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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

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.

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.

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.

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.

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.

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.

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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We numerically investigate the dynamics of recollision of an electron in high-order harmonic generation (HHG) for an H atom and a molecular ion H2+ using a short (ten optical cycles), and intense (I0>=1014  W/cm2), z-polarized linear laser pulse with wavelength 800 nm by accurately solving the three-dimensional time-dependent Schrödinger equation. A time-frequency analysis obtained via Gabor transforms is employed to identify electron recollision and recombination times responsible for the generation of harmonics. We find that the HHG spectra are mainly attributed to the recollision of an inner electron wave packet with the parent ion in agreement with the classical recollision model. A time delay of the electron recollision occurs between wave packets in inner and outer regions, near to and far from the parent ion, due to different phase of the acceleration (as well as dipole velocity) of the electron. Inner wave packets at recollision contain mainly short and long trajectories whereas outer wave packets contain only single trajectories. Lower-order harmonics are generated mainly by single recollisions near field extrema, i.e., in strong electric fields whereas higher-order harmonics are generated by double trajectories with different intensities. In the case of H2+ at a critical nuclear distance for charge resonance enhanced ionization, we also find that HHG mainly comes from contributions of the inner electron wave packet, but with more complex recollision trajectories due to the presence of more than one Coulomb center. Triple recollision trajectories are shown to occur generally for the latter.

Alignment-dependent nonsequential double ionization of N2 in intense laser fields: The role of different valence orbitals
XinYan Jia, WeiDong Li, Jie Liu, and J. Chen
Published 6 November 2009 (4 pages)
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The alignment-dependent nonsequential double ionization (NSDI) of diatomic molecule N2 in intense fields is studied using the S-matrix theory. Our results show that the valence orbitals play an important role in alignment-dependent NSDI process: in addition to the contribution from the outmost 1piu orbital of N2+ in all the alignment angles, the suboutmost 3sigmag orbital plays an important role at small alignment angles, which is more pronounced at high intensity. Taking into account the experimental alignment condition, the ratio of N22+:N+2 for the parallel alignment to that for perpendicular alignment obtained, including the 3sigmag orbital, is closer to the experimental result than that without considering this orbital.

Matter waves and collective properties of cold atoms and molecules

First and second sound in a strongly interacting Fermi gas
E. Taylor, H. Hu, X.-J. Liu, L. P. Pitaevskii, A. Griffin, and S. Stringari
Published 2 November 2009 (7 pages)
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Using a variational approach, we solve the equations of two-fluid hydrodynamics for a uniform and trapped Fermi gas at unitarity. In the uniform case, we find that the first and second sound modes are remarkably similar to those in superfluid helium, a consequence of strong interactions. In the presence of harmonic trapping, first and second sound become degenerate at certain temperatures. At these points, second sound hybridizes with first sound and is strongly coupled with density fluctuations, giving a promising way of observing second sound. We also discuss the possibility of exciting second sound by generating local heat perturbations.

Critical velocity of superfluid flow through single-barrier and periodic potentials
Gentaro Watanabe, F. Dalfovo, F. Piazza, L. P. Pitaevskii, and S. Stringari
Published 3 November 2009 (9 pages)
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We investigate the problem of an ultracold atomic gas in the superfluid phase flowing in the presence of a potential barrier or a periodic potential. We use a hydrodynamic scheme in the local density approximation (LDA) to obtain an analytic expression for the critical current as a function of the barrier height or the lattice intensity, which applies to both Bose and Fermi superfluids. In this scheme, the stationary flow becomes energetically unstable when the local superfluid velocity is equal to the local sound velocity at the point where the external potential is maximum. We compare this prediction with the results of the numerical solutions of the Gross-Pitaevskii and Bogoliubov-de Gennes equations. We discuss the role of long-wavelength excitations in determining the critical velocity. Our results allow one to identify the different regimes of superfluid flow, namely, the LDA hydrodynamic regime, the regime of quantum effects beyond LDA for weak barriers and the regime of tunneling between weakly coupled superfluids for strong barriers. We finally discuss the relevance of these results in the context of current experiments with ultracold gases.

