We study the creation of electron-positron pairs from the vacuum induced by a combination of a static electric field and an alternating field. We find that the overall pair production can be increased by two orders of magnitude compared to the yields associated with each field individually. We examine the interesting case where both fields are spatially localized, permitting us to examine the time evolution of the spatial density for the created particle pairs. We find that there are a variety of competing mechanisms that contribute to the total yield.

The modeling of complex atomic spectra is a difficult task, due to the huge number of levels and lines involved. In the presence of a magnetic field, the computation becomes even more difficult. The anomalous Zeeman pattern is a superposition of many absorption or emission profiles with different Zeeman relative strengths, shifts, widths, asymmetries, and sharpnesses. We propose a statistical approach to study the effect of a magnetic field on the broadening of spectral lines and transition arrays in atomic spectra. In this model, the $\sigma$ and $\pi$ profiles are described using the moments of the Zeeman components, which depend on quantum numbers and Landé factors. A graphical calculation of these moments, together with a statistical modeling of Zeeman profiles as expansions in terms of Hermite polynomials are presented. It is shown that the procedure is more efficient, in terms of convergence and validity range, than the Taylor-series (TS) expansion in powers of the magnetic field, which was suggested in the past. Finally, a simple approximate method to estimate the contribution of a magnetic field to the width of transition arrays is proposed. It relies on our recently published recursive technique for the numbering of $LS$ terms of an arbitrary configuration.

We present a self-consistent $t$-matrix theory for Bose-Einstein-condensed systems within the Hartree-Fock-Bogoliubov (HFB) approximation. Using the Lippmann-Schwinger equation for a $t$ matrix describing the collision between two particles via an interparticle potential, we derive a set of equations for the normal and anomalous self-energies in the HFB approximation expressed in terms of the $t$ matrix. These equations are solved for a hard-sphere potential. A result is then obtained which is valid over the full range of density, reducing to the exact expressions at low densities and to the Brueckner-Sawada theory at high densities. The spectrum is gapless and linear in small momentum, but does not have any roton minimum in the large-momentum region even for high densities such as those of ${}^4$He.

We calculate the entanglement of the ground state of alkaline-earth-metal fermionic atoms confined in one-dimensional optical lattices. This system can be described using the Kondo lattice model plus a harmonic confining potential, and we adopt the density-matrix renormalization group to study its ground state. We find that the local von Neumann entropy is constant in the insulating domains, and a one-to-one correspondence with the variance of the local density is observed. We show that the average entropy and its derivative are useful tools for identifying quantum transitions in impurity systems.

This paper demonstrates how we can generate coherent light of lasing characteristics from a system made of a two-level system and a nanoparticle. The lasing is achieved by embedding the whole system in a polaritonic band-gap material. The study is conducted on silver nanoparticles embedded within silicon carbide.

We show that a waveguide with multiple gratings can have a modal dispersion relation which supports frozen light. This means that light can be coupled efficiently to low group velocity modes of an optical waveguide or can even have finite coupling to zero group velocity modes. These effects are associated with stationary points in the dispersion of the form $\omega -{\omega }_o\propto {(k-k_o)}^m$, for integer order $m>1$, around a center frequency ${\omega }_o$ and wave number $k_o$. Stationary points of any order can be created, not only regular band edges ($m=2$), but also degenerate band edges ($m>2$ and even) and stationary inflection points ($m$ odd). Using the perturbation theory of matrices in Jordan normal form, the modes and their properties are calculated analytically. Efficient coupling is shown to stem from evanescent modes which must accompany the presence of high-order stationary points with $m>2$.

We propose a simple modification of the Keldysh theory of ionization of an atom in a few-cycle laser pulse field. The results obtained agree well with the ab initio simulations carried out by Rohringer and Santra [Phys. Rev. A 79, 053402 (2009)] for the Ne atom and the Ne${}^{+}$ ion, both in the ground state and in the excited state. This is a confirmation of the model of the inelastic tunnel effect that is one of the major many-body effects in tunnel ionization theory.

