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  • 1.
    Bezett, Alice
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kobyakov, Dmitry
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Magnetic Richtmyer-Meshkov instability in a two-component Bose-Einstein condensate2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 82, no 4, p. 043608-Article, review/survey (Refereed)
  • 2.
    Cetoli, Alberto
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Correlations and superfluidity of a one-dimensional Bose gas in a quasiperiodic potential2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 6, p. 063635-063642Article in journal (Refereed)
    Abstract [en]

    We consider the correlations and superfluid properties of a Bose gas in an external potential. Using a Bogoliubov scheme, we obtain expressions for the correlation function and the superfluid density in an arbitrary external potential. These expressions are applied to a one-dimensional system at zero temperature subject to a quasiperiodic modulation. The critical parameters for the Bose glass transition are obtained using two different criteria and the results are compared. The Lifshitz glass is seen to be the limiting case for vanishing interactions.

  • 3. De Sarlo, Luigi
    et al.
    Fallani, Leonardo
    Lye, Jessica E.
    Modugno, Michele
    Saers, Robert
    Umeå University, Faculty of Science and Technology, Physics.
    Fort, Chiara
    Inguscio, Massimo
    Unstable regimes for a Bose-Einstein condensate in an optical lattice2005In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, p. 013603-Article in journal (Refereed)
  • 4.
    Dion, Claude
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jonsell, Svante
    Stockholm University.
    Kastberg, Anders
    University of Nice.
    Sjölund, Peder
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bimodal momentum distribution of laser-cooled atoms in optical lattices2016In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, no 5, article id 053416Article in journal (Refereed)
    Abstract [en]

    We study, numerically and experimentally, the momentum distribution of atoms cooled in optical lattices. Using semiclassical simulations, we show that this distribution is bimodal, made up of a central feature corresponding to "cold," trapped atoms, with tails of "hot," untrapped atoms, and that this holds true also for very shallow potentials. Careful analysis of the distribution of high-momentum untrapped atoms, both from simulations and experiments, shows that the tails of the distribution do not follow a normal law, hinting at a power-law distribution and nonergodic behavior. We also revisit the phenomenon leading to the existence of an optimal cooling point, i.e., a potential depth below which the temperature of the atoms starts increasing.

  • 5.
    Eriksson, Daniel
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Physics.
    Stenflo, Lennart
    Umeå University, Faculty of Science and Technology, Physics.
    Possibility to measure elastic photon-photon scattering in vacuum2004In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, no 1, p. 013808-Article in journal (Refereed)
    Abstract [en]

    Photon-photon scattering in vacuum due to the interaction with virtual electron-positron pairs is a consequence of quantum electrodynamics. A way for detecting this phenomenon has been devised based on interacting modes generated in microwave wave guides or cavities [G. Brodin, M. Marklund, and L. Stenflo, Phys. Rev. Lett. 87, 171801 (2001)]. Here we materialize these ideas, suggest a concrete cavity geometry, make quantitative estimates and propose experimental details. It is found that detection of photon-photon scattering can be within the reach of present day technology.

  • 6. Harvey, C. N.
    et al.
    Gonoskov, A.
    Marklund, M.
    Wallin, Erik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Narrowing of the emission angle in high-intensity Compton scattering2016In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 93, no 2, article id 022112Article in journal (Refereed)
    Abstract [en]

    We consider the emission spectrum of high-energy electrons in an intense laser field. At high intensities (a0∼200) we find that the QED theory predicts a narrower angular spread of emissions than the classical theory. This is due to the classical theory overestimating the energy loss of the particles, resulting in them becoming more susceptible to reflection in the laser pulse.

