In the face of the recent development of the time-domain detection method for short pulses, typically less than 1 microsecond, we consider a feasible method for a continuous variable quantum swapping gate between light polarization and atomic spin ensemble by three successive Faraday rotation interactions using short pulses. This method does not need any particular initial states and its fidelity can ideally run up to one. It also works as a quantum memory of light and a device to generate a nonclassical state of the spin. To evaluate a quantum nature of the proposed swapping gate, we calculated its average fidelity for a set of the coherent states with a Gaussian distribution and confirmed the efficiency as a quantum gate as well as a quantum memory of light.

Discrete localized structures are found in optical cavities with focusing saturable nonlinearity. Families of both bright and gray solitons exhibit multistability as they develop internal shelves in the pinning region around a Maxwell point. Both saturability and discreteness are required for these solitons to be observable, but they can occur for either zero or finite losses.

We realize an experimental implementation of one step of a quantum random walk at the single-photon level. After a single step, it is already possible to observe the difference between the quantum and classical random walk. The single photon is obtained using twin photons from parametric down-conversion, in which the detection of one photon of the idler prepares a single-photon state of the signal. We used two different experimental setups, one based on an interferometer and a second using a birefringent crystal. The physical process behind these effects is the spatial reshaping of the single-photon wave packet, which is the spatial analog of the time reshaping observed in tunneling experiments.

We prove conjectures on the relative entropy of entanglement (REE) for two families of multipartite qubit states. Thus, analytical expressions of the REE for these families of states can be given. The first family of states is composed of mixtures of some permutation-invariant multiqubit states. The results generalized to multiqudit states are also shown to hold. The second family of states contains Dür’s bound entangled states. Along the way, we discuss the relation of the REE to two other measures: robustness of entanglement and the geometric measure of entanglement, slightly extending previous results.

In building a quantum-information processor (QIP), the challenge is to coherently control a large quantum system well enough to perform an arbitrary quantum algorithm and to be able to correct errors induced by decoherence. Nuclear magnetic resonance (NMR) QIPs offer an excellent testbed on which to develop and benchmark tools and techniques to control quantum systems. Two main issues to consider when designing control methods are accuracy and efficiency, for which two complementary approaches have been developed so far to control qubit registers with liquid-state NMR methods. The first applies optimal control theory to numerically optimize the control fields to implement unitary operations on low-dimensional systems with high fidelity. The second technique is based on the efficient optimization of a sequence of imperfect control elements so that implementation of a full quantum algorithm is possible while minimizing error accumulation. This paper summarizes our work in implementing both of these methods. Furthermore, we show that taken together, they form a basis to design quantum control methods for a block-architecture QIP so that large system size is not a barrier to implementing optimal control techniques.

Electron-impact ionization cross sections are calculated for the ground and metastable states of ${\text{B}}^{+}$. Comparisons between perturbative distorted-wave and nonperturbative close-coupling calculations find reductions in the direct ionization cross sections due to long-range electron correlation effects of approximately 10% for the ground state and approximately 15% for the metastable state. Previous crossed-beams experiments, with a metastable to ground ratio of between 50% and 90%, are found to be in reasonable agreement with metastable state close-coupling results. New crossed-beams experiments, with a metastable to ground ratio of only 9%, are found to be in reasonable agreement with ground state close-coupling results. Combined with previous work on neutral B and ${\text{B}}^{2+}$, the nonperturbative close-coupling calculations provide accurate ionization cross sections for the study of edge plasmas in controlled fusion research.

We predict a huge dynamical boost from collective electron motion on the valence $3\mathit{s}$ photoionization of Mg in the $\text{Mg}@{\text{C}}_{60}$ molecule. Calculations have been performed in a framework of time-dependent local density approximation that includes coupling between dipole-allowed final state channels. Our results qualitatively agree with the recent predictions of a semiclassical model, however, significantly differing in magnitude. In particular, a strong dependence on the nature of the caged atom is found which is at odds with the prediction of the semiclassical result.

We introduce a species of the discrete nonlinear Schrödinger (DNLS) equation, which is a model for a self-attractive Bose-Einstein condensate confined in a combination of a cigar-shaped trap and deep optical lattice acting in the axial direction. The equation is derived as a discretization of the respective nonlinear nonpolynomial Schrödinger equation. Unlike previously considered varieties of one-dimensional DNLS equations, the present discrete model admits on-site collapse. We find two families of unstaggered on-site-centered discrete solitons, stable and unstable ones, which include, respectively, broad and narrow solitons, their stability exactly complying with the Vakhitov-Kolokolov criterion. Unstable on-site solitons either decay or transform themselves into robust breathers. Intersite-centered unstaggered solitons are unstable to collapse; however, they may be stabilized by the application of a sufficiently strong kick, which turns them into moving localized modes. Persistently moving solitons can be readily created too by the application of the kick to stable on-site unstaggered solitons. In the same model, staggered solitons, which are counterparts of gap solitons in the continuum medium, are possible if the intrinsic nonlinearity is self-repulsive. All on-site staggered solitons are stable, while intersite ones have a small instability region. The staggered solitons are immobile.

The polarization and spatiotemporal dynamics of coherent pulses propagating in a doped birefringent nonlinear fiber are modeled with the inclusion of the local Lorentz field. The numerical solutions describe the full set of equations for circularly polarized components of a coherent optical pulse coupled to the inhomogeneously ensemble of doped resonance atoms. The local field is shown to degrade the stability of the $4\pi$-decay process. The development of a steady-state signal from the series of multiple photon echoes generated in active fiber is observed in simulations.