We demonstrate the conversion of cold Cs${}_2$ molecules initially distributed over several vibrational levels of the lowest triplet state $a^3{\Sigma }_u^{+}$ into the singlet ground state $X^1{\Sigma }_g^{+}$. This conversion is realized by a broadband laser exciting the molecules to a well-chosen state from which they may decay to the singlet state through two sequential single-photon emission steps: The first photon populates levels with mixed triplet-singlet character, making possible a second spontaneous emission down to several vibrational levels of the $X^1{\Sigma }_g^{+}$ states. By adding an optical scheme for vibrational cooling, a substantial fraction of molecules are transferred to the ground vibrational level of the singlet state. The efficiency of the conversion process, with and without vibrational cooling, is discussed at the end of the article. The presented conversion is general in scope and could be extended to other molecules.

We revisit the upper bound of quantum discord given by the von Neumann entropy of the measured subsystem. Using the Koashi-Winter relation, we obtain a tradeoff between the amount of classical correlation and quantum discord in the tripartite pure states. The difference between the quantum discord and its upper bound is interpreted as a measure on the classical correlative capacity. Further, we give the explicit characterization of the quantum states saturating the upper bound of quantum discord, through the equality condition for the Araki-Lieb inequality. We also demonstrate that the saturating of the upper bound of quantum discord precludes any further correlation between the measured subsystem and the environment.

We study the core effect of nonhydrogenic alkaline-earth-metal barium in its diamagnetic spectrum by one photon transition from the ground state $6s^2$ ${}^1{S}_0$ experimentally and theoretically. The non-Coulombic potential of the ion core introduces an extra energy shift compared with hydrogen and a level anticrossing between different $n$ manifolds, characterized by the quantum defect of the concerned angular momentum states. With a complex rotation coordinate technique and a $B$-spline expansion method, we develop a matrix-form Hamiltonian based on an effective potential incorporating the angular-dependent quantum defect into the angular rotation term. The nonhydrogen core effects are investigated by sweeping the quantum defects of different channels in the calculation. Results show that quantum defects of $p$ and $f$ states have a undeniable effect on the intensities and positions of the spectral lines, although barium is closely hydrogenlike in the energy range examined. The anticrossing spectral lines are also identified with the aid of theoretical calculations. The calculations are in good agreement with the experimental observations.

We present a new formulation of monotonically convergent algorithms which allows us to optimize both the control duration and the field fluence. A standard algorithm designs a control field of fixed duration, which both brings the system close to the target state and minimizes its fluence, whereas here we include in addition the optimization of the duration in the cost functional. We apply this new algorithm to the control of spin systems in nuclear magnetic resonance. We show how to implement controlled-not gates in systems of two and four coupled spins.

The mechanism of the formation of doorway quasistates (transient species) prior to the start of ionization of the two-electron molecular H${}_2$ system subjected to an eight-cycle ultrashort intense laser pulse is investigated by solving exactly the one-dimensional (1D) electronic time-dependent Schrödinger equation, including nuclear motion semiclassically. Space of the electron density of the two-electron 1D H${}_2$ system around the nuclei is partitioned into four physicochemically significant partitions, including homolytic $(e_1{\mathrm{H}}_{\alpha }^{+}-{\mathrm{H}}_{\beta }^{+}{e}_2)\sim (e_2{\mathrm{H}}_{\alpha }^{+}-{\mathrm{H}}_{\beta }^{+}{e}_1)$ and ionic $({\mathrm{H}}_{\alpha }^{+}-{\mathrm{H}}_{\beta }^{-})\sim ({\mathrm{H}}_{\alpha }^{-}-{\mathrm{H}}_{\beta }^{+})$ transient species. The underlying mechanisms responsible for the formation and evolution of the homolytic and ionic transient species are explored based on probing variation of the two-electron norms of these states and their corresponding time-dependent high-order harmonic generation spectra.

