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Physical Review A

(Atomic, Molecular, and Optical Physics)

November 2009

Volume 80, Number 5 , partial issue

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RAPID COMMUNICATIONS

Fundamental concepts
Rapid

Electromagnetic interactions between parallel dielectric-diamagnetic cylinders
K. Tatur and L. M. Woods
Published 13 November 2009 (4 pages)
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The interaction energy due to electromagnetic field fluctuations between two infinitely long straight parallel dielectric-diamagnetic cylinders immersed in a medium is considered. We make use of the mode summation method for the calculations. We investigate the energy dependence on the cylindrical radial curvature and dielectric response of the involved materials. It is shown that the sign of the interaction energy can be changed by a suitable choice of the dielectric properties of the involved objects. The condition for the relation of the material's dielectric properties for repulsive interaction is obtained to be the same as the one for planar materials.
Rapid

Phase shift of an adiabatic rotating magnetic field in Ramsey atom interferometry for m=0 sodium-atom spin states
Atsushi Takahashi, Hiromitsu Imai, Kazuya Numazaki, and Atsuo Morinaga
Published 20 November 2009 (4 pages)
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The pi-phase shift between |F=1,mF=0> and |F=2,mF=0> states for an adiabatic rotation of a magnetic field observed in Ramsey atom interferometry arises from the negative sign of the transition amplitude between the |1,0> and |2,0> states when the wave functions are rotated to the opposite direction. Using a two-photon Raman atom interferometer with a cold ensemble of sodium atoms, the phase shift for a half-rotation of the magnetic field was confirmed to be pi  rad with an uncertainty of 4%, and the phase shift for a noncyclic adiabatic rotation of the magnetic field was investigated.

Quantum information
Rapid

Long-distance entanglement and quantum teleportation in coupled-cavity arrays
Salvatore M. Giampaolo and Fabrizio Illuminati
Published 6 November 2009 (4 pages)
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We introduce quantum spin models whose ground states allow for sizable entanglement between distant spins. We discuss how spin models with global end-to-end entanglement realize quantum teleportation channels with optimal compromise between scalability and resilience to thermal decoherence and can be implemented straightforwardly in suitably engineered arrays of coupled optical cavities.

Atomic and molecular collisions and interactions
Rapid

Radio-frequency dressing of multiple Feshbach resonances
A. M. Kaufman, R. P. Anderson, Thomas M. Hanna, E. Tiesinga, P. S. Julienne, and D. S. Hall
Published 3 November 2009 (4 pages)
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We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in 87Rb using radio-frequency (rf) radiation. We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.
Rapid

Intershell trielectronic recombination with K-shell excitation in Kr30+
C. Beilmann, O. Postavaru, L. H. Arntzen, R. Ginzel, C. H. Keitel, V. Mäckel, P. H. Mokler, M. C. Simon, H. Tawara, I. I. Tupitsyn, J. Ullrich, J. R. Crespo López-Urrutia, and Z. Harman
Published 4 November 2009 (4 pages)
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We report the observation of trielectronic recombination with simultaneous excitation of a K-shell and an L-shell electron, hence involving three active electrons. This process was identified in the x-ray emission spectrum of recombining highly charged Kr ions. An energy resolution three times higher than any reported for this collision energy range around 10 keV resulted in the separation of the associated lines from the stronger dielectronic resonances. For Kr30+, intershell trielectronic recombination contributions of nearly 6% to the total resonant photorecombination rate were found.

Clusters (including fullerenes)
Rapid

Efficient fusion neutron generation from heteronuclear clusters in intense femtosecond laser fields
H. Y. Lu, J. S. Liu, C. Wang, W. T. Wang, Z. L. Zhou, A. H. Deng, C. Q. Xia, Y. Xu, X. M. Lu, Y. H. Jiang, Y. X. Leng, X. Y. Liang, G. Q. Ni, R. X. Li, and Z. Z. Xu
Published 16 November 2009 (4 pages)
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We demonstrate experimentally the efficient fusion neutron generation from Coulomb explosion (CE) of laser irradiated large-size heteronuclear deuterated methane clusters. A conversion efficiency of 2.1×106  neutrons/J of incident laser energy is obtained with a 120 mJ, 70 fs laser pulse. It is 50 times higher than that of homonuclear deuterium clusters of similar size. This enhancement is attributed to the significant increase in the deuteron kinetic energies by fourfold due to energetic boosting and overrun effects during CE of heteronuclear clusters. The yield of 5.5×106 neutrons per pulse is obtained with a 100 TW, 50 fs driving laser pulse at an intensity of 1.5×1019  W/cm2. This work may facilitate the development of a high-flux table-top neutron source.

Atomic and molecular processes in external fields, including interactions with strong fields and short pulses
Rapid

Vibrational cooling of cesium molecules using noncoherent broadband light
Dimitris Sofikitis, Ridha Horchani, Xiaolin Li, Marin Pichler, Maria Allegrini, Andrea Fioretti, Daniel Comparat, and Pierre Pillet
Published 11 November 2009 (4 pages)
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We demonstrate selective vibrational population transfer in cold cesium dimers using a simple approach based on the use of a shaped incoherent broadband diode laser near threshold. Optical pumping into a single vibrational level is accomplished with an incoherent light source by eliminating transitions from the targeted vibrational level. The broadband spectrum of the laser is wide enough to electronically excite several vibrational states of the molecule simultaneously. This method is relatively inexpensive, simple, and flexible to allow for development of new applications, in particular, the preparation of optically closed molecular system, opening the way to direct laser cooling of molecules.
Rapid

Analysis of angular dependence of strong-field tunneling ionization for CO2
Song-Feng Zhao, Cheng Jin, Anh-Thu Le, T. F. Jiang, and C. D. Lin
Published 17 November 2009 (4 pages)
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We analyzed the discrepancy of the angular dependence of strong-field ionization for CO2 among the different theoretical calculations and experiments. Using a more accurate ground-state wave function of CO2 in the asymptotic region, we showed that the accuracy in the earlier tunneling ionization theory of Tong et al. [Phys. Rev. A 66, 033402 (2002)] is much improved. We also concluded that the angular dependence deduced from the experiment of Pavičić et al. [Phys. Rev. Lett. 98, 243001 (2007)] appears to be too narrowly distributed.
Rapid

Field-free permanent molecular planar alignment
M. Lapert, E. Hertz, S. Guérin, and D. Sugny
Published 20 November 2009 (4 pages)
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We show the existence of a permanent molecular planar alignment in field-free conditions. We present different control strategies using shaped laser pulses to reach this state. The strategies are robust with respect to the temperature and can be implemented with the state of the art technology. They can be applied not only to linear molecules but also to symmetric or asymmetric top molecules along the most polarizable molecular axis. We propose potential applications of this planar alignment such as the increase of the adsorption on a surface.

