Showing papers in "European Physical Journal D in 2011"

Matter rogue wave in Bose-Einstein condensates with attractive atomic interaction

Abstract: We investigate the matter rogue wave in Bose-Einstein condensates with attractive interatomic interaction analytically and numerically. Our results show that the formation of rogue wave is mainly due to the accumulation of energy and atoms toward to its central part; and the decay rate of atoms in unstable matter rogue wave can be effectively controlled by modulating the trapping frequency of external potential. The numerical simulation demonstrate that even a small periodic perturbation with small modulation frequency can induce the generation of a near-ideal matter rogue wave. We also give an experimental protocol to observe this phenomenon in Bose-Einstein condensates.

Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

Abstract: We analyze the properties of a single impurity immersed in a Fermi sea. At positive energy and scattering lengths, we show that the system possesses a well-defined but metastable excitation, the repulsive polaron, and we calculate its energy, quasiparticle residue and effective mass. From a thermodynamic argument we obtain the number of particles in the dressing cloud, illustrating the repulsive character of the polaron. Identifying the important 2- and 3-body decay channels, we furthermore calculate the lifetime of the repulsive polaron. The stability conditions for the formation of fully spin polarized (ferromagnetic) domains are then examined for a binary mixture of atoms with a general mass ratio. Our results indicate that mass imbalance lowers the critical interaction strength for phase-separation, but that very short quasiparticle decay times will complicate the experimental observation of itinerant ferromagnetism. Finally, we present the spectral function of the impurity for various coupling strengths and momenta.

Production of a dual-species Bose-Einstein condensate of Rb and Cs atoms

Abstract: We report the simultaneous production of Bose-Einstein condensates (BECs) of 87Rb and 133Cs atoms in separate optical traps. The two samples are mixed during laser cooling and loading but are separated by 400 μm for the final stage of evaporative cooling. This is done to avoid considerable interspecies three-body recombination, which causes heating and evaporative loss. We characterize the BEC production process, discuss limitations, and outline the use of the dual-species BEC in future experiments to produce rovibronic ground state molecules, including a scheme facilitated by the superfluid-to-Mott-insulator (SF-MI) phase transition.

High-resolution mass spectrometric study of pure helium droplets, and droplets doped with krypton

Abstract: Mass spectra of doped and undoped helium droplets are presented. The high resolution of the time-of-flight spectrometer (m/Δm ≅ 5000) makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the size distribution of He n +. The yield of He4 + shows the well-known enhancement relative to other small cluster ions when the expansion changes from sub- to supercritical, provided the electron energy exceeds a value of 40 ± 1 eV, the threshold for formation of electronically excited ions. Upon doping with krypton, pure Kr n + cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed clusters He m Kr+ indicate closure of a solvation shell at m = 12.

Quantum dissension: Generalizing quantum discord for three-qubit states

Abstract: We introduce the notion of quantum dissension for a three-qubit system as a measure of quantum correlations We use three classically equivalent expressions of three-variable mutual information Their differences are zero classically but not so in quantum domain It generalizes the notion of quantum discord to a multipartite system There can be multiple definitions of the dissension depending on the nature of projective measurements done on the subsystems As an illustration, we explore the consequences of these multiple definitions and compare them for three-qubit pure and mixed GHZ and W states We find that unlike discord, dissension can be negative This is because measurement on a subsystem may enhance the correlations in the rest of the system Furthermore, when we consider a bipartite split of the system, the dissension reduces to discord This approach can pave a way to generalize the notion of quantum correlations in the multiparticle setting

Dense coding using cluster states and its application on deterministic secure quantum communication

Abstract: This work proposes a dense coding using four-particle cluster states. Based on the dense coding, an efficient deterministic secure quantum communication (DSQC) is devised. The quantum bit efficiency of the proposed DSQC is better than the other DSQCs also using four-particle cluster states.

Head-on collision of ion acoustic solitary waves in electron-positron-ion plasma with superthermal electrons and positrons

Abstract: The head-on collision of ion acoustic solitary waves are studied in an electron-positron-ion plasma composed of superthermal electrons, superthermal positrons, and cold ions using the extended Poincare-Lighthill-Kuo (PLK) method. The effects of the ratio of electron to positron temperature, the spectral index of electron and positron, and the concentration of positron component on the phase shift are studied. It is found that the presence of superthermal electrons and superthermal positrons play a significant role on the collision of ion acoustic solitary waves. It is also been observed that the temperature ratio plays a significant role on the collision of ion acoustic solitary waves.