Within the framework of exact quantum electrodynamics in dispersing and absorbing media, we have studied the quantum state of the radiation emitted from an initially in the upper state, prepared two-level atom in a high-$\mathit{Q}$ cavity, including the regime where the emitted photon belongs to a wave packet that simultaneously covers the areas inside and outside the cavity. For both continuing atom-field interaction and short-term atom-field interaction, we have determined the spatiotemporal shape of the excited outgoing wave packet and calculated the efficiency of the wave packet to carry a one-photon Fock state. Furthermore, we have made contact with quantum noise theories where the intracavity field and the field outside the cavity are regarded as approximately representing independent degrees of freedom such that two separate Hilbert spaces can be introduced.

On traversing materials with absorbing dyes, weak optical beams decay exponentially (a Beer profile), while intense beams develop in time a profile that is spatially linear until at great depth it becomes spatially exponential. This anomalous, deep penetration, due to photobleaching of surface layers, is important for heavy dye loading and intense beams, for instance in photo-actuation. We address the problem of the evolution in time from initial Beer’s Law to a finally deeply-penetrating optical profile in dyes. Our largely analytic solution of the coupled, nonlinear, partial differential equations governing the spatiotemporal decay of the Poynting flux and the nonlinear population dynamics of the photo-active molecules under intense irradiation has application to optomechanical devices.

We investigate the entanglement properties of a three-parameter family of states that are part of the magic simplex of two qutrits, which is a simplex of states that are mixtures of maximally entangled two-qutrit Bell states. Using entanglement witnesses, we reveal large regions of bound entangled and separable states.

A simple algebraic procedure is described for deriving Maxwell-Bloch-type equations from single-atom cavity quantum electrodynamics (cavity QED) master equations via orthogonal projection onto a manifold of semiclassical states. In particular, the usual Maxwell-Bloch equations are obtained—up to a state-dependent correction factor of order unity—straightforwardly from the unconditional Jaynes-Cummings master equation. The technique of projecting onto a semiclassical manifold can also be applied with conditional master equations (quantum filters), leading to stochastic simulation models that include multiplicative noise terms associated with fluctuations of the atomic dipole. The utility of such models is briefly explored in the context of single-atom absorptive bistability.

We investigate experimentally the high-order harmonic generation from aligned ${\text{CO}}_2$ molecules and demonstrate that the modulation inversion of the harmonic yield with respect to molecular alignment can be altered dramatically by fine-tuning the intensity of the driving laser pulse for harmonic generation. The results can be modeled by employing the strong field approximation including a ground state depletion factor. The laser intensity is thus proved to be a parameter that can control the high-harmonic emission from aligned molecules.

Two particular subjects tackled in the recent paper by Schirmer and Dreuw (SD) [J. Schirmer and A. Dreuw, Phys. Rev. A 75, 022513 (2007)] are commented on by us. By providing a convincing example, we answer positively the question posed by SD whether a local operator can be reconstructed from the knowledge of its particle-hole matrix elements (when the number of particle states exceeds one). In connection with the problem of the utmost reduction of the density-functional theory (DFT) to a solution of one equation for the density amplitude—the radical Kohn-Sham (rKS) approach—used by SD as a valuable tool to investigate the time-independent and time-dependent DFT, we demonstrate that the calculational scheme implementing the rKS formulation is rather uncompetitive with the traditional KS scheme.

We propose an efficient technique for creation of cluster states of trapped ions. The ions are addressed collectively, in only two interaction steps, by a pair of pulses tuned near the blue- and red-sideband transitions. The technique is very suitable for four-ion cluster states, for which the initial condition is the product state ∣0000⟩. Five- and six-ion cluster states require an initial preparation of one or two Bell states. Larger cluster states can be prepared by using four-ion cluster states as building blocks. The technique is insensitive to vibrational motion and hence to heating.

Conclusive discrimination of quantum states is expressible as a sequence of optimal weak measurements. The sequence of probabilities of successful discrimination realized in this process is shown to be a martingale, a model of a fair game. For discriminating between two pure states, this martingale is constant; for at least one strictly mixed state or more than two pure states, it is constant just on average. In all these latter cases, therefore, the realized discrimination probability conditioned on prior measurements may be greater than the optimal success probability of the original ensemble. So in these cases superoptimal discrimination is conditionally achievable. This phenomenon is demonstrated by examples.

We investigate the monogamy nature of entanglement in a three-qubit system. A monogamy inequality is presented to describe the exclusive relation between the $A\text{−}B$ two-qubit concurrence $C_{AB}$ and the $AB\text{−}C$ three-qubit concurrence $C_{\left(AB\right)C}$, which represents the entanglement between qubits $\mathit{A}$ and $\mathit{B}$ as a whole and the third qubit $\mathit{C}$. It is found that the entanglement between any two qubits in a three-qubit system is limited by the entanglement between these two qubits and another qubit. As a consequence, we present the upper bounds for the concurrence $C_{AB}$, when the concurrence between qubits $\mathit{A}$ and $\mathit{C}$ $\left(C_{AC}\right)$ and the concurrence between qubits $\mathit{B}$ and $\mathit{C}$ $\left(C_{BC}\right)$ are both given or one of the two is provided.

The stochastic variational method is used with a confining potential of the form $V=\lambda r^{12}$ to generate correlated basis sets capable of accurately describing low-energy elastic scattering. Stabilization concepts are then used to extract the phase shifts and other scattering observables. The method is applied to positron scattering from H and He, and Ps scattering from H and He. The results of the calculations are in uniformly good agreement with previous high quality calculations or with experiment. In particular, the scattering length for Ps-He scattering is found to be $1.566a_0$ while the zero-energy ${{}_{1}Z}_{\text{eff}}$ was 0.1157. The Ps-He $s$-wave phase shift is found to be incompatible with experimental momentum transfer cross sections obtained by the Doppler broadening technique.