Matter waves and collective properties of cold atoms and molecules
Rapid

Stable fractional vortices in the cyclic states of Bose-Einstein condensates
J. A. M. Huhtamäki, T. P. Simula, M. Kobayashi, and K. Machida
Published 4 November 2009 (4 pages)
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We propose methods to create fractional vortices in the cyclic state of an F=2 spinor Bose-Einstein condensate by manipulating its internal spin structure using pulsed microwave and laser fields. The stability of such vortices is studied as a function of the rotation frequency of the confining harmonic trap both in pancake- and cigar-shaped condensates. We find a range of parameters for which the so-called (1/3) vortex state is energetically favorable. Such fractional vortices could be created in condensates of 87Rb atoms using current experimental techniques facilitating probing of topological defects with non-Abelian statistics.
Rapid

Mott transitions in ternary flavor mixtures of ultracold fermions on optical lattices
E. V. Gorelik and N. Blümer
Published 11 November 2009 (4 pages)
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Ternary flavor mixtures of ultracold fermionic atoms in an optical lattice are studied in the case of equal repulsive on-site interactions U>0. The corresponding SU(3) invariant Hubbard model is solved numerically exactly within dynamical mean-field theory using multigrid Hirsch-Fye quantum Monte Carlo simulations. We establish Mott transitions close to integer filling at low temperatures and show that the associated signatures in the compressibility and pair occupancy persist to high temperatures, i.e., they should be accessible to experiments. In addition, we present spectral functions and discuss the properties of a “semicompressible” state observed for large U near half filling.
Rapid

Quantum turbulence cascades in the Gross-Pitaevskii model
Davide Proment, Sergey Nazarenko, and Miguel Onorato
Published 12 November 2009 (4 pages)
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We present a numerical study of quantum turbulence within the three-dimensional Gross-Pitaevskii equation, concentrating on the direct energy cascade in the case of a forced-dissipated system. We show that the behavior of the system is very sensitive to the properties of the model at the scales greater than the forcing scale, and we identify three different regimes: (1) a nonstationary regime with condensation and transition from a four-wave to a three-wave interaction process when the largest scales are not dissipated, (2) a steady weak wave turbulence regime when largest scales are dissipated with a friction-type dissipation, and (3) a state with a scale-by-scale balance of the linear and the nonlinear time scales when the large-scale dissipation is a hypoviscosity.

Quantum optics, physics of lasers, nonlinear optics, classical optics
Rapid

Impact of amplifying media on the Casimir force
Agnes Sambale, Stefan Yoshi Buhmann, Ho Trung Dung, and Dirk-Gunnar Welsch
Published 9 November 2009 (4 pages)
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On the basis of macroscopic quantum electrodynamics, a theory of Casimir forces in the presence of linearly amplifying bodies is presented which provides a consistent framework for studying the effect of, e.g., amplifying left-handed metamaterials on dispersion forces. It is shown that the force can be given in terms of the classical Green tensor and that it can be decomposed into a resonant component associated with emission processes and an off-resonant Lifshitz-type component. We explicitly demonstrate that our theory extends additive approaches beyond the dilute-gas limit.
Rapid

Attraction and repulsion of parallel femtosecond filaments in air
Hua Cai, Jian Wu, Peifen Lu, Xueshi Bai, Liang'en Ding, and Heping Zeng
Published 16 November 2009 (4 pages)
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We present an approach to explore and control nonlinear interactions between two orthogonally polarized femtosecond filaments launched parallel in air. The self-phase and cross-phase modulations due to the Kerr effect and cross-(de)focusing induced by the plasma and molecular alignment were distinctly identified resulting in attraction and repulsion of parallel filaments with different spatiotemporal proximities. Their interaction ranges were analyzed by comparing the interaction-induced displacements of parallel filaments at different initial separations. As proved by field-free displacements of parallel filaments around the impulsive rotational Raman excitation of the diatomic molecules in air, the molecular alignment exhibited a relatively longer interaction range than that of the plasma defocusing and the Kerr effect.

ARTICLES

Fundamental concepts

Smearing effect due to the spread of a probe particle on the Brownian motion near a perfectly reflecting boundary
Masafumi Seriu and Chun-Hsien Wu
Published 4 November 2009 (8 pages)
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Quantum fluctuations of an electromagnetic vacuum are investigated in a half-space bounded by a perfectly reflecting plate by introducing a probe described by a charged wave-packet distribution in time direction. The wave-packet distribution of the probe enables one to investigate the smearing effect upon the measured vacuum fluctuations caused by the quantum nature of the probe particle. It is shown that the wave-packet spread of the probe particle significantly influences the measured velocity dispersion of the probe. In particular, the asymptotic late-time behavior of its z component <Deltavz<sup>2</sup>> for the wave-packet case is quite different from the test point-particle case (z is the coordinate normal to the plate). The result for the wave packet is <Deltavz<sup>2</sup>>~1/tau2 at late time (tau is the measuring time), instead of the reported late-time behavior <Deltavz<sup>2</sup>>~1/z2 for a point-particle probe. This result can be quite significant for further investigations on the measurement of vacuum fluctuations.

Unambiguous comparison of quantum measurements
Mario Ziman, Teiko Heinosaari, and Michal Sedlák
Published 5 November 2009 (10 pages)
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The goal of comparison is to reveal the difference of compared objects as fast and reliably as possible. In this paper we formulate and investigate the unambiguous comparison of unknown quantum measurements represented by nondegenerate sharp positive operator valued measures. We distinguish between measurement devices with a priori labeled and unlabeled outcomes. In both cases we can unambiguously conclude only that the measurements are different. For the labeled case it is sufficient to use each unknown measurement only once and the average conditional success probability decreases with the Hilbert space dimension as 1/d. If the outcomes of the apparatuses are not labeled, then the problem is more complicated. We analyze the case of two-dimensional Hilbert space. In this case single shot comparison is impossible and each measurement device must be used (at least) twice. The optimal test state in the two-shot scenario gives the average conditional success probability 4/9. Interestingly, the optimal experiment detects unambiguously the difference with nonvanishing probability for any pair of observables.

Generalized Liouville time-dependent perturbation theory
Jeremy Moix, Eli Pollak, and Jiushu Shao
Published 5 November 2009 (4 pages)
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A generalized time-dependent perturbation theory is derived for superoperators. Instead of using the “standard” breakup of the Hamiltonian into a known zeroth order term and a correction, we use the approximate superpropagator to define the correction superoperator which is then used to obtain a series representation of the exact Liouville operator. The theory reduces to known limits and may be used for a perturbation expansion of classical Wigner and Husimi dynamics as well as for recent phase-space-based semiclassical approximations. The theory is demonstrated for a model quartic potential.

Area law for the entropy of low-energy states
Lluís Masanes
Published 10 November 2009 (8 pages)
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It is often observed in the ground state of quantum lattice systems with local interactions that the entropy of a large region is proportional to its surface area. In some cases, this area law is corrected with a logarithmic factor. This contrasts with the fact that in almost all states of the Hilbert space, the entropy of a region is proportional to its volume. This paper shows that low-energy states have (at most) an area law with the logarithmic correction, provided two conditions hold: (i) the state has sufficient decay of correlations and (ii) the number of eigenstates with vanishing energy density is not exponential in the volume. These two conditions are satisfied by many relevant systems. The central idea of the argument is that energy fluctuations inside a region can be observed by measuring the exterior and a superficial shell of the region.

Effects of squeezing on quantum nonlocality of superpositions of coherent states
Chang-Woo Lee and Hyunseok Jeong
Published 10 November 2009 (6 pages)
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We analyze effects of squeezing upon superpositions of coherent states (SCSs) and entangled coherent states for Bell-inequality tests. We find that external squeezing can always increase the degrees of Bell violations, if the squeezing direction is properly chosen, for the case of photon parity measurements. On the other hand, when photon on/off measurements are used, the squeezing operation can enhance the degree of Bell violations only for moderate values of amplitudes and squeezing. We point out that a significant improvement is required over currently available squeezed SCSs in order to directly demonstrate a Bell-inequality violation in a real experiment.

Genuine tripartite entanglement in quantum brachistochrone evolution of a three-qubit system
Bao-Kui Zhao, Fu-Guo Deng, Feng-Shou Zhang, and Hong-Yu Zhou
Published 11 November 2009 (6 pages)
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We explore the connection between quantum brachistochrone (time-optimal) evolution of a three-qubit system and its residual entanglement called three-tangle. The result shows that the entanglement between two qubits is not required for some brachistochrone evolutions of a three-qubit system. However, the evolution between two distinct states cannot be implemented without its three-tangle, except for the trivial cases in which less than three qubits attend evolution. Although both the probability density function of the time-averaged three-tangle and that of the time-averaged squared concurrence between two subsystems become more and more uniform with the decrease in angles of separation between an initial state and a final state, the features of their most probable values exhibit a different trend.