  • 7.
    Harvey, Christopher
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Radiation damping in pulsed Gaussian beams2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 1, p. 013412-013424Article in journal (Refereed)
    Abstract [en]

    We consider the effects of radiation damping on the electron dynamics in a Gaussian-beam model of a laser field. For high intensities, i.e., with dimensionless intensity a0≫1, it is found that the dynamics divides into three regimes. For low-energy electrons (low initial γ factor, γ0) the radiation damping effects are negligible. At higher energies, but still at 2γ0<a0, the damping alters the final displacement and the net energy change of the electron. For 2γ0>a0 one is in a regime of radiation-reaction-induced electron capture. This capture is found to be stable with respect to the spatial properties of the electron beam and results in a significant energy loss of the electrons. In this regime the plane-wave model of the laser field provides a good description of the dynamics, whereas for lower energies the Gaussian-beam and plane-wave models differ significantly. Finally the dynamics is considered for the case of an x-ray free-electron laser field. It is found that the significantly lower intensities of such fields inhibit the damping effects.

  • 8.
    Ilderton, Anton
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pair annihilation in laser pulses: Optical versus x-ray free-electron laser regimes2011In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 84, no 3, p. 032119-Article in journal (Refereed)
    Abstract [en]

    We discuss the theory and phenomenology of pair annihilation, within an ultrashort laser pulse, to a single photon. The signature of this process is the unidirectional emission of single photons with a fixed energy. We show that the cross section is significantly larger than for two-photon pair annihilation in vacuum, with x-ray free-electron laser parameters admitting a much clearer signal than optical beams.

  • 9.
    Jensen, L.M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Nilsen, H.M.
    Watanabe, G.
    BCS-BEC crossover in atomic Fermi gases with a narrow resonance2006In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, no 4, p. 043608-Article in journal (Refereed)
  • 10.
    Jonsell, Svante
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Froelich, P.
    Eriksson, S.
    Strasburger, K.
    Strong nuclear force in cold antihydrogen-helium collisions2004In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 70, no 6, p. 062708-Article in journal (Refereed)
    Abstract [en]

    We calculate cross sections for elastic scattering and annihilation in antihydrogen-helium collisions at low energies. The calculations are based on the Born-Oppenheimer approximation, and incorporate the effects of the strong interaction through a scattering length approach. We find that the strong nuclear force not only causes significant annihilation, but also cannot be neglected in the elastic channel. In the zero energy limit we obtain the scattering length a=−7.69−3.80i a.u. for ground state antihydrogen-helium collisions. Annihilation is found to dominate over elastic scattering up to a temperature about 3 K. Loosely bound metastable antihydrogen-helium states are also investigated, and it is found that a number of relatively long-lived states with up to three units of angular momentum exist.

  • 11.
    Kobyakov, Dmitry
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bezett, A.
    Institute for Theoretical Physics, Utrecht University, The Netherlands.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Department of Applied Physics, Division of Condensed Matter Theory, Chalmers University of Technology.
    Bychkov, Vitaliy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Turbulence in binary Bose-Einstein condensates generated by highly nonlinear Rayleigh-Taylor and Kelvin-Helmholtz instabilities2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 89, p. 013631-Article in journal (Refereed)
    Abstract [en]

    Quantum turbulence (QT) generated by the Rayleigh-Taylor instability in binary immiscible ultracold 87Rb atoms at zero temperature is studied theoretically. We show that the quantum vortex tangle is qualitatively different from previously considered superfluids, which reveals deep relations between QT and classical turbulence. The present QT may be generated at arbitrarily small Mach numbers, which is a unique property not found in previously studied superfluids. By numerical solution of the coupled Gross-Pitaevskii equations we find that the Kolmogorov scaling law holds for the incompressible kinetic energy. We demonstrate that the phenomenon may be observed in the laboratory.

  • 12.
    Kobyakov, Dmitry
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bezett, Alice
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quantum swapping of immiscible Bose-Einstein condensates as an alternative to the Rayleigh-Taylor instability2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 1, p. 013630-Article, review/survey (Refereed)
    Abstract [en]

    We consider a two-component Bose-Einstein condensate in a quasi-one-dimensional harmonic trap, where the immiscible components are pressed against each other by an external magnetic force. The zero-temperature nonstationary Gross-Pitaevskii equations are solved numerically; analytical models are developed for the key steps in the process. We demonstrate that if the magnetic force is strong enough, then the condensates may swap their places in the trap due to dynamic quantum interpenetration of the nonlinear matter waves. The swapping is accompanied by development of a modulational instability leading to quasiturbulent excitations. Unlike the multidimensional Rayleigh-Taylor instability in a similar geometry of two-component quantum fluid systems, quantum interpenetration has no classical analog. In a two-dimensional geometry a crossover between the Rayleigh-Taylor instability and the dynamic quantum interpenetration is investigated.