Nonlinear Floquet states and quasienergies of a Bose-Einstein condensate in a driven double-well potential
Qiongtao Xie (谢琼涛) and Wenhua Hai (海文华)
Published 4 November 2009 (6 pages)
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We investigate nonlinear Floquet states and quasienergies of a Bose-Einstein condensate in a double-well potential subject to a high-frequency driving field. A multiple time scale method is used to obtain the nonlinear Floquet states and quasienergies in both weak and strong nonlinearity regimes. It is shown that there are more nonlinear Floquet states without linear counterparts in the strong nonlinearity regime than in the weak nonlinearity regime, and they are phase-locked to an integer multiple of the external driving frequency. The validity of the effective description in the two different regimes is discussed. The stability criterion for these nonlinear Floquet states is found analytically and confirmed numerically.

Role of temperature effects in the phenomenon of ultraslow electromagnetic pulses in Bose-Einstein condensates of alkali-metal atoms
Yurii Slyusarenko and Andrii Sotnikov
Published 6 November 2009 (9 pages)
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We study the temperature dependence of optical properties of dilute gases of alkali-metal atoms in the state with Bose-Einstein condensates. The description is constructed in the framework of the microscopic approach that is based on the Green's-functions formalism. We find the expressions for the scalar Green's functions describing a linear response of a condensed gas to a weak external electromagnetic field (laser). It is shown that these functions depend on the temperature, other physical properties of a system, and on the frequency detuning of a laser. We compare the relative contributions of the condensate and noncondensate particles in the system response. The influence of the temperature effects is studied by the example of two- and three-level systems. We show that in these cases, which are most commonly realized in the present experiments, the group velocity and the absorption rate of pulses practically do not depend on the gas temperature in the region from the absolute zero to the critical temperature. We discuss also the cases when the temperature effects can play a significant role in the phenomenon of slowing of electromagnetic pulses in a gas of alkali-metal atoms with Bose-Einstein condensates.

Polaron-to-molecule transition in a strongly imbalanced Fermi gas
M. Punk, P. T. Dumitrescu, and W. Zwerger
Published 6 November 2009 (10 pages)
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A single down-spin fermion with an attractive zero-range interaction with a Fermi sea of up-spin fermions forms a polaronic quasiparticle. The associated quasiparticle weight vanishes beyond a critical strength of the attractive interaction, where a many-body bound state is formed. From a variational wave function in the molecular limit, we determine the critical value for the polaron-to-molecule transition. The value agrees well with the diagrammatic Monte Carlo results of Prokof'ev and Svistunov and is consistent with recent rf-spectroscopy measurements of the quasiparticle weight by Schirotzek et al. [Phys. Rev. Lett. 102, 230402 (2009)]. In addition, we calculate the contact coefficient of the strongly imbalanced gas, using the adiabatic theorem of Tan and discuss the implications of the polaron-to-molecule transition for the phase diagram of the attractive Fermi gas at finite imbalance.

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

Strong reduction of laser power noise by means of a Kerr nonlinear cavity
Alexander Khalaidovski, André Thüring, Henning Rehbein, Nico Lastzka, Benno Willke, Karsten Danzmann, and Roman Schnabel
Published 2 November 2009 (7 pages)
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We demonstrated the power noise reduction of a continuous-wave laser field by means of an effective third-order Kerr nonlinear cavity. In contrast to conventional noise reduction schemes relying on linear cavities, a strong noise suppression at Fourier frequencies below the linewidth of the nonlinear cavity was possible. The laser light was reflected off a Kerr nonlinear cavity that had a half width half maximum linewidth of 4.5 MHz. The cavity was operated slightly off-resonance at approximately half of the maximum power buildup, close to its so-called critical state; a power noise reduction of up to 32 dB at Fourier frequencies below 1 MHz was observed after reflection. The effective third-order nonlinearity was a so-called cascaded second-order nonlinearity of MgO:LiNbO3. The laser had a power of 0.75 W at the wavelength of 1064 nm.

Orientation-dependent spontaneous emission rates of a two-level quantum emitter in any nanophotonic environment
Willem L. Vos, A. Femius Koenderink, and Ivan S. Nikolaev
Published 2 November 2009 (7 pages)
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We study theoretically the spontaneous emission rate of a two-level quantum emitter in any nanophotonic system. We derive a general representation of the rate on the orientation of the transition dipole by only invoking symmetry of the Green function. The rate depends quadratically on orientation and is determined by rates along three principal axes, which greatly simplifies visualization: emission rate surfaces provide insight on how preferred orientations for enhancement (or inhibition) depend on emission frequency and location, as shown for a mirror, a plasmonic sphere, and a photonic band-gap crystal. Moreover, insight is provided on means to “switch” the emission rates by actively controlling the orientation of the emitters' transition dipole.