Solutions of [script P][script T]-symmetric tight-binding chain and its equivalent Hermitian counterpart
L. Jin and Z. Song
Published 11 November 2009 (7 pages)
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We study the non-Hermitian quantum mechanics for the discrete system. This paper gives an exact analytic single-particle solution for an N-site tight-binding chain with two conjugated imaginary potentials ±igamma at two end sites, which Hamiltonian has parity-time symmetry ([script P][script T] symmetry). Based on the Bethe ansatz results, it is found that, in single-particle subspace, this model is comprised of two phases: an unbroken symmetry phase with a purely real energy spectrum in the region gamma[precedes]gammac and a spontaneously broken symmetry phase with N−2 real and two imaginary eigenvalues in the region gamma[succeeds]gammac. The behaviors of eigenfunctions and eigenvalues in the vicinity of gammac are investigated. It is shown that the boundary of two phases possesses the characteristics of exceptional point. We also construct the equivalent Hermitian Hamiltonian of the present model in the framework of metric-operator theory. We find out that the equivalent Hermitian Hamiltonian can be written as another bipartite lattice model with real long-range hoppings.

Reconstruction of Markovian master equation parameters through symplectic tomography
Bruno Bellomo, Antonella De Pasquale, Giulia Gualdi, and Ugo Marzolino
Published 12 November 2009 (6 pages)
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In open quantum systems, phenomenological master equations with unknown parameters are often introduced. Here we propose a time-independent procedure based on quantum tomography to reconstruct the potentially unknown parameters of a wide class of Markovian master equations. According to our scheme, the system under investigation is initially prepared in a Gaussian state. At an arbitrary time t, in order to retrieve the unknown coefficients one needs to measure only a finite number (ten at maximum) of points along three time-independent tomograms. Due to the limited amount of measurements required, we expect our proposal to be especially suitable for experimental implementations.

Quantum mechanics of a constrained electrically charged particle in the presence of electric currents
Bjørn Jensen and Rossen Dandoloff
Published 17 November 2009 (6 pages)
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We discuss the dynamics of a classical spinless quantum particle carrying electric charge and constrained to move on a nonsingular static surface in ordinary three-dimensional space in the presence of arbitrary configurations of time independent electric currents. Starting from the canonical action in the embedding space we show that a charged particle with charge q couples to a term linear in qA3M, where A3 is the transverse component of the electromagnetic vector potential and M is the mean curvature in the surface. This term cancels exactly a curvature contribution to the orbital magnetic moment of the particle. It is shown that particles, independently of the value of the charge, in addition to the known couplings to the geometry also couple to the mean curvature in the surface when a Neumann type of constraint is applied on the transverse fluctuations of the wave function. In contrast to a Dirichlet constraint on the transverse fluctuations a Neumann type of constraint on these degrees of freedom will in general make the equations of motion nonseparable. The exceptions are the equations of motion for electrically neutral particles on surfaces with constant mean curvature. In the presence of electric currents the equation of motion of a charged particle is generally nonseparable independently of the coupling to the geometry and the boundary constraints.

Long quantum transition times due to unstable semiclassical dynamics
D. G. Levkov and A. G. Panin
Published 18 November 2009 (17 pages)
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Quantum transitions are described semiclassically as motions of systems along (complex) trajectories. We consider the cases when the semiclassical trajectories are unstable and find that durations of the corresponding transitions are large. In addition, we show that the probability distributions over transition times have unusual asymmetric form in cases of unstable trajectories. We investigate in detail three types of processes related to unstable semiclassical dynamics. First, we analyze recently proposed mechanism of multidimensional tunneling where transitions proceed by formation and subsequent decay of classically unstable “states.” The second class of processes includes one-dimensional activation transitions due to energy dispersion. In this case the semiclassical transition-time distributions have universal form. Third, we investigate long-time asymptotics of transition-time distributions in the case of overbarrier wave packet transmissions. We show that behavior of these asymptotics is controlled by unstable semiclassical trajectories which linger near the barrier top.

State tomography of a qubit through scattering of a probe qubit
Antonella De Pasquale, Kazuya Yuasa, and Hiromichi Nakazato
Published 18 November 2009 (6 pages)
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We discuss the state tomography of a fixed qubit (a spin-1/2 target particle), which is in general in a mixed state, through one-dimensional scattering of a probe qubit off the target. Two strategies are presented by making use of different degrees of freedom of the probe, i.e. spin and momentum. Remarkably, the spatial degree of freedom of the probe can be utilized for optimizing the tomographic schemes.

Environmentally induced shift of the quantum arrival time
M. Genkin, F. Ferro, and E. Lindroth
Published 18 November 2009 (6 pages)
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Using a simple model potential, we study the effects of weak Markovian dissipation on the quantum arrival time. The interaction with the environment is incorporated into the dynamics through a Markovian master equation of Lindblad type, which allows us to compare time-of-arrival distributions and approximate crossing probabilities for different dissipation strengths and temperatures. We also establish a connection to an earlier study where quantum tunneling with dissipation was investigated, which leads us to some conclusions concerning the formulation of the continuity equation in the Lindblad theory.

Extraction of a squeezed state in a field mode via repeated measurements on an auxiliary quantum particle
Bruno Bellomo, Giuseppe Compagno, Hiromichi Nakazato, and Kazuya Yuasa
Published 20 November 2009 (7 pages)
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The dynamics of a system, consisting of a particle initially in a Gaussian state interacting with a field mode, under the action of repeated measurements performed on the particle, is examined. It is shown that regardless of its initial state the field is distilled into a squeezed state. The dependence on the physical parameters of the dynamics is investigated.

Quantum information

Equivalence between discrete quantum walk models in arbitrary topologies
F. M. Andrade and M. G. E. da Luz
Published 2 November 2009 (4 pages)
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Coin and scattering are the two major formulations for discrete quantum walks models, each believed to have its own advantages in different applications. Although they are related in some cases, it was an open question their equivalence in arbitrary topologies. Here we present a general construction for the two models for any graph and also for position dependent transition amplitudes. We then prove constructively their unitary equivalence. Defining appropriate projector operators, we moreover show how to obtain the probabilities for one model from the evolution of the other.

Preparation of Dicke states in an ion chain
D. B. Hume, C. W. Chou, T. Rosenband, and D. J. Wineland
Published 2 November 2009 (5 pages)
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We have investigated theoretically and experimentally a method for preparing Dicke states in trapped atomic ions. We consider a linear chain of N ion qubits that is prepared in a particular Fock state of motion |m>. The m phonons are removed by applying a laser pulse globally to the N qubits and converting the motional excitation to m flipped spins. The global nature of this pulse ensures that the m flipped spins are shared by all the target ions in a state that is a close approximation to the Dicke state |DN<sup>(m)</sup>>. We calculate numerically the fidelity limits of the protocol and find small deviations from the ideal state for m=1 and m=2. We have demonstrated the basic features of this protocol by preparing the Bell state |D2<sup>(1)</sup>> in two 25Mg+ target ions trapped simultaneously with an 27Al+ ancillary ion.

Inseparability criteria based on matrices of moments
Adam Miranowicz, Marco Piani, Pawel Horodecki, and Ryszard Horodecki
Published 3 November 2009 (10 pages)
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Inseparability criteria for continuous and discrete bipartite quantum states based on moments of annihilation and creation operators are studied by developing the idea of Shchukin-Vogel criterion [Phys. Rev. Lett. 95, 230502 (2005)]. If a state is separable, then the corresponding matrix of moments is separable too. Thus, we derive generalized criteria based on the separability properties of the matrix of moments. In particular, a criterion based on realignment of moments in the matrix is proposed as an analog of the standard realignment criterion for density matrices. Other inseparability inequalities are obtained by applying positive maps to the matrix of moments. Usefulness of the Shchukin-Vogel criterion to describe bipartite-entanglement of more than two modes is demonstrated: we obtain various three-mode inseparability criteria, including some previously known ones, which were originally derived from the Cauchy-Schwarz inequality.