Entanglement or separability: the choice of how to factorize the algebra of a density matrix

Abstract: Quantum entanglement has become a resource for the fascinating developments in quantum information and quantum communication during the last decades. It quantifies a certain nonclassical correlation property of a density matrix representing the quantum state of a composite system. We discuss the concept of how entanglement changes with respect to different factorizations of the algebra which describes the total quantum system. Depending on the considered factorization a quantum state appears either entangled or separable. For pure states we always can switch unitarily between separability and entanglement, however, for mixed states a minimal amount of mixedness is needed. We discuss our general statements in detail for the familiar case of qubits, the GHZ states, Werner states and Gisin states, emphasizing their geometric features. As theorists we use and play with this free choice of factorization, which for an experimentalist is often naturally fixed. For theorists it offers an extension of the interpretations and is adequate to generalizations, as we point out in the examples of quantum teleportation and entanglement swapping.

Arbitrated quantum signature with an untrusted arbitrator

Abstract: In an arbitrated signature scheme, all communications involve a so called arbitrator who has access to the contents of the messages. The security of most arbitrated signature schemes depends heavily on the trustworthiness of the arbitrators. In this paper we show how to construct an arbitrated quantum signature protocol of classical messages with an untrusted arbitrator. Its security is analyzed and it is proved to be secure even if the arbitrator is compromised. In addition, the proposed protocol does not require a direct quantum link between any two communicating users, which is an appealing advantage in the implementation of a practical quantum distributed communication network.

Designing spin-spin interactions with one and two dimensional ion crystals in planar micro traps

Abstract: We discuss the experimental feasibility of quantum simulation with trapped ion crystals, using magnetic field gradients. We describe a micro structured planar ion trap, which contains a central wire loop generating a strong magnetic gradient of about 20 T/m in an ion crystal held about 160 μ ma bove the surface. On the theoretical side, we extend a proposal about spin-spin interactions via magnetic gradi- ent induced coupling (MAGIC) (J. Phys. B At. Mol. Opt. Phys. 42, 154009 (2009)). We describe aspects where planar ion traps promise novel physics: spin-spin coupling strengths of transversal eigenmodes ex- hibit significant advantages over the coupling schemes in longitudinal direction that have been previously investigated. With a chip device and a magnetic field coil with small inductance, a resonant enhancement of magnetic spin forces through the application of alternating magnetic field gradients is proposed. Such resonantly enhanced spin-spin coupling may be used, for instance, to create Schrodinger cat states. Fi- nally we investigate magnetic gradient interactions in two-dimensional ion crystals, and discuss frustration effects in such two-dimensional arrangements.

Arbitrary magnetosonic solitary waves in spin 1/2 degenerate quantum plasma

Abstract: Linear and nonlinear compressional magnetosonic waves are studied in magnetized degenerate spin-1/2 Fermi plasmas. Starting from the basic equations of a quantum magnetoplasma we develop the system of quantum magnetohydrodynamic (QMHD) equations. Spin effects are incorporated via spin force and macroscopic spin magnetization current. Sagdeev potential approach is employed to derive the nonlinear energy integral equation which admits the rarefactive solitary structure in the subAlfvenic region. The quantum diffraction due to Bohm potential does not affect the amplitude of soliton but has a direct effect on its width. The width of soliton is broadened with the increase in the quantization of the system due to quantum diffraction. However, the nonlinear wave amplitude is reduced with the increase in the value of magnetization energy due to electron spin-1/2 effects. The degeneracy effect due to quantum plasma beta enhances the amplitude of magnetosonic soliton. The importance of the work relevant to compact astrophysical bodies is pointed out.

Quantum key distribution protocol using dense coding of three-qubit W state

Abstract: The three-qubit W state, with an important feature that each pair of it’s qubits has the same and maximum amount of bipartite entanglement, can be reduced to an entangled 2-qubit system if one of its qubits is lost. Recently, Xue et al. proposed a three-party quantum secret sharing (QSS) protocol based on the three-qubit W state [Chinese Phys. 15, 7 (2006)]. Also, Joo et al. proposed a pair-wise quantum key distribution protocol among three users based on a special measurement on the three-qubit W state [eprint arXiv:quant-ph/0204003v2 (2002)]. This study aims to propose a novel quantum key distribution protocol (QKDP) for arbitrary two communications based on the dense coding and the special measurement of three-qubit W state with the X basis and the Z basis.