  • 13.
    Kobyakov, Dmitry
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bezett, A
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Parametric resonance of capillary waves at the interface between two immiscible Bose-Einstein condensates2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 86, no 2, p. 023614-Article in journal (Refereed)
    Abstract [en]

    We study the parametric resonance of capillary waves on the interface between two immiscible Bose-Einstein condensates pushed towards each other by an oscillating force. Guided by analytical models, we solve numerically the coupled Gross-Pitaevskii equations for a two-component Bose-Einstein condensate at zero temperature. We show that, at moderate amplitudes of the driving force, the instability is stabilized due to nonlinear modifications of the oscillation frequency. When the amplitude of the driving force is large enough, we observe a detachment of droplets from the Bose-Einstein condensates, resulting in the generation of quantum vortices (skyrmions). We analytically investigate the vortex dynamics, and conditions of quantized vortex generation.

  • 14.
    Kobyakov, Dmitry
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bezett, Alice
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Akkerman, Vyacheslav
    Princeton University.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Interface dynamics of a two-component Bose-Einstein condensate driven by an external force2011In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 83, no 4, p. 043623-Article in journal (Refereed)
    Abstract [en]

    The dynamics of an interface in a two-component Bose-Einstein condensate driven by a spatially uniform time-dependent force is studied. Starting from the Gross-Pitaevskii Lagrangian, the dispersion relation for linear waves and instabilities at the interface is derived by means of a variational approach. A number of diverse dynamical effects for different types of driving force is demonstrated, which includes the Rayleigh-Taylor instability for a constant force, the Richtmyer-Meshkov instability for a pulse force, dynamic stabilization of the Rayleigh-Taylor instability and onset of the parametric instability for an oscillating force. Gaussian Markovian and non-Markovian stochastic forces are also considered. It is found that the Markovian stochastic force does not produce any average effect on the dynamics of the interface, while the non-Markovian force leads to exponential perturbation growth.

  • 15. Kolliopoulos, G.
    et al.
    Bergues, B.
    Schröder, H.
    Carpeggiani, P. A.
    Veisz, Laszlo
    Max-Planck-Institut für Quantenoptik, Garching, Germany.
    Tsakiris, G. D.
    Charalambidis, D.
    Tzallas, P.
    Revealing quantum path details in high-field physics2014In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 90, no 1, article id 013822Article in journal (Refereed)
    Abstract [en]

    The fundamental mechanism underlying harmonic emission in the strong-field regime is governed by tunnel ionization of the atom, followed by the motion of the electron wave packet in the continuum, and finally by its recollision with the atomic core. Due to the quantum nature of the process, the properties of the electron wave packet strongly correlate with those of the emitted radiation. Here, by spatially resolving the interference pattern generated by overlapping the harmonic radiation emitted by different interfering electron quantum paths, we have succeeded in unravelling the intricacies associated with the recollision process. This has been achieved by mapping the spatial extreme-ultraviolet (EUV)-intensity distribution onto a spatial ion distribution, produced in the EUV focal area through the linear and nonlinear processes of atoms. By in situ manipulation of the intensity-dependent motion of the electron wave packets, we have been able to directly measure the difference between the harmonic emission times and electron path lengths resulting from different electron trajectories. Due to the high degree of accuracy that the present approach provides, we have been able to demonstrate the quantum nature of the recollision process. This is done by quantitatively correlating the photoemission time and the electron quantum path-length differences, taking into account the energy-momentum transfer from the driving laser field into the system. This information paves the way for electron-photon correlation studies at the attosecond time scale, while it puts the recollision process from the semiclassical prospective into a full quantum-mechanical context.