Quantum discord between relatively accelerated observers
Animesh Datta
Published 4 November 2009 (5 pages)
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We calculate the quantum discord between two free modes of a scalar field, which start in a maximally entangled state and then undergo a relative constant acceleration. In a regime where there is no distillable entanglement due to the Unruh effect, we show that there is a finite amount of quantum discord, which is a measure of purely quantum correlations in a state over and above quantum entanglement. Even in the limit of infinite acceleration of the observer detecting one of the modes, we provide evidence for a nonzero amount of purely quantum correlations. We discuss our result in the context of secure quantum communications involving eavesdroppers in noninertial frames.

Minimum error discrimination problem for pure qubit states
Boris F. Samsonov
Published 4 November 2009 (11 pages)
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The necessary and sufficient conditions for minimization of the generalized rate error for discriminating among N pure qubit states are reformulated in terms of Bloch vectors representing the states. For the direct optimization problem, an algorithmic solution to these conditions is indicated. A solution to the inverse optimization problem is given. General results are widely illustrated by particular cases of equiprobable states and N=2,3,4 pure qubit states given with different prior probabilities.

Power of symmetric extensions for entanglement detection
Miguel Navascués, Masaki Owari, and Martin B. Plenio
Published 6 November 2009 (15 pages)
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In this paper, we present progress on the study of the symmetric extension criterion for separability. First, we show that a perturbation of order O(1/N) is sufficient and, in general, necessary to destroy the entanglement of any state admitting an N Bose-symmetric extension. On the other hand, the minimum amount of local noise necessary to induce separability on states arising from N Bose-symmetric extensions with positive partial transpose (PPT) decreases at least as fast as O(1/N2). From these results, we derive upper bounds on the time and space complexity of the weak membership problem of separability when attacked via algorithms that search for PPT-symmetric extensions. Finally, we show how to estimate the error we incur when we approximate the set of separable states by the set of (PPT) N-extendable quantum states in order to compute the maximum average fidelity in pure state estimation problems, the maximal output purity of quantum channels, and the geometric measure of entanglement.

General algorithm for manipulating nonlinear and linear entanglement witnesses by using exact convex optimization
M. A. Jafarizadeh, K. Aghayar, and A. Heshmati
Published 9 November 2009 (13 pages)
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We consider the problem of obtaining the entanglement witnesses for a given quantum system. We present a method to convert this problem to a standard convex optimization problem by defining a feasible region. Then we develop a generic two-step algorithm for this problem which can be applied to the entanglement detection of N-partite quantum system. This permits an organized construction of nonlinear and linear entanglement witnesses for a given quantum system. To demonstrate the capability of this approach, we apply our results to several three-qubit systems which in some cases lead to necessary and sufficient conditions for separability.

High-performance diamond-based single-photon sources for quantum communication
Chun-Hsu Su, Andrew D. Greentree, and Lloyd C. L. Hollenberg
Published 9 November 2009 (11 pages)
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Quantum communication places stringent requirements on single-photon sources. Here we report a theoretical study of the cavity Purcell enhancement of two diamond point defects, the nickel-nitrogen (NE8) and silicon-vacancy (SiV) centers, for high-performance, near on-demand single-photon generation. By coupling the centers strongly to high-finesse optical photonic-band-gap cavities with modest quality factor Q=O(104) and small mode volume V=O(lambda3), these system can deliver picosecond single-photon pulses at their zero-phonon lines with probabilities of 0.954 (NE8) and 0.812 (SiV) under a realistic optical excitation scheme. The undesirable blinking effect due to transitions via metastable states can also be suppressed with O(10−4) blinking probability. We analyze the application of these enhanced centers, including the previously studied cavity-enhanced nitrogen-vacancy (NV) center, to long-distance Bennett-Brassard 1984 protocol quantum key distribution (QKD) in fiber-based, open-air terrestrial and satellite-ground setups. In this comparative study, we show that they can deliver performance comparable with decoy state implementation with weak coherent sources, and are most suitable for open-air communication.

Weight of quadratic forms and graph states
Alessandro Cosentino and Simone Severini
Published 9 November 2009 (6 pages)
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We prove a connection between Schmidt rank and weight of quadratic forms. This provides a new tool for the classification of graph states based on entanglement. Our main tool arises from a reformulation of previously known results concerning the weight of quadratic forms in terms of graph states properties. As a byproduct, we obtain a straightforward characterization of the weight of functions associated with pivot-minor of bipartite graphs.

Gaussian-state quantum-illumination receivers for target detection
Saikat Guha and Baris I. Erkmen
Published 10 November 2009 (4 pages)
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The signal half of an entangled twin beam, generated using spontaneous parametric downconversion, interrogates a region of space that is suspected of containing a target and has high loss and high (entanglement-breaking) background noise. A joint measurement is performed on the returned light and the idler beam that was retained at the transmitter. An optimal quantum receiver, whose implementation is not yet known, was shown to achieve 6 dB gain in the error-probability exponent relative to that achieved with a single coherent-state (classical) laser transmitter and the optimum receiver. We present two structured optical receivers that achieve up to 3 dB gain in the error exponent over that attained with the classical sensor. These are designs of quantum-optical sensors for target detection, which can be readily implemented in a proof-of-concept experiment, that appreciably outperform the best classical sensor in the low-signal-brightness, high-loss, and high-noise operating regime.

Geometric quantum gates in liquid-state NMR based on a cancellation of dynamical phases
Yukihiro Ota, Yoshito Goto, Yasushi Kondo, and Mikio Nakahara
Published 10 November 2009 (7 pages)
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A proposal for applying nonadiabatic geometric phases to quantum computing, called double-loop method [S.-L. Zhu and Z. D. Wang, Phys. Rev. A 67, 022319 (2003)], is demonstrated in a liquid-state nuclear magnetic-resonance quantum computer. Using a spin-echo technique, the original method is modified so that quantum gates are implemented in a standard high-precision nuclear magnetic-resonance system for chemical analysis. We show that a dynamical phase is successfully eliminated and a one-qubit quantum gate is realized although the gate fidelity is not high.

High-threshold universal quantum computation on the surface code
Austin G. Fowler, Ashley M. Stephens, and Peter Groszkowski
Published 11 November 2009 (14 pages)
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We present a comprehensive and self-contained simplified review of the quantum computing scheme of Raussendorf et al. [Phys. Rev. Lett. 98, 190504 (2007); N. J. Phys. 9, 199 (2007)], which features a two-dimensional nearest-neighbor coupled lattice of qubits, a threshold error rate approaching 1%, natural asymmetric and adjustable strength error correction, and low overhead arbitrarily long-range logical gates. These features make it one of the best and most practical quantum computing schemes devised to date. We restrict the discussion to direct manipulation of the surface code using the stabilizer formalism, both of which we also briefly review, to make the scheme accessible to a broad audience.

Optimal Lewenstein-Sanpera decomposition of two-qubit states using semidefinite programming
Guo Chuan Thiang, Philippe Raynal, and Berthold-Georg Englert
Published 11 November 2009 (6 pages)
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We use the language of semidefinite programming and duality to derive necessary and sufficient conditions for the optimal Lewenstein-Sanpera decomposition (LSD) of two-qubit states. We first provide a simple and natural derivation of the Wellens-Kuś equations for full-rank states. Then, we obtain a set of necessary and sufficient conditions for the optimal decomposition of rank-3 states. This closes the gap between the full-rank case, where optimality conditions are given by the Wellens-Kuś equations, and the rank-2 case, where the optimal decomposition is analytically known. We also give an analytic expression for the optimal LSD of a special class of rank-3 states. Finally, our formulation ensures efficient numerical procedures to return the optimal LSD for any arbitrary two-qubit state.