Feshbach resonances in the 6 Li- 40 K Fermi-Fermi mixture: elastic versus inelastic interactions

Abstract: We present a detailed theoretical and experimental study of Feshbach resonances in the 6Li-40K mixture. Particular attention is given to the inelastic scattering properties, which have not been considered before. As an important example, we thoroughly investigate both elastic and inelastic scattering properties of a resonance that occurs near 155 G. Our theoretical predictions based on a coupled channels calculation are found in excellent agreement with the experimental results. We also present theoretical results on the molecular state that underlies the 155 G resonance, in particular concerning its lifetime against spontaneous dissociation. We then present a survey of resonances in the system, fully characterizing the corresponding elastic and inelastic scattering properties. This provides the essential information to identify optimum resonances for applications relying on interaction control in this Fermi-Fermi mixture.

Prospects for sympathetic cooling of molecules in electrostatic, ac and microwave traps

Abstract: We consider how trapped molecules can be sympathetically cooled by ultracold atoms. As a prototypical system, we study LiH molecules co-trapped with ultracold Li atoms. We calculate the elastic and inelastic collision cross sections of 7LiH + 7Li with the molecules initially in the ground state and in the first rotationally excited state. We then use these cross sections to simulate sympathetic cooling in a static electric trap, an ac electric trap, and a microwave trap. In the static trap we find that inelastic losses are too great for cooling to be feasible for this system. The ac and microwave traps confine ground-state molecules, and so inelastic losses are suppressed. However, collisions in the ac trap can take molecules from stable trajectories to unstable ones and so sympathetic cooling is accompanied by trap loss. In the microwave trap there are no such losses and sympathetic cooling should be possible.

The prospects of sympathetic cooling of NH molecules with Li atoms

Abstract: We calculate the quartet potential energy surface for Li+NH and use it to calculate elastic and spin-relaxation cross sections for collisions in magnetically trappable spin-stretched states. The potential is strongly anisotropic but spin-relaxation collisions are still suppressed by centrifugal barriers when both species are in spin-stretched states. In the ultracold regime, both the elastic and inelastic cross sections fluctuate dramatically as the potential is varied because of Feshbach resonances. The potential-dependence is considerably reduced at higher energies. The major effect of using an unconverged basis set in the scattering calculations is to shift the resonances without changing their general behaviour. We have calculated the ratio of elastic and spin-relaxation cross sections, as a function of collision energy and magnetic field, for a variety of potential energy surfaces. Most of the surfaces produce ratios that are favorable for sympathetic cooling, at temperatures below about 20 mK.

Atom-dimer and dimer-dimer scattering in fermionic mixtures near a narrow Feshbach resonance

Abstract: We develop a diagrammatic approach for solving few-body problems in heteronuclear fermionic mixtures near a narrow interspecies Feshbach resonance. We calculate s-, p-, and d-wave phaseshifts for the scattering of an atom by a weakly-bound dimer. The fermionic statistics of atoms and the composite nature of the dimer lead to a strong angular momentum dependence of the atom-dimer interaction, which manifests itself in a peculiar interference of the scattered s- and p-waves. This effect strengthens with the mass ratio and is remarkably pronounced in 40K-(40K-6Li) atom-dimer collisions. We calculate the scattering length for two dimers formed near a narrow interspecies resonance. Finally, we discuss the collisional relaxation of the dimers to deeply bound states and evaluate the corresponding rate constant as a function of the detuning and collision energy.

Correlation between entanglement and spin density in nitrogen-vacancy center of diamond

Abstract: Many-body wavefunctions were utilized to calculate von Neumann’s entropy as an entanglement measurement for neutral and negatively charged nitrogen vacancy (NV) centers in diamond. A generalized Hubbard Hamiltonian which considers e-e interaction terms completely was used to calculate many-electron wavefunctions of the ground and excited states. Correlation between entanglement and spin density distributed on neighboring atoms of NV is presented. The behavior of spin density and entanglement under relaxations of neighboring atoms is the same for all investigated ground and excited states. The results suggest that the spin density may be used to quantify the entanglemnt and vice versa.

Trapping of a supersonic beam in a traveling magnetic wave

Abstract: Here we report on a new approach to the magnetic deceleration of supersonic beams, based on the generation of a propagating wave of magnetic field. Atoms and molecules possessing a magnetic dipole moment, in so-called low field seeking quantum states, are trapped around a node of the propagating wave. The wave travels at a desired velocity in the direction of the supersonic beam, which can be chosen to match a velocity class populated in the beam. An additional quadrupole guide provides transverse confinement, independently of the decelerator itself. Our technique has been conceived to generate a smooth motion of the magnetic wave, which should optimize the efficiency of the trapping during a future Zeeman deceleration of the beam. We demonstrate the trapping of metastable argon atoms in a magnetic wave traveling at selected, constant velocities.

