The evolution of the lower bound of entanglement proposed by Chen et al. [Phys. Rev. Lett. 95, 210501 (2005)] in high-dimensional bipartite systems under dissipation is studied. Discontinuities for the time derivative of this bound are found depending on the initial conditions for entangled states. These abrupt changes along the evolution of the entanglement bound appear as precursors of sudden death.

We investigate the photodetachment process in the negative fluorine ion in a strong linearly polarized laser field. Angle-resolved momentum distributions of photoelectrons are measured with the use of an imaging technique for a wide range of laser frequencies and peak intensities. The nonmonotonic structure recorded in photoelectron spectra is interpreted in terms of the quantum interference effect predicted by a Keldysh-like theory. In particular, the dependence of the interference term on the laser parameters is used to explain the origin of the observed spectral features. Our results unambiguously show that the length gauge is the proper one to use in the frame of the strong-field approximation.

We report on the observation of collective atomic recoil lasing and superradiant Rayleigh scattering with ultracold and Bose-Einstein condensed atoms in an optical ring cavity. Both phenomena are based on instabilities evoked by the collective interaction of light with cold atomic gases. This publication clarifies the link between the two effects. The observation of superradiant behavior with thermal clouds as hot as several tens of $\mu \mathrm{K}$ proves that the phenomena are driven by the cooperative dynamics of the atoms, which is strongly enhanced by the presence of the ring cavity.

We construct a model for the atomic detection in the context of cavity quantum electrodynamics (QED) used to study coherence properties of superpositions of states of an electromagnetic mode. Analytic expressions for the atomic ionization are obtained, considering the imperfections of the measurement process due to the probabilistic nature of the interactions between the ionization field and the atoms. We provide for a dynamical content for the available expressions for the counting rates considering limited efficiency of detectors. Moreover, we include false countings. The influence of these imperfections on the information about the state of the cavity mode is obtained. In order to test the adequacy of our approach, we investigate a recent experiment reported by Maître [X. Maître et al., Phys. Rev. Lett. 79, 769 (1997)] and we obtain excellent agreement with the experimental results.

We observed self-formation of looped random lasing paths in a laser-diode-pumped wide-aperture thin-slice solid-state laser with imperfect reflective end surfaces. Observed lasing patterns, resulting from random scatterers on the surfaces, showed transverse intensity distributions possessing exponential spatial autocorrelation functions. It is shown that such a speckle type of lasing pattern formation takes place when the pump position is tuned such that the surface irregularity possesses an exponential autocorrelation function whose delay length is smaller than the lasing beam diameter.

The physics of quantum walks on graphs is formulated in Hamiltonian language, both for simple quantum walks and for composite walks, where extra discrete degrees of freedom live at each node of the graph. It is shown how to map between quantum walk Hamiltonians and Hamiltonians for qubit systems and quantum circuits; this is done for both single-excitation and multiexcitation encodings. Specific examples of spin chains, as well as static and dynamic systems of qubits, are mapped to quantum walks, and walks on hyperlattices and hypercubes are mapped to various gate systems. We also show how to map a quantum circuit performing the quantum Fourier transform, the key element of Shor’s algorithm, to a quantum walk system doing the same. The results herein are an essential preliminary to a Hamiltonian formulation of quantum walks in which coupling to a dynamic quantum environment is included.

We show that no universal quantum cloning machine exists that can broadcast an arbitrary mixed qubit with a constant fidelity. Based on this result, we investigate the dependent quantum cloner in the sense that some parameter of the input qubit ${\rho }_s(\theta ,\omega ,\lambda )$ is regarded as constant in the fidelity. For the case of constant $\omega$, we establish the $1\to 2$ optimal symmetric dependent cloner with a fidelity $1∕2$. It is also shown that the $1\to M$ optimal quantum cloning machine for pure qubits is also optimal for mixed qubits, when $\lambda$ is the unique parameter in the fidelity. For general $N\to M$ broadcasting of mixed qubits, the situation is very different.

We propose and prove protocols of combined and controlled remote implementations of partially unknown quantum operations belonging to the restricted sets [A. M. Wang, Phys. Rev. A 74, 032317 (2006)] using Greenberger-Horne-Zeilinger (GHZ) states. We present the protocols in detail in the cases of one qubit, with two senders and with one controller, respectively. Then we study the variations of protocols with many senders, or with many controllers, or with both many senders and controllers using a multipartite GHZ state. Furthermore, we extend these protocols to the cases of multiqubits. Because our protocols have to request that the senders work together and transfer the information in turn or receive the repertoire of extra supercontrollers, or/and the controller(s) open the quantum channel and distribute the passwords in different ways, they definitely have the strong security in remote quantum information processing and communications. Moreover, the combined protocol with many senders is helpful to arrive at the power of remote implementations of quantum operations to the utmost extent in theory, since the different senders may have different operational resources and different operational rights in practice, and the controlled protocol with many controllers is able to enhance security and increase applications of remote implementations of quantum operations in engineering, since it has some common features in a controlled process.