Learning and testing algorithms for the Clifford group
Richard A. Low
Published 11 November 2009 (6 pages)
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Given oracle access to an unknown unitary C from the Clifford group and its conjugate, we give an exact algorithm for identifying C with O(n) queries, which we prove is optimal. We then extend this to all levels of the Gottesman-Chuang hierarchy (also known as the [script C]k hierarchy). Further, for unitaries not in the hierarchy itself but known to be close to an element of the hierarchy, we give a method of finding this close element. We also present a Clifford testing algorithm that decides whether a given black-box unitary is close to an element of the Clifford group or far from every element of the Clifford group.

Maximally entangled three-qubit states via geometric measure of entanglement
Sayatnova Tamaryan, Tzu-Chieh Wei, and DaeKil Park
Published 11 November 2009 (10 pages)
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Bipartite maximally entangled states have the property that the largest Schmidt coefficient reaches its lower bound. However, for multipartite states, the standard Schmidt decomposition generally does not exist. We use a generalized Schmidt decomposition and the geometric measure of entanglement to characterize three-qubit pure states and derive a single-parameter family of maximally entangled three-qubit states. The paradigmatic Greenberger-Horne-Zeilinger (GHZ) and W states emerge as extreme members in this family of maximally entangled states. This family of states possesses different trends of entanglement behavior: in going from GHZ to W states, the geometric measure, the relative entropy of entanglement, and the bipartite entanglement all increase monotonically whereas the three-tangle and bipartition negativity both decrease monotonically.

Driving quantum systems into decoherence-free subspaces by Lyapunov control
X. X. Yi, X. L. Huang, Chunfeng Wu, and C. H. Oh
Published 12 November 2009 (5 pages)
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We present a scheme to drive a finite-dimensional quantum system into the decoherence-free subspaces (DFS) by Lyapunov control. Control fields are established by Lyapunov function. This proposal works well for both closed and open quantum systems, with replacing the DFS by desired subspaces for closed systems. An example which consists of a four-level system with three degenerate states driven by three lasers is presented to gain further insight on the scheme. Numerical simulations for the dynamics of the system are performed and the results are good.

Realization of topological quantum computation with planar codes
Su-Peng Kou
Published 12 November 2009 (9 pages)
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In this paper, the degenerate ground states of Z2 topological order on a plane with holes (the so-called planar codes) are used as the protected code subspace to build a topological quantum computer by tuning their quantum tunneling effect. Using a designer Hamiltonian—the Kitaev toric-code model as an example, we study quantum tunneling effects of the planar codes and obtain its effective theory. Finally, we show how to do topological quantum computation including the initialization, the unitary transformation and the measurement.

Globally controlled universal quantum computation with arbitrary subsystem dimension
Gerardo A. Paz-Silva, Gavin K. Brennen, and Jason Twamley
Published 12 November 2009 (8 pages)
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We present a scheme to perform universal quantum computation using global control with an arbitrary subsystem dimension (not necessarily finite). The scheme is developed over a one spatial dimension N-element array, using only mirror-symmetric logical encoding, global pulses for single subsystem operations, uniform Ising-type interaction, and an all zero initial pure state. A mechanism using ancillary degrees of freedom for subsystem specific measurement is also presented.

Information propagation through quantum chains with fluctuating disorder
Christian K. Burrell, Jens Eisert, and Tobias J. Osborne
Published 12 November 2009 (5 pages)
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We investigate the propagation of information through one-dimensional nearest-neighbor interacting quantum spin chains in the presence of external fields which fluctuate independently on each site. We study two fundamentally different models: (i) a model with general nearest-neighbor interactions in a field which fluctuates in both strength and direction and (ii) the XX chain placed in a fluctuating field aligned in the z direction. In both cases we find that information propagation is suppressed in a way which is quite different from the suppression observed when the XX model is placed in a statically disordered field.

Optimum unambiguous discrimination of linearly independent pure states
Shengshi Pang (庞盛世) and Shengjun Wu (吴盛俊)
Published 12 November 2009 (14 pages)
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Given n linearly independent pure states and their prior probabilities, we study the optimum unambiguous state discrimination problem. We derive the conditions for the optimum measurement strategy to achieve the maximum average success probability and establish two sets of equations that must be satisfied by the optimum solution in different situations. We also provide the detailed steps to find the optimum measurement strategy. The method and results we obtain are given a geometrical illustration with a numerical example. Furthermore, using these equations, we derive a formula which shows a clear analytical relation between the optimum solution and the n states to be discriminated. We also solve a generalized equal-probability measurement problem analytically. Finally, as another application of our result, the unambiguous discrimination problem of three pure states is studied in detail and analytical solutions are obtained for some interesting cases.

Generation of polarization-entangled photons using type-II doubly periodically poled lithium niobate waveguides
K. Thyagarajan, J. Lugani, S. Ghosh, K. Sinha, A. Martin, D. B. Ostrowsky, O. Alibart, and S. Tanzilli
Published 13 November 2009 (8 pages)
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In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions for two spontaneous parametric down-conversion processes, namely, ordinary pump photon down conversion to either extraordinary signal and ordinary idler paired photons or to ordinary signal and extraordinary idler paired photons. In contrast to single type-II phase matching, these two processes, when enabled together, can lead to the direct production of cross-polarization-entangled states for nondegenerate signal and idler wavelengths. Such a scheme should be of great interest in applications requiring polarization-entangled nondegenerate paired photons with, for instance, one of the entangled photons at an appropriate wavelength being used for local operation or for quantum storage in an atomic ensemble and the other one at the typical wavelength of 1550 nm for propagation through an optical fiber.

Discrimination with error margin between two states: Case of general occurrence probabilities
H. Sugimoto, T. Hashimoto, M. Horibe, and A. Hayashi
Published 17 November 2009 (9 pages)
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We investigate a state discrimination problem which interpolates minimum error and unambiguous discrimination by introducing a margin for the probability of error. We closely analyze discrimination of two pure states with general occurrence probabilities. The optimal measurements are classified into three types. One of the three types of measurement is optimal depending on parameters (occurrence probabilities and error margin). We determine the three domains in the parameter space and the optimal discrimination success probability in each domain in a fully analytic form. It is also shown that when the states to be discriminated are multipartite, the optimal success probability can be attained by local operations and classical communication. For discrimination of two mixed states, an upper bound of the optimal success probability is obtained.

NMR multiple quantum coherences in quasi-one-dimensional spin systems: Comparison with ideal spin-chain dynamics
Wenxian Zhang, Paola Cappellaro, Natania Antler, Brian Pepper, David G. Cory, Viatcheslav V. Dobrovitski, Chandrasekhar Ramanathan, and Lorenza Viola
Published 17 November 2009 (13 pages)
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The 19F spins in a crystal of fluorapatite have often been used to experimentally approximate a one-dimensional spin system. Under suitable multipulse control, the nuclear-spin dynamics may be modeled to first approximation by a double-quantum one-dimensional Hamiltonian, which is analytically solvable for nearest-neighbor couplings. Here, we use solid-state nuclear magnetic resonance techniques to investigate the multiple quantum coherence dynamics of fluorapatite, with an emphasis on understanding the region of validity for such a simplified picture. Using experimental, numerical, and analytical methods, we explore the effects of long-range intrachain couplings, cross-chain couplings, as well as couplings to a spin environment, all of which tend to damp the oscillations of the multiple quantum coherence signal at sufficiently long times. Our analysis characterizes the extent to which fluorapatite can faithfully simulate a one-dimensional quantum wire.