Diffusing opinions in bounded confidence processes

Abstract: We study the effects of diffusing opinions on the Deffuant et al. model for continuous opinion dynamics. Individuals are given the opportunity to change their opinion, with a given probability, to a randomly selected opinion inside an interval centered around the present opinion. We show that diffusion induces an order-disorder transition. In the disordered state the opinion distribution tends to be uniform, while for the ordered state a set of well defined opinion clusters are formed, although with some opinion spread inside them. If the diffusion jumps are not large, clusters coalesce, so that weak diffusion favors opinion consensus. A master equation for the process described above is presented. We find that the master equation and the Monte Carlo simulations do not always agree due to finite-size induced fluctuations. Using a linear stability analysis we can derive approximate conditions for the transition between opinion clusters and the disordered state. The linear stability analysis is compared with Monte Carlo simulations. Novel interesting phenomena are analyzed.

Separability criteria for several classes of n-partite quantum states

Abstract: In this paper, we mainly discuss the separability of n-partite quantum states using elements of density matrices. Practical separability criteria for different classes of n-qubit and n-qudit quantum states are obtained. Some of them are also sufficient conditions for genuine entanglement of n-partite quantum states. Moreover, one of the resulting criteria is also necessary and sufficient for a class of n-partite states.

Structural and magnetic properties of Nin (n = 2–21) clusters

Abstract: Extensive studies of structural and magnetic properties of pure Ni n (n=2–21) clusters are carried out using density functional theory with generalized gradient approximation. A new structural growth pattern is reported and the icosahedron sequence is not the global minima structures for size n < 16. The evolution pattern of Ni5–Ni10 clusters is based on square pyramid. For cluster between n = 9 and n = 14, structures of them are trilayered stacking pattern. Especially for Ni13, it seriously deviates from icosahedral pattern which was frequently assumed in previous studies. Ni10, Ni13 and Ni14 are pieces of a double tetrahedron. For cluster size larger than 16, icosahedron based configurations are found to be the dominative structure pattern with exception of Ni19. But trilayered structure is a strong competitor due to small difference in energy between this pattern and icosahedral family. The comparison of magnetic behavior between the calculations and experimental results are conducted. The average coordination mainly dominates on the magnetic behavior in the size range we considered.

Reciprocal invisibility cloak based on complementary media

Abstract: The first invisibility cloak was proposed by Pendry et al [Science 312, 1780 (2006)] But the object enclosed in this original cloak is “blind", that is, it cannot see the outside world, since no electromagnetic waves can reach within the cloaked space Based on the concept of complementary media, we propose a reciprocal invisibility cloak, in which the hidden object can see the outside world, but its presence cannot be detected by electromagnetic wave The performance of the cloak has been verified by full-wave simulations

Modeling the formation of alkali clusters attached to helium nanodroplets and the abundance of high-spin states

Abstract: The doping process of helium nanodroplets with alkali atoms has been modeled in order to study deviations from the Poissonian statistics of measured pick-up statistics which are important for assigning cluster or complex sizes in many experimental studies Several, formally unexplained findings are reproduced and their origin has been analyzed: derivations from the expected functional form of the initial incline, the suppression of the formation of lithium clusters, the influence of the functional form and width of droplet size distributions Furthermore, the controversially discussed formation of high-spin alkali clusters on helium droplets has been calculated within the model The selection of high-spin states comes out to depend strongly on the experimental conditions, and is in general not pronounced for cluster sizes ≥ 3 The enhancement factor of 50 of high-spin states reported in earlier experiments is reproduced when choosing the conditions of these experiments

Ab initio study of spectroscopic properties of the calcium hydride molecular ion

Abstract: In the present work, all adiabatic potential energy curves, spectroscopic constants and dipole moments of CaH+ molecular ion dissociating below the ionic limit Ca2+H− are presented. These curves are determined by an ab initio approach involving a non-empirical pseudo-potential for the Ca core, core-valence correlation accounted in operator form with two types of core polarization potentials (CPP) and full valence Configuration Interaction. The molecule is thus treated as a two-electron system. The potential energy curves and the spectroscopic constants are presented. In addition, the permanent and transition dipole moments are calculated for most of the states and reveal the underlying ionic states. A rather good agreement with the available theoretical works in the literature is obtained for the spectroscopic constants of the lowest states of the CaH+ molecule.

'Measurement of quantum mechanical operators' revisited

Abstract: The Wigner-Araki-Yanase (WAY) theorem states a remarkable limitation to quantum mechanical measurements in the presence of additive conserved quantities. Discovered by Wigner in 1952, this limitation is known to induce constraints on the control of individual quantum systems in the context of information processing. It is therefore important to understand the precise conditions and scope of the WAY theorem. Here we elucidate its crucial assumptions, briefly review some generalizations, and show how a particular extension can be obtained by a simple modification of the original proofs. We also describe the evolution of the WAY theorem from a strict no-go verdict for certain, highly idealized, precise measurements into a quantitative constraint on the accuracy and approximate repeatability of imprecise measurements.

