matching to vacuum can easily be achieved. Finally we discuss the tensorial properties of the medium response and derive expressions for the dependence of the refractive index on the propagation direction.

We investigate the creation of squeezed states of a vibrating membrane or a movable mirror in an optomechanical system. An optical cavity is driven by the squeezed light and couples via the radiation pressure to the membrane or mirror, effectively providing a squeezed heat bath for the mechanical oscillator. Under the conditions of laser cooling to the ground state, we find an efficient transfer of squeezing with roughly 60% of light squeezing conveyed to the membrane or mirror (on a dB scale). We determine the requirements on the carrier frequency and the bandwidth of squeezed light. Beyond the conditions of ground-state cooling, we predict mechanical squashing to be observable in current systems.

We propose a scheme of obtaining Ramsey fringes based on pulsed coherent optical information storage. Unlike the usual frequency references where the Ramsey fringes are obtained by detecting the population of a certain state, we get the Ramsey fringes by detecting the atomic coherence. The central line of the Ramsey fringes can be used as a frequency reference in an absorption-cell-based atomic frequency standard. This scheme is free of light shifts as the interrogating process is separated from the optical pumping process, and the cavity pulling effect is negligible due to the low $Q$ requirement. Encoding the Ramsey interference into the retrieval light pulse has the merit of high signal-to-noise ratio. This scheme is promising for building small, compact, and stable atomic clocks.

We report the experimental realization of entanglement localization which restores the polarization entanglement completely redirected after a linear coupling with incoherent and noisy surrounding photon. The method based only on measurements of the surrounding photon after the coupling and on postselection can localize the entanglement back to original systems for any linear coupling.

Impulsive Raman transition on the $B{}^3\Pi \left(0_u^{+}\right)$ state of the iodine molecule is reported. Several rovibrational states in the $B$ state are populated with the second harmonics of a neodymium-doped yttrium lithium fluoride (Nd:YLF) laser ($\le 250\text{ns}$, 527 nm). The population alteration with $\Delta v=\pm 1$ and $\pm 2$ are induced with a Ti:sapphire laser ($\sim 30\text{fs}$, 780 nm, ${10}^{12}–{10}^{14}\text{W}/{\text{cm}}^2$) and detected by the $B\text{−}X$ dispersed fluorescence spectrum. The alteration is attributed to the Raman transition on the basis of the spectral profile, the pressure dependence, and the laser power dependence. The observed Raman transition is concluded to be a continuum resonance Raman transition mediated by upper repulsive states. Competing dissociation and ionization processes are also discussed by analyzing the laser power dependence.

We consider the time evolution of the electric and magnetic field operators for a two-level atom, interacting with the electromagnetic field, placed near an infinite perfectly conducting wall. We solve iteratively the Heisenberg equations for the field operators and obtain the electric and magnetic energy density operators around the atom (valid for any initial state). Then we explicitly evaluate them for an initial state with the atom in its bare ground state and the field in the vacuum state. We show that the results can be physically interpreted as the superposition of the fields propagating directly from the atom and the fields reflected on the wall. Relativistic causality in the field propagation is discussed. Finally we apply these results to the calculation of the dynamical Casimir-Polder interaction energy in the far zone between two atoms when a boundary condition such as a conducting wall is present. Magnetic contributions to the interatomic Casimir-Polder interaction in the presence of the wall are also considered. We show that in the limit of large times, the known results of the stationary case are recovered.

We present two sets of computable entanglement measures for multipartite systems where each subsystem can have different degrees of freedom (so-called qudits). One set, called “separability” measure, reveals which of the subsystems are separable or entangled. For that we have to extend the concept of $k$ separability for multipartite systems to a unambiguous separability concept which we call ${\gamma }_k$ separability. The second set of entanglement measures reveals the “kind” of entanglement, i.e., if it is bipartite, tripartite, …, $n$-partite entangled and is denoted as the “physical” measure. We show how lower bounds on both sets of measures can be obtained by the observation that any entropy may be rewritten via operational expressions known as $m$ concurrences. Moreover, for different classes of bipartite or multipartite qudit systems we compute the bounds explicitly and discover that they are often tight or equivalent to positive partial transposition.