Characterizing entanglement sources
Pavel Lougovski and S. J. van Enk
Published 18 November 2009 (11 pages)
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We discuss how to characterize entanglement sources with finite sets of measurements. The measurements do not have to be tomographically complete and may consist of POVMs rather than von Neumann measurements. Our method yields a probability that the source generates an entangled state as well as estimates of any desired calculable entanglement measures, including their error bars. We apply two criteria, namely, Akaike's information criterion and the Bayesian information criterion, to compare and assess different models (with different numbers of parameters) describing entanglement-generating devices. We discuss differences between standard entanglement-verification methods and our present method of characterizing an entanglement source.

Communication in quantum networks of logical bus topology
T. Brougham, G. M. Nikolopoulos, and I. Jex
Published 19 November 2009 (9 pages)
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Perfect state transfer (PST) is discussed in the context of passive quantum networks with logical bus topology, where many logical nodes communicate using the same shared media without any external control. The conditions under which a number of point-to-point PST links may serve as building blocks for the design of such multinode networks are investigated. The implications of our results are discussed in the context of various Hamiltonians that act on the entire network and are capable of providing PST between the logical nodes of a prescribed set in a deterministic manner.

Multimode states in decoy-based quantum-key-distribution protocols
Wolfram Helwig, Wolfgang Mauerer, and Christine Silberhorn
Published 19 November 2009 (8 pages)
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Every security analysis of quantum-key distribution (QKD) relies on a faithful modeling of the employed quantum states. Many photon sources, such as for instance a parametric down-conversion (PDC) source, require a multimode description but are usually only considered in a single-mode representation. In general, the important claim in decoy-based QKD protocols for indistinguishability between signal and decoy states does not hold for all sources. We derive bounds on the single-photon transmission probability and error rate for multimode states and apply these bounds to the output state of a PDC source. We observe two opposing effects on the secure key rate. First, the multimode structure of the state gives rise to a new attack that decreases the key rate. Second, more contributing modes change the photon number distribution from a thermal toward a Poissonian distribution, which increases the key rate.

Inhomogeneous quantum walks
Noah Linden and James Sharam
Published 20 November 2009 (11 pages)
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We study a natural construction of a general class of inhomogeneous quantum walks (namely, walks whose transition probabilities depend on position). Within the class we analyze walks that are periodic in position and show that, depending on the period, such walks can be bounded or unbounded in time; in the latter case we analyze the asymptotic speed. We compare the construction to others in the existing literature. As an example we give a quantum version of a nonirreducible classical walk: the Pólya Urn.

Atomic and molecular structure and dynamics

Orthopositronium lifetime at O(alpha) and O(alpha3 ln alpha) in closed form
B. A. Kniehl, A. V. Kotikov, and O. L. Veretin
Published 4 November 2009 (11 pages)
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Recently, the O(alpha) and O(alpha3 ln alpha) radiative corrections to the orthopositronium lifetime have been presented in closed analytical form, in terms of basic irrational numbers that can be evaluated numerically to arbitrary precision [B. A. Kniehl, A. V. Kotikov, and O. L. Veretin, Phys. Rev. Lett. 101, 193401 (2008)]. Here, we present the details of this calculation and reveal the nature of these new constants. We also list explicit transformation formulas for generalized polylogarithms of weight four, which may be useful for other applications.

Local Hartree-exchange and correlation potential defined by local force equations
M. Ruggenthaler and D. Bauer
Published 5 November 2009 (7 pages)
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From the local force equation of quantum mechanics and by expanding the interacting wave function in terms of the noninteracting Kohn-Sham wave function, we derive differential equations defining the local Hartree-exchange and the local correlation potential of density functional theory. The derived equations apply to time-dependent as well as to time-independent problems. Approximations to the correlation part of the interacting wave function result in approximations to the effective potential which take the Hartree-exchange forces into account exactly. Under some assumptions, we find explicit expressions for the local effective potential. We analyze the local Hartree-exchange-only approximation for the ground and the first two excited states of an exactly solvable one-dimensional model system of helium and compare the results to an optimized effective potential approach.

Valence-electron configuration of Fe, Cr, and Ni in binary and ternary alloys from Kbeta-to-Kalpha x-ray intensity ratios
I. Han and L. Demir
Published 10 November 2009 (6 pages)
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Kbeta-to-Kalpha x-ray intensity ratios of Fe, Cr, and Ni have been measured in pure metals and in alloys of FexNi1−x (x=0.8, 0.7, 0.6, 0.5, 0.4, 0.3, and 0.2), NixCr1−x (x=0.8, 0.6, 0.5, 0.4, and 0.2), FexCr1−x (x=0.9, 0.7, and 0.5), and FexCryNi1−(x+y) (x=0.7−y=0.1, x=0.5−y=0.2, x=0.4−y=0.3, x=0.3−y=0.3, x=0.2−y=0.2, and x=0.1−y=0.2) following excitation by 22.69 keV x rays from a 10 mCi 109Cd radioactive point source. The valence-electron configurations of these metals were determined by corporation of measured Kbeta-to-Kalpha x-ray intensity ratios with the results of multiconfiguration Dirac-Fock calculation for various valence-electron configurations. Valence-electron configurations of 3d transition metals in alloys indicate significant differences with respect to the pure metals. Our analysis indicates that these differences arise from delocalization and/or charge transfer phenomena in alloys. Namely, the observed change of the valence-electron configurations of metals in alloys can be explained with the transfer of 3d electrons from one element to the other element and/or the rearrangement of electrons between 3d and 4s,4p states of individual metal atoms.

Spectroscopy of 2s1/2−2p3/2 transitions in W65+ through W71+
Y. Podpaly, J. Clementson, P. Beiersdorfer, J. Williamson, G. V. Brown, and M. F. Gu
Published 11 November 2009 (5 pages)
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A high-resolution flat-crystal spectrometer was used on the SuperEBIT electron beam ion trap to measure the energies of the 2s1/2−2p3/2 transitions in lithiumlike through fluorinelike tungsten. These transitions are strongly affected by energy shifts due to quantum electrodynamics (QED). SuperEBIT was run at an electron energy of 103.2±0.5  keV and an electron beam current of 150 mA to generate the respective charge states; hydrogenlike aluminum and neonlike krypton were used as calibration elements. The spectra were analyzed with and the results compared to calculations based on the flexible atomic code. Good agreement was found. The measurements yielded line positions with a precision of 1–2 eV, which test QED calculations to 5%–10%.

Interaction of the 6p2 1S0 broad resonance with 5dnd J=0 autoionizing resonances in barium
M. A. Kalyar, M. Rafiq, and M. A. Baig
Published 12 November 2009 (8 pages)
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We present even-parity autoionizing resonances in barium using two-step laser excitation via 5d6p 1P1 intermediate level covering the energy region from the first ionization threshold to the 5d 2D5/2 limit. The data are achieved using an atomic beam apparatus in conjunction with a time-of-flight mass spectrometer and a Nd:YAG pumped dye laser system. True line shape of the 6p2 1S0 autoionizing resonance at 44 850±50  cm−1, its width (980±50  cm−1), and the absolute value of photoionization cross section (185±35  Mb) are reported. A combination of parallel and perpendicular polarization vectors of the exciting and the ionizing dye lasers reveals unambiguous J-value assignments of the excited states. The interactions between the 6p2 1S0 broad feature with the 5d5/2nd J=0 Rydberg series have been simulated using the phase-shifted multichannel quantum defect theory.