Suppression of transverse instabilities of dark solitons and their dispersive shock waves
Andrea Armaroli, Stefano Trillo, and Andrea Fratalocchi
Published 3 November 2009 (8 pages)
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We investigate the impact of nonlocality, owing to diffusive behavior, on transverse instabilities of a dark stripe propagating in a defocusing cubic medium. The nonlocal response turns out to have a strongly stabilizing effect both in the case of a single soliton input and in the regime where dispersive shock waves develop (multisoliton regime). Such conclusions are supported by the linear stability analysis and numerical simulation of the propagation.

Spectrally partially coherent propagation-invariant fields
Kimmo Saastamoinen, Jari Turunen, Pasi Vahimaa, and Ari T. Friberg
Published 3 November 2009 (11 pages)
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Polychromatic propagation-invariant fields (localized waves) with variable spectral coherence properties are considered. A bridge is provided between coherent propagation-invariant pulses, including Bessel pulses, X-waves and focus wave modes, and stationary polychromatic propagation-invariant fields using a difference-frequency spectral-correlation model. In the space-time domain, this assumption leads to a representation of a partially coherent field as a continuum of spectrally fully coherent, mutually independent pulses originating at different instants of time. The influence of partial spectral coherence on temporal coherence and pulse shape as well as intensity distribution and transverse spatial coherence is illustrated by considering fields with propagation-invariant Bessel-mode monochromatic components.

Nonlinear optical dynamics in nonideal gases of interacting colloidal nanoparticles
R. El-Ganainy, D. N. Christodoulides, E. M. Wright, W. M. Lee, and K. Dholakia
Published 3 November 2009 (6 pages)
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We show that many-body effects in stabilized nanocolloidal suspensions can have a profound effect on their optical nonlinearity. By considering the screened Coulomb repulsions between nanoparticles, we find that the nonlinear optical behavior of these colloids can range from polynomial to exponential depending on their composition and chemistry. The dynamics and stability properties of optical beams propagating in such nonideal gas environments of interacting colloidal particles are investigated. Our analysis provides a theoretical foundation for understanding the recently observed super-Kerr nonlinear optical response of such systems.

Optical frequency stabilization of a 10 GHz Ti:sapphire frequency comb by saturated absorption spectroscopy in 87rubidium
D. C. Heinecke, A. Bartels, T. M. Fortier, D. A. Braje, L. Hollberg, and S. A. Diddams
Published 4 November 2009 (7 pages)
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The high power per mode of a recently developed 10 GHz femtosecond Ti:sapphire frequency comb permits nonlinear Doppler-free saturation spectroscopy in 87Rb with a single mode of the comb. We use this access to the natural linewidth of the rubidium D2 line to effectively stabilize the optical frequencies of the comb with an instability of 7×10−12 in 1 s of averaging. The repetition rate is stabilized to a microwave reference leading to a stabilized and atomically referenced comb. The frequency stability of the 10 GHz comb is characterized using optical heterodyne with an independent self-referenced 1 GHz comb. In addition, we present alternative stabilization approaches for high repetition rate frequency combs and evaluate their expected stabilities.

Loss compensation in metal-dielectric structures in negative-refraction and super-resolving regimes
M. A. Vincenti, D. de Ceglia, V. Rondinone, A. Ladisa, A. D'Orazio, M. J. Bloemer, and M. Scalora
Published 5 November 2009 (7 pages)
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We study the influence of gain on negative refraction and super-resolution in transparent resonant metal-dielectric photonic band gap structures in the visible and near infrared ranges. We find that while the introduction of gain can compensate for losses caused by the excitation of surface waves, it also improves the resolving characteristics of the lens and leads to gain-tunable super-resolution.

Spontaneous symmetry breaking and circulation by optically bound microparticle chains in Gaussian beam traps
J. M. Taylor and G. D. Love
Published 6 November 2009 (6 pages)
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It has been known for some time that simple “optically bound” chains of dielectric microparticles can form in a counter propagating Gaussian beam optical trap. Here we report experimental observations of more complex trapped states, which do not reflect the underlying symmetry of the optical beam trap they are confined in. We discuss both stationary off-axis trapping and dynamic motion. We confirm the results using a rigorous Mie scattering model and also give a physical explanation for these static and dynamic off-axis trapped states.