Scattering of Stark-decelerated OH radicals with rare gas atoms

Abstract: We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH (X2Π3/2, J = 3/2, f) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D2 as a function of the collision energy between ∼70 cm−1 and 400 cm−1. The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-to-state inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by coupled channel scattering calculations based on accurate ab initio potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, G. Meijer, Science 313, 1617 (2006)], Ar [L. Scharfenberg, J. Klos, P.J. Dagdigian, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Chem. Chem. Phys. 12, 10660 (2010)], He, and D2 [M. Kirste, L. Scharfenberg, J. Klos, F. Lique, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Rev. A 82, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.

Large atom number dual-species magneto-optical trap for fermionic 6Li and 40K atoms

Abstract: We present the design, implementation and characterization of a dual-species magneto-optical trap (MOT) for fermionic 6Li and 40K atoms with large atom numbers. The MOT simultaneously contains 5.2 × 109 6Li-atoms and 8.0 × 109 40K-atoms, which are continuously loaded by a Zeeman slower for 6Li and a 2D-MOT for 40K. The atom sources induce capture rates of 1.2 × 109 6Li-atoms/s and 1.4 × 109 40K-atoms/s. Trap losses due to light-induced interspecies collisions of ∼65% were observed and could be minimized to ∼10% by using low magnetic field gradients and low light powers in the repumping light of both atomic species. The described system represents the starting point for the production of a large-atom number quantum degenerate Fermi-Fermi mixture.

Role of the volume and surface breakdown in a formation of microdischarges in a steady-state DBD

Abstract: The results of an experimental study on a spatial-time behavior of microdischarges (MDs) in steady-state dielectric barrier discharge (DBD) are presented. MDs of DBD have a spatial “memory”, i.e. every subsequent MD appears exactly at the same place that was occupied by the preceding MD. In most cases each MD appears at its fixed place only once by every half-period (HP). Spatial “memory” is derived from slow recombination of plasma in the MDs channels for a period between two neighbor HPs. In steady-state DBD each plasma column was formed only one-time due to local avalanche-streamer breakdown in the very first (initial) gas gap breakdown under inception voltage $U^*$ . After that DBD is sustained under voltage lower than $U^*$ . For the plane-to-plane DBD having the restricted electrode area there is a critical voltage U 1: DBD is in a steady-state if U > U 1 but the DBD decays slowly at voltages below U 1. The decay takes many HPs and occurs due to decreasing the number of MDs inside the gap because of their Brownian motion from central region to the outside of the discharge area. In steady-state DBD there is no correlation between an appearance of alone MD and phase of the applied voltage – each MD has a great scatter in its appearance at the HP. This scatter is attributed to the dispersion in a threshold voltage for local surface breakdowns around the MD base. So, in steady-state DBD the MD volume plasma is responsible for an existence of spatial “memory” (i.e. where the MD appears) but the surface charge distribution around MD is responsible for MD time dispersion (i.e. when the MD appears).

State-dependent lattices for quantum computing with alkaline-earth-metal atoms

Abstract: Recent experimental progress with Alkaline-Earth atoms has opened the door to quantum computing schemes in which qubits are encoded in long-lived nuclear spin states, and the metastable electronic states of these species are used for manipulation and readout of the qubits. Here we discuss a variant of these schemes, in which gate operations are performed in nuclear-spin-dependent optical lattices, formed by near-resonant coupling to the metastable excited state. This provides an alternative to a previous scheme [Phys. Rev. Lett. 101, 170504 (2008)], which involved independent lattices for different electronic states. As in the previous case, we show how existing ideas for quantum computing with Alkali atoms such as entanglement via controlled collisions can be freed from important technical restrictions. We also provide additional details on the use of collisional losses from metastable states to perform gate operations via a lossy blockade mechanism.

Propagation of Airy-Gaussian beams in a quadratic-index medium

Abstract: The propagation properties of the Airy-Gaussian beams are introduced in a quadratic-index medium analytically and numerically. The linear momentum, the beam width, the beam center of the Airy-Gaussian beam are analyzed. The Airy-Gaussian beam shape and curvature repeats itself at intervals proportional to \hbox{$1/\sqrt{az_{R}}$}1/azR due to the potential barrier. The bend (the transversely acceleration) and the periodicity of the Airy-Gaussian beam depend on the parameters χ0 and azR.