Line-broadening studies of excited diatomic homoatomic and heteroatomic Rydberg molecules formed by potassium, rubidium, and cesium atoms
C. Vadla, V. Horvatic, and K. Niemax
Published 19 November 2009 (10 pages)
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Recent absorption measurements of the self-broadening of Rb principal series lines [C. H. Greene, E. L. Hamilton, H. Crowell, C. Vadla, and K. Niemax, Phys. Rev. Lett. 97, 233002 (2006)] verified the existence of minima in highly excited long-range potentials of Rb2 which were predicted theoretically as bound Rydberg molecule [C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour, Phys. Rev. Lett. 85, 2458 (2000)]. The present paper reports on the extension of these experimental studies to principal series lines of K broadened by K as well as Cs broadened by Cs. In addition, the broadening of some lower members of the K, Rb, and Cs principal line series due to heteroatomic interactions was investigated. The analysis of the measured satellite structures in the quasistatic line wings and their comparison to the previously published analogous data for Rb*+Rb system lead to the conclusion that one can expect existence of potential minima in homo- and heteroatomic alkali-metal A*+B systems in a wide range of principal quantum numbers, starting with K, Rb, and Cs excited to the third resonance states. The positions and strengths of the satellite structures indicate that the potential minima are due to scattering of the valence electron by ground-state perturbers.

Hylleraas-configuration-interaction study of the 2 2S ground state of neutral lithium and the first five excited 2S states
James S. Sims and Stanley A. Hagstrom
Published 19 November 2009 (8 pages)
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High-precision Hylleraas-configuration-interaction (Hy-CI) method variational calculations are reported for the 2 2S ground state of neutral lithium. The nonrelativistic energy is calculated to be −7.478 060 323 451 9  hartree, demonstrating that the Hy-CI technique is capable of sub-nanohartree accuracy for three-electron systems. A Hylleraas expansion without linked products of odd powers of rij gives −7.478 060 323 452  hartree, showing the relative unimportance of such terms for lithium at the nanohartree level of accuracy. Hy-CI calculations are also reported for the 3 2S, 4 2S, 5 2S, 6 2S, and 7 2S lithium excited states.

Atomic and molecular collisions and interactions

Double ionization of helium in collision with 20–500-keV/amu Cq+ and Oq+ (q=1–3) ions
X. R. Zou, J. X. Shao, X. M. Chen, Y. Cui, Z. M. Gao, Y. Z. Yin, B. W. Ding, Z. Li, and D. Y. Yu
Published 4 November 2009 (6 pages)
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Target ionization and projectile charge changing were investigated for 20–500 keV/u Cq+, Oq++He (q=1–3) collisions. Double- to single-ionization ratios R21 of helium associated with no projectile charge change (direct ionization), single-electron capture, and single-electron loss were measured. The cross-section ratio R21 depends strongly on the collision velocity v, the projectile charge state q, and the outgoing reaction channel. Meanwhile, a model extended from our previous work [J. X. Shao, X. M. Chen, and B. W. Ding, Phys. Rev. A 75, 012701 (2007)] is presented to interpret the above-mentioned dependences. Good agreement is found between the model and the experimental data.

Two-dimensional dipolar scattering
Christopher Ticknor
Published 5 November 2009 (4 pages)
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We characterize the long-range dipolar scattering in two dimensions. We use the analytic zero energy wave function including the dipolar interaction; this solution yields universal dipolar scattering properties in the threshold regime. We also study the semiclassical dipolar scattering and find universal dipolar scattering for this energy regime. For both energy regimes, we discuss the validity of the universality and give physical examples of the scattering.

Rabi spectroscopy and excitation inhomogeneity in a one-dimensional optical lattice clock
S. Blatt, J. W. Thomsen, G. K. Campbell, A. D. Ludlow, M. D. Swallows, M. J. Martin, M. M. Boyd, and J. Ye
Published 6 November 2009 (11 pages)
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We investigate the influence of atomic motion on precision Rabi spectroscopy of ultracold fermionic atoms confined in a deep one-dimensional optical lattice. We analyze the spectral components of longitudinal sideband spectra and present a model to extract information about the transverse motion and sample temperature from their structure. Rabi spectroscopy of the clock transition itself is also influenced by atomic motion in the weakly confined transverse directions of the optical lattice. By deriving Rabi flopping and Rabi line shapes of the carrier transition, we obtain a model to quantify trap-state-dependent excitation inhomogeneities. The inhomogeneously excited ultracold fermions become distinguishable, which allows s-wave collisions. We derive a detailed model of this process and explain observed density shift data in terms of a dynamic mean-field shift of the clock transition.

Electron scattering with a SiF2 molecule using the R-matrix method
Hema Munjal and K. L. Baluja
Published 6 November 2009 (6 pages)
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Differential, integral, and momentum-transfer cross sections for the elastic scattering of electrons by SiF2 have been calculated for the incident electron energy range of 0–10 eV. These results are obtained by using the R-matrix method. The close-coupling expansion of the wave function of the scattering used in the R-matrix formalism includes the lowest six target states. We have also calculated the excitation cross sections from the ground state X 1A1 to the first five excited states 3B1, 1B1, 3B2, 3A1, and 1B2 from their respective thresholds up to 10 eV. The target states are represented by configuration-interaction wave functions that account for correlation effects. We detect a shape resonance in 2B1 symmetry and a core-excited shape resonance in 2A2 symmetry. We have included up to g-partial waves in the scattering calculation. For this polar molecule, higher partial wave contribution is accounted for by invoking a Born-closure approximation. Elastic cross-section results are compared with the other theoretical works available.

Quantization function for potentials with −1/r4 tails
Patrick Raab and Harald Friedrich
Published 10 November 2009 (10 pages)
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For deep potentials with attractive tails proportional to −1/r4, as occur in the interaction between a charged ion and a neutral polarizable atom (or molecule), we present an analytical expression for the tail contribution to the quantization function F(E) which determines the near-threshold bound-state energies En according to the quantization rule nth−n=F(En). Its usefulness is demonstrated in applications to a model Lennard-Jones potential and to the H2+ molecular ion.

Fully relativistic time-dependent close-coupling method for electron-impact ionization of atomic ions
M. S. Pindzola, J. A. Ludlow, F. Robicheaux, J. Colgan, and C. J. Fontes
Published 13 November 2009 (6 pages)
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A fully relativistic time-dependent close-coupling method is developed based on Dirac's covariant formulation of quantum mechanics. The expansion of a one-electron wave function in spin-orbit eigenfunctions yields the well-known coupled Dirac equations in two radial wave functions, while the expansion of a two-electron wave function in coupled spin-orbit eigenfunctions yields close-coupled Dirac equations in four radial wave functions. The time-dependent Dirac equations are solved directly using numerical methods that avoid the Fermi doubling pathology. Test calculations are carried out using the one-electron coupled equations for j=(1/2) elastic potential scattering from Ne9+ at 2.00 keV and using the two-electron close-coupled equations for J=0,1 ionization of Ne9+ at 4.15 keV.

Electron transfer in fast atomic collisions in the presence of an x-ray laser
A. B. Voitkiv
Published 18 November 2009 (5 pages)
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We consider electron capture in fast collisions between a proton and hydrogen in the presence of an intense x-ray laser whose angular frequency omega is close to v2/2, where v is the collision velocity. We show that in such a case laser-induced capture becomes possible and that the latter proceeds via both induced photon emission and photon absorption channels and can, in principle, compete with kinematic and radiative electron capture.

Relativistic convergent close-coupling method: Calculation of electron scattering from hydrogenlike ions
Christopher J. Bostock, Dmitry V. Fursa, and Igor Bray
Published 18 November 2009 (8 pages)
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We report on the extension of the recently formulated relativistic convergent close-coupling (RCCC) method to include the Breit and Møller interactions. The inclusion of these relativistic effects ensures that the RCCC method is now capable of calculating electron scattering excitation and ionization cross sections for highly charged ions. We have calculated the polarization of the Lyman-alpha1 x-ray line emitted by hydrogenlike Ti21+, Ar17+, and Fe25+ ions excited by electron impact. We find that account of Breit relativistic corrections is important to resolve the discrepancy between experiment and theoretical calculations. For the much heavier hydrogenlike U91+ ion where the Møller interaction becomes important we present the estimate of the polarization of the Lyman-alpha1 x-ray line and performed a calculation of the total ionization cross section.