The secure key rates of decoy-state quantum key distribution (QKD) using the heralded single-photon source (HSPS) are recalculated in the case of intensity fluctuations. By numerical simulations, we show that the HSPS is a good source for decoy-state QKD experiments not only because it has larger upper bound of transmission distance than the usual weak-coherent source (WCS) but also because it is more robust against intensity fluctuations than the WCS.

We have calculated parity-nonconserving $7s-6d_{3/2}$ amplitude $E_{\text{PNC}}$ in ${}^{223}\text{R}{\text{a}}^{+}$ using high-precision relativistic all-order method where all single and double excitations of the Dirac-Fock wave functions are included to all orders of perturbation theory. Detailed study of the uncertainty of the parity-nonconserving amplitude is carried out; additional calculations are performed to estimate some of the missing correlation corrections. A systematic study of the parity-conserving atomic properties, including the calculation of the energies, transition matrix elements, lifetimes, hyperfine constants, and quadrupole moments of the $6d$ states, as well as dipole and quadrupole ground-state polarizabilities, is carried out. The results are compared with other theoretical calculations and available experimental values.

Using ${{}^{201}\text{H}\text{g}}^{+}$ contained within a linear quadrupole rf ion trap that is part of an atomic clock, we report the results of high-resolution spectroscopy on the hyperfine structure of ${{}^{201}\text{H}\text{g}}^{+}$. We measure the absolute ground-state hyperfine interval to be $29.954365821130\left(171\right)\left(62\right)\left(10\right)\text{GHz}$, more than 8 orders of magnitude improvement over the previous measurement. The first error estimate in parentheses is the statistical error in the shifted line center measurement, the second is systematic uncertainty, and the third is calibration uncertainty in the hydrogen maser reference standard. By comparison to the already accurately known ground-state hyperfine interval for ${{}^{199}\text{H}\text{g}}^{+}$, we are able to derive a value for the hyperfine anomaly in singly ionized mercury, $\Delta \left(S_{1/2},{{}^{199}\text{H}\text{g}}^{+},{{}^{201}\text{H}\text{g}}^{+}\right)=-0.0016257\left(5\right)$, now limited by knowledge of the nuclear magnetic-moment ratio.

We analyze atomic photoelectron momentum distributions induced by bichromatic and monochromatic laser fields within the strong-field approximation (SFA), separable Coulomb-Volkov approximation (SCVA), and ab initio treatment. We focus on the high frequency regime—the smallest frequency used is larger than the ionization potential of the atom. We observe a remarkable agreement between the ab initio and velocity gauge SFA results while the velocity gauge SCVA fails to agree. Reasons of such a failure are discussed.

We report the numerical calculations for the multiple ionization of rubidium in a strong laser field. We consider multiply charged ion formation and take into account some of the many-body effects, i.e., the inelastic tunneling effect, the collective tunneling, and $m$ relaxation of an ionic core. In the case of strong fields, with intensities in excess of ${10}^{17}\text{W}/{\text{cm}}^2$, the free motion of a photoelectron is affected by the magnetic component of the laser field. Because of this, the formation of multiply charged ions as a result of a rescattering process becomes improbable. We present our findings for ${\text{Rb}}^{10+}$ and ${\text{Rb}}^{11+}$ ions, where a significant contribution of collective tunneling was revealed, which was not observed in the weaker fields previously investigated [A. S. Kornev, E. B. Tulenko, and B. A. Zon, Phys. Rev. A 68, 043414 (2003); Phys. Rev. A 69, 065401 (2004)].

We generalize the concept of quantum phase transitions, which is conventionally defined for a ground state and usually applied in the thermodynamic limit, to one for metastable states in finite-size systems. In particular, we treat the one-dimensional Bose gas on a ring in the presence of both interactions and rotation. To support our study, we bring to bear mean-field theory, i.e., the nonlinear Schrödinger equation, and linear perturbation or Bogoliubov–de Gennes theory. Both methods give a consistent result in the weakly interacting regime: there exist two topologically distinct quantum phases. The first is the typical picture of superfluidity in a Bose-Einstein condensate on a ring: average angular momentum is quantized and the superflow is uniform. The second is where one or more dark solitons appear as stationary states, breaking the symmetry, the average angular momentum becomes a continuous quantity. The phase of the condensate can therefore be continuously wound and unwound.