We have investigated the effect of weakly coupled hot plasmas on the bound states and resonance states of the $pp\mu$ molecular ion. The plasma effect is taken care of by employing a screened Coulomb (Yukawa-type) potential, and highly correlated wave functions are used. The ${}^{1}S^e$ and ${}^{3}P^o$ resonances of the $pp\mu$ molecular ion in plasmas for different screening parameters below the $p\mu \left(2S\right)$ threshold are calculated using the stabilization method. The bound ${}^{1}S^e$ and ${}^{3}P^o$ state energies of the $pp\mu$ molecular ion are determined using a variational method. The $p\mu \left(1S\right)$ and $p\mu \left(2S\right)$ threshold energies for various screening parameters are also reported here.

Ion-induced molecular fragmentation of $\mathrm{CO}$ has been studied using time-of-flight mass spectroscopy with position sensitive detectors in multihit coincidence mode. ${\mathrm{Ar}}^{8+}$ ions having a velocity of 1.1 a.u. were used as projectiles. The features observed in the kinetic energy release (KER) spectra for all the detected fragmentation channels are discussed in light of the existing and calculated ab initio potential energy curves. The preference of the symmetric breakup over the asymmetric one is clearly observed. For fragmentation channels originating from the same parent molecular ion, it is observed that the most probable KER value is higher for the dissociation channel having a higher charge on the oxygen ion. Occurrence of sharp peaks in KER spectra of some of the fragmentation channels hints towards the existence of shallow (possibly metastable) excited states of ${\mathrm{CO}}^{q+}$ $(q=4,5)$ molecular ions and calls for further theoretical investigations.

We study the confining properties of two-dimensional quantum-reflection traps, which are important for the transverse motion in atomic waveguides. For square geometry, the effect of nonseparability of the Schrödinger equation due to the corners is shown to be small. The survival probability due to quantum reflection is very similar for square and circular geometries.

In this paper we demonstrate a technique of utilizing magnetically trapped neutral ${}^{87}\mathrm{Rb}$ atoms to measure the magnitude and direction of stray electric fields emanating from surface contaminants. We apply an alternating external electric field that adds to (or subtracts from) the stray field in such a way as to resonantly drive the trapped atoms into a mechanical dipole oscillation. The growth rate of the oscillation’s amplitude provides information about the magnitude and sign of the stray field gradient. Using this measurement technique, we are able to reconstruct the vector electric field produced by surface contaminants. In addition, we can accurately measure the electric fields generated from adsorbed atoms purposely placed onto the surface and account for their systematic effects, which can plague a precision surface-force measurement. We show that baking the substrate can reduce the electric fields emanating from adsorbate and that the mechanism for reduction is likely surface diffusion, not desorption.

We study phonon-mediated transitions between translational levels of an atom in a surface-induced potential. We present a general master equation governing the dynamics of the translational states of the atom. In the framework of the Debye model, we derive compact expressions for the rates for both upward and downward transitions. Numerical calculations for the transition rates are performed for a deep silica-induced potential allowing for a large number of bound levels as well as free states of a cesium atom. The total absorption rate is shown to be determined mainly by the bound-to-bound transitions for deep bound levels and by bound-to-free transitions for shallow bound levels. Moreover, the phonon emission and absorption processes can be orders of magnitude larger for deep bound levels as compared to the shallow bound ones. We also study various types of transitions from free states. We show that, for thermal atomic cesium with a temperature in the range from $100\mu \mathrm{K}\text{to}400\mu \mathrm{K}$ in the vicinity of a silica surface with a temperature of $300\mathrm{K}$, the adsorption (free-to-bound decay) rate is about two times larger than the heating (free-to-free upward decay) rate, while the cooling (free-to-free downward decay) rate is negligible.

We analyze the dynamics of a two-level system subject to driving by large-amplitude external fields, focusing on the resonance properties in the case of driving around the region of avoided level crossing. In particular, we consider three main questions that characterize resonance dynamics: (1) the resonance condition, (2) the frequency of the resulting oscillations on resonance, and (3) the width of the resonance. We identify the regions of validity of different approximations. In a large region of the parameter space, we use a geometric picture in order to obtain both a simple understanding of the dynamics and quantitative results. The geometric approach is obtained by dividing the evolution into discrete time steps, with each time step described by either a phase shift on the basis states or a coherent mixing process corresponding to a Landau-Zener crossing. We compare the results of the geometric picture with those of a rotating wave approximation. We also comment briefly on the prospects of employing strong driving as a useful tool to manipulate two-level systems.