Electron-scattering cross sections for collisions with tetrahydrofuran from 50 to 5000 eV
M. Fuss, A. Muñoz, J. C. Oller, F. Blanco, D. Almeida, P. Limão-Vieira, T. P. D. Do, M. J. Brunger, and G. García
Published 19 November 2009 (6 pages)
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In this paper, we report on total electron tetrahydrofuran (C4H8O) scattering cross-section measurements for energies in the range from 50 to 5000 eV with experimental errors of about 5%. In addition, integral elastic and inelastic cross sections have been calculated over a broad energy range (1–10 000  eV), with an optical potential method assuming a screening-corrected independent atom representation. Partial and total ionization cross sections have been also obtained by combining simultaneous electron and ion measurements with a time-of-flight analysis of the ionic induced fragmentation. Finally, an average energy distribution of secondary electrons has been derived from these measurements in order to provide data for modeling electron-induced damage in biomolecular systems.

Feshbach projection operator approach to positron annihilation
S. d'A. Sanchez, M. A. P. Lima, and M. T. do N. Varella
Published 20 November 2009 (9 pages)
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We present a theory of vibrationally enhanced positron annihilation on molecules based on the Feshbach projection operator formalism. A key aspect of the present approach is the fact that no direct vibrational excitation is assumed, i.e., the attachment mechanism is electronic in nature, arising from positron-electron correlation-polarization forces, and energy transfer to the nuclei essentially follows from the difference between the potential-energy surfaces of the isolated target and the positron-molecule compound; moreover, no a priori assumption is made on the character of the transient (bound or virtual state). An approximate relation between the annihilation parameter Zeff and the vibrationally summed cross section is presented, as well as a hierarchy of approximations that may allow for elaborate model calculations. We also discuss how important aspects of the annihilation process are taken into account in the present theory, such as isotope effects, vibrational energy redistribution and relative strengths among vibrational resonances. For completeness, semiempirical model calculations for acetylene and ethylene are presented. Despite the stringent approximations employed in this simplest version of the theory, fair agreement with experimental data was obtained in the vicinity of 0-->1 thresholds.

Clusters (including fullerenes)

Structures and composition-dependent polarizabilities of open- and closed-shell GanAsm semiconductor clusters
Panaghiotis Karamanis, Phillipe Carbonnière, and Claude Pouchan
Published 2 November 2009 (12 pages)
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A systematic investigation of the structures and the dependence of the dipole polarizabilities on the composition of closed and open shell gallium arsenic clusters is presented. Our investigation focuses on nine gallium arsenide (GaAs) clusters with five and six total number of atoms of systematically varying composition. These clusters are the smallest species of small GaAs clusters which have attracted substantial attention due to their strongly oscillating experimental polarizability values. The ground states of those clusters have been determined using a global approach which combines molecular dynamics and an automatic procedure of comparing and selecting cluster structures based on pattern recognition techniques. The polarizabilities have been studied by means of coupled cluster techniques complemented by a semi-empirical hybrid functional which includes corrections from perturbation theory. Our global structural investigation found two different structures for Ga4As2 and Ga1As5 which are lower in energy than the previously reported ones. The performed polarizability investigation suggests that open shell GaAs systems composed of five atoms are not more polarizable than closed-shell clusters built by six atoms as is indirectly implied by the reported experimental data. Also, the polarizabilities of those species increase as a function of the number of Ga in a monotonic but not systematic manner. The observed increase type is explained by the large atomic polarizabilities of Ga and in terms of the particular structural and bonding features of a given cluster. Furthermore, the comparison between our theoretical values and earlier experimental polarizability estimations clearly shows that for the five-atomic clusters, the reported experimental polarizability is not largely overestimated as was previously believed. Our results for the six-atomic cluster demonstrate that the polarizability per atom of a six atomic GaAs cluster of any composition is larger than the polarizability of the bulk material, contrary to what has been demonstrated by the experiment.

Ionization dynamics of cluster targets irradiated by x-ray free-electron-laser light
Tatsufumi Nakamura, Yuji Fukuda, and Yasuaki Kishimoto
Published 4 November 2009 (6 pages)
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Interactions of x-ray free-electron laser (XFEL) light with a single cluster target are numerically investigated by using a three-dimensional particle-in-cell code. The plasma dynamics as well as relevant atomic processes are taken into account, such as photoionization, the Auger effect, collisional ionization and relaxation, and field ionization. It is found that as the XFEL intensity increases to as high as ~1021  (photons/pulse)/mm2, the field ionization, which is the dominant ionization process over the other atomic processes, leads to rapid target ionization. The target damage due to the irradiation by XFEL light is numerically evaluated, which gives an estimation of the XFEL intensity so as to suppress the target damage within a tolerable range for imaging.

Atomic and molecular processes in external fields, including interactions with strong fields and short pulses

Evolution of squeezed states under the Fock-Darwin Hamiltonian
Jaime E. Santos, Nuno M. R. Peres, and João M. B. Lopes dos Santos
Published 2 November 2009 (8 pages)
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We develop a complete analytical description of the time evolution of squeezed states of a charged particle under the Fock-Darwin (FD) Hamiltonian and a time-dependent electric field. This result generalizes a relation obtained by Infeld and Plebański for states of the one-dimensional harmonic oscillator. We relate the evolution of a state-vector subjected to squeezing to that of state which is not subjected to squeezing and for which the time evolution under the simple harmonic oscillator dynamics is known (e.g., an eigenstate of the Hamiltonian). A corresponding relation is also established for the Wigner functions of the states, in view of their utility in the analysis of cold-ion experiments. In Appendix A, we compute the response functions of the FD Hamiltonian to an external electric field, using the same techniques as in the main text.

Coherent control in the classical limit: Symmetry breaking in an optical lattice
Michael Spanner, Ignacio Franco, and Paul Brumer
Published 3 November 2009 (7 pages)
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The quantum-to-classical transition of a symmetry-breaking coherent control scenario is computationally demonstrated in an optical lattice arrangement. Control is shown to survive in the classical limit and, for small effective [h-bar], to be comparable in magnitude to quantum control. Moderate decoherence is seen to eliminate structure from the momentum space distribution, but not to cause loss of control. The proposed scenario is designed so as to be demonstrable experimentally in a moving or shaken one-dimensional optical lattice.

Nonlinear Compton scattering with a laser pulse
Madalina Boca and Viorica Florescu
Published 5 November 2009 (14 pages)
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We investigate the scattering of intense laser radiation on free electrons using a semiclassical relativistic approach. The laser field is described as an ideal pulse with a finite duration, a fixed direction of propagation, and indefinitely extended in the plane perpendicular to it. This allows the use of Volkov solutions and leads to a transition amplitude which is a product of a three-dimensional delta function with a linear combination of three one-dimensional integrals that we evaluate numerically. We give the general expression of the emitted photon spectrum as a function of frequency and direction valid for any initial geometry of the electron-laser beam scattering and for arbitrary shape, duration, and polarization of the laser pulse averaged over the initial electron spin and summed over the emitted photon and ejected electron polarizations. At a fixed photon scattering angle, one obtains a continuous frequency distribution with a succession of maxima located near the discrete values corresponding to the monochromatic case. We present results for head-on collisions and circularly polarized laser pulses. Our figures illustrate the dependence of the photon spectrum on pulse parameters (duration, shape, and maximum intensity) and the role of the initial electron energy. For a few-cycle linearly polarized pulse we also explore the effect of the carrier-envelope phase.

Recollision dynamics of electron wave packets in high-order harmonic generation
Kai-Jun Yuan and André D. Bandrauk
Published 6 November 2009 (12 pages)
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