In this paper, we study the homogeneous one-dimensional bosonic gas interacting via a repulsive contact potential by using the improved Gaussian approximation. We obtain the gapless excitation spectrum of the Bogoliubov mode. Our result is in good agreement with the exact numerical calculation based on the Bethe ansatz. We speculate that the improved Gaussian approximation could be a quantitatively good approximation for higher-dimensional systems.

We in this paper investigate the phase diagram associated with the Bardeen-Cooper-Schrieffer–Bose-Einstein-condensate (BCS-BEC) crossover of a three-component ultracold superfluid Fermi gas of different chemical potentials and equal masses in two dimensions. The gap-order parameter and number densities are found analytically by using the functional-path-integral method. The balance of pairing will be broken in the free space due to the unequal chemical potentials. We obtain the same particle-number density and condensed fraction in the BCS-superfluid phase as those in a recent paper [Phys. Rev. A 83, 033630 (2011)] while the Sarma phase of the coexistence of normal and superfluid Fermi gases is characteristic of an inhomogeneous system. The minimum ratio of the BCS-superfluid phase becomes 1/3 in the BCS limit corresponding to the zero ratio in the two-component system in which the critical point of phase separation is ${\epsilon }_B/{\epsilon }_F=2$, but it becomes 3 in the three-component case.

We show how the spatial decoherence properties of coherently split beams of charged and neutral particles are influenced by their (quantum) electromagnetic interaction with a metallic surface. Our investigation makes use of macroscopic quantum electrodynamics based on linear-response theory, which provides an alternative to microscopic models of the decoherence mechanism. We find strikingly simple expressions for the decoherence rate that admit an interpretation in terms of which path information due to image-charge interactions.

The high-resolution inelastic x-ray scattering has been applied to study the excitation mechanism of atomic neon. The different electric multipolar transitions of neon, i.e., the electric monopolar, dipolar, and quadrupolar ones, were clearly resolved, and their squared form factors were determined. It is found that for the monopolar and quadrupolar transitions the present inelastic x-ray scattering results are in agreement with the ones measured by high-energy electron scattering, while for the dipolar transition large discrepancies between the results measured by these two methods were observed. Such phenomena are well elucidated with the aid of the second-order Born approximation: Born amplitude of a transition near zero momentum transfer is the key factor which determines the behavior of the intramolecular multiple scattering in the high-energy electron scattering. It is revealed that the intramolecular multiple scattering in high-energy electron scattering must be considered for the dipole-allowed transition, while it is negligible for the dipole-forbidden ones.

We give an explicit parametrization of the set of mixed quantum states and of the set of mixed classical states for a spin-1 Hilbert space. The boundary of the set of mixed classical states is described as a two-parameter family of ellipsoids.

Quantum correlation can be created by a local operation from some initially classical states. We prove that the necessary and sufficient condition for a local trace-preserving channel to create quantum correlation is that it is not a commutativity-preserving channel. This condition is valid for arbitrary finite dimension systems. We also derive the explicit form of commutativity-preserving channels. For a qubit, a commutativity-preserving channel is either a completely decohering channel or a mixing channel. For a three-dimensional system (qutrit), a commutativity-preserving channel is either a completely decohering channel or an isotropic channel.

The distance between a quantum state and its closest state not having a certain property has been used to quantify the amount of correlations corresponding to that property. This approach allows a unified view of the various kinds of correlations present in a quantum system. In particular, using relative entropy as a distance measure, total correlations can be meaningfully separated into a quantum part and a classical part thanks to an additive relation involving only the distances between states. Here we investigate a unified view of correlations using as a distance measure the square norm, which has already been used to define the so-called geometric quantum discord. We thus also consider geometric quantifiers for total and classical correlations, finding, for a quite general class of bipartite states, their explicit expressions. We analyze the relationship among geometric total, quantum, and classical correlations, and we find that they no longer satisfy a closed additivity relation.