Anderson localization of matter waves was recently observed with cold atoms in a weak one-dimensional disorder realized with laser speckle potential [Billy et al., Nature (London) 453, 891 (2008)]. The latter is special in that it does not have spatial frequency components above certain cutoff $q_c$. As a result, the Lyapunov exponent (LE) or inverse localization length vanishes in Born approximation for particle wave vector $k>\frac{1}{2}{q}_c$, and higher orders become essential. These terms, up to the fourth order, are calculated analytically and compared with numerical simulations. For very weak disorder, LE exhibits a sharp drop at $k=\frac{1}{2}{q}_c$. For moderate disorder (a) the drop is less dramatic than expected from the fourth-order approximation and (b) LE becomes very sensitive to the sign of the disorder skewness (which can be controlled in cold atom experiments). Both observations are related to the strongly non-Gaussian character of the speckle intensity.

We investigate the rotational properties of a Bose-Einstein condensate (BEC) in an effective magnetic field. The corresponding gauge potential is optically generated, and based on the adiabatic motion of the atoms. We demonstrate that the nucleation of vortices is seeded by instabilities in surface excitations and show that this picture also holds when the applied effective magnetic field is not homogeneous. The eventual configuration of vortices in the cloud depends on the geometry of the applied field.

Analysis of the effects of dephasing on quantum features of the cavity radiation of externally pumped correlated emission laser is presented. It turns out that entanglement and nonclassical photon number correlations are strongly reliant on the rate at which the atomic coherent superposition decays specially when the amplitude of the driving radiation is smaller. It is, particularly, found that almost perfectly entangled light can be generated while the rate of dephasing and the amplitude of the external driving radiation are kept low in case the atoms are initially prepared in possible maximum coherence. It is also found that the nonclassical behavior of the photon correlation associated with the violation of Cauchy-Schwarz inequality decreases with the rate of dephasing except for some values of the amplitude of the driving radiation.

Using the cold target recoil ion momentum spectroscopy technique, we have measured state-selective projectile scattering angles for single and double electron captures in collisions of protons and ${\text{He}}^{1,2+}$ projectiles with a helium target for incident energies of 60–630 keV/u. We also report theoretical results obtained by means of four-body one-channel distorted-wave models (continuum distorted-wave Born final state, continuum distorted-wave Born initial state, and Born distorted wave) and find mixed agreement with the measured data.

We propose and theoretically analyze an experimental configuration in which lasers induce three-spin interactions between trapped ions. By properly choosing the intensities and frequencies of the lasers, three-spin couplings may be dominant or comparable to two-spin terms and magnetic fields. In this way, trapped ions can be used to study exotic quantum phases which do not have a counterpart in nature. We study the conditions for the validity of the effective three-spin Hamiltonian and predict qualitatively the quantum phase diagram of the system.

We investigate feedback control of linear quantum systems subject to feedback-loop time delays. In particular, we examine the relation between the potentially achievable control performance and the time delays and provide theoretical guidelines for the future experimental setup in two physical systems, which are typical in this research field. The evaluation criterion for the analysis is given by the optimal control performance formula, the derivation of which is from the classical control theoretic results about the input-output delay systems.

We analyze unambiguous identification of coherent states of an electromagnetic field. In particular, we study possible generalizations of an optical setup proposed in Sedlák et al. [Phys. Rev. A 76, 022326 (2007)]. We show how the unambiguous identification of coherent states can be performed in a general case when multiple copies of unknown and reference states are available. Under the condition that the experimental setup consists only of linear optical elements and photodetectors, we prove the optimality of the setup. We also investigate whether reference states after the measurement can be “recovered” and further used for subsequent unambiguous identification tasks. We show that in spite of the fact that the recovered reference states are disturbed by measurements, they can be repeatedly used for unambiguous identifications. We analyze the influence of particular type of noise in the preparation of the unknown and the reference coherent states on the performance of our unambiguous-identification setup.