We consider the thermodynamics of a homogeneous superfluid dilute Bose gas in the presence of weak quenched disorder. Following the zero-temperature approach of Huang and Meng, we diagonalize the Hamiltonian of a dilute Bose gas in an external random $\delta$-correlated potential by means of a Bogoliubov transformation. We extend this approach to finite temperature by combining the Popov and the many-body $T$-matrix approximations. This approach permits us to include the quasiparticle interactions within this temperature range. We derive the disorder-induced shifts of the Bose-Einstein critical temperature and of the temperature for the onset of superfluidity by approaching the transition points from below, i.e., from the superfluid phase. Our results lead to a phase diagram consistent with that of the finite-temperature theory of Lopatin and Vinokur which was based on the replica method, and in which the transition points were approached from above.

We present exact solutions for two nonlinear models each of which describes parametric down conversion of photons as well as the Kerr effect. The Hamiltonians for these models are related to the dual Hahn finite orthogonal polynomials. Explicit expressions are obtained for the spectra and for the eigenvectors of the Hamiltonians. A discussion of the physical characteristics of the systems is presented.

We study the influence of a lossless material medium on the coherent storage and quantum-state transfer of a quantized probe light in an ensemble of $\Lambda$-type atoms. The medium is modeled as uniformly distributed two-level atoms with the same energy level spacing, coupling to a probe light. This coupled system can be simplified to a collection of two-mode polaritons which couple to one transition of the $\Lambda$-type atoms. We show that, when the other transition of $\Lambda$-type atoms is controlled by a classical light, electromagnetically induced transparency can also occur for the polaritons. In this case the coherent storage and quantum transfer for photon states are achievable through novel dark states with respect to the polaritons. By calculating the corresponding dispersion relation, we find that the ensemble of three-level atoms with $\Lambda$-type transitions may serve as quantum memory for it slows or even stops light propagation through the mechanism of electromagnetically induced transparency.

Using a rapidly convergent composite basis of Frankowski-Pekeris and Frankowski functions, we have accurately calculated the nodal surfaces of low-lying excited states of the helium atom to investigate Bressanini and Reynolds’ conjecture [D. Bressanini and P. J. Reynolds, Phys. Rev. Lett. 95, 110201 (2005)] that these nodal surfaces are rigorously independent of the interelectronic angle ${\theta }_{12}$. We find that in fact there is a slight dependence of the nodal surfaces on ${\theta }_{12}$, but it is so small that the assumption of strict independence may well yield extremely useful approximations in fixed-node quantum Monte Carlo calculations. We explain how Kato’s cusp conditions determine the qualitative features of these nodal surfaces, which can accurately be modeled using the familiar ansatz of a symmetric or antisymmetric linear combination of products of hydrogenic orbitals, with some adjustments of the parameters. We explain why a similar near independence of the nodal surfaces on the angular variables can be expected for the ground and singly excited states of the lithium atom, but generally not for larger atoms.

Given a quantum channel $\Phi$ in a Hilbert space $H$, set ${\widehat{H}}_{\Phi }\left(\rho \right)={\min }_{{\rho }_{av}=\rho }{\Sigma }_{j=1}^k{\pi }_{j}S\mathbf{\left(}\Phi \left({\rho }_j\right)\mathbf{\right)}$, where ${\rho }_{av}={\Sigma }_{j=1}^k{\pi }_j{\rho }_j$, the minimum is taken over all probability distributions $\pi =\left\{{\pi }_j\right\}$ and states ${\rho }_j$ in $H$, and $S\left(\rho \right)=-\mathrm{Tr}\rho \log \rho$ is the von Neumann entropy of a state $\rho$. The strong superadditivity conjecture states that ${\widehat{H}}_{\Phi \otimes \Psi }\left(\rho \right)⩾{\widehat{H}}_{\Phi }\mathbf{\left(}{\mathrm{Tr}}_K\left(\rho \right)\mathbf{\right)}+{\widehat{H}}_{\Psi }\mathbf{\left(}{\mathrm{Tr}}_H\left(\rho \right)\mathbf{\right)}$ for two channels $\Phi$ and $\Psi$ in Hilbert spaces $H$ and $K$, respectively. We have proved the strong superadditivity conjecture for the quantum depolarizing channel in any dimensions.