  • 16.
    Kuopanportti, Pekko
    et al.
    Aalto, Finland.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Huhtamaki, Jukka A. M.
    Aalto, Finland; Okayama, Japan.
    Pietila, Ville
    Aalto, Finland; Sydney NSW 2052, Australia.
    Mottonen, Mikko
    Aalto, Finland; Sydney NSW 2052, Australia.
    Core sizes and dynamical instabilities of giant vortices in dilute Bose-Einstein condensates2010In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 81, no 2, article id 023603Article in journal (Refereed)
    Abstract [en]

    Motivated by a recent demonstration of cyclic addition of quantized vorticity into a Bose-Einstein condensate, the vortex pump, we study dynamical instabilities and core sizes of giant vortices. The core size is found to increase roughly as a square-root function of the quantum number of the vortex, whereas the strength of the dynamical instability either saturates to a fairly low value or increases extremely slowly for large quantum numbers. Our studies suggest that giant vortices of very high angular momenta may be achieved by gradually increasing the operation frequency of the vortex pump.

  • 17.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mott-insulator dynamics2011In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 84, no 3, p. 6-Article in journal (Refereed)
    Abstract [en]

    The hydrodynamics of a lattice Bose gas in a time-dependent external potential is studied in a mean-field approximation. The conditions under which a Mott insulating region can melt, and the local density can adjust to the new potential, are determined. In the case of a suddenly switched potential, it is found that the Mott insulator stays insulating and the density will not adjust if the switch is too abrupt. This comes about because too rapid currents result in Bloch oscillation-type current reversals. For a stirrer moved through a Mott insulating cloud, it is seen that only if the stirrer starts in a superfluid region and the velocity is comparable to the time scale set by the tunneling will the Mott insulator be affected.

  • 18.
    Lundh, Emil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cetoli, Alberto
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hydrodynamic theory of giant vortices in trapped unitary Fermi gases2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 80, no 2, p. 023610-Article in journal (Refereed)
    Abstract [en]

    The rotational properties are studied for a unitary superfluid gas of fermions at zero temperature. Using a hydrodynamic approach, the conditions for the formation of a giant vortex are discussed. It is found that in the present approximation, an anharmonic addition to the usual harmonic-oscillator type of trap potential is necessary for the energetic stability of a giant vortex. To determine the conditions quantitatively, a Thomas-Fermi approach is compared with numerical solutions in three dimensions.

  • 19.
    Lundh, Emil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Martikainen, J-P
    Kelvin-Helmholtz instability in two-component Bose gases on a lattice2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 2, p. 023628-Article in journal (Refereed)
    Abstract [en]

    We explore the stability of the interface between two phase-separated Bose gases in relative motion on a lattice. Gross-Pitaevskii-Bogoliubov theory and the Gutzwiller ansatz are employed to study the short-and long-time stability properties. The underlying lattice introduces effects of discreteness, broken spatial symmetry, and strong correlations, all three of which are seen to have considerable qualitative effects on the Kelvin-Helmholtz instability. Discreteness is found to stabilize low flow velocities because of the finite energy associated with displacing the interface. Broken spatial symmetry introduces a dependence not only on the relative flow velocity but also on the absolute velocities. Strong correlations close to a Mott transition will stop the Kelvin-Helmholtz instability from affecting the bulk density and creating turbulence; instead, the instability will excite vortices with Mott-insulator-filled cores.

  • 20.
    Lundh, Emil
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rammer, Jörgen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Effective action approach to a trapped Bose gas2002In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 66, no 3, p. 033607-12 pArticle in journal (Refereed)
    Abstract [en]

    The effective-action formalism is applied to a gas of bosons. The equations describing the condensate and the excitations are obtained using the loop expansion for the effective action. For a homogeneous gas, the expansion in terms of the diluteness parameter is identified in terms of the loop expansion. The loop expansion and the limits of validity of the well-known Bogoliubov [J. Phys. (Moscow) 11, 23 (1947)] and Popov, (Zh. Éksp. Teor. Fiz. 47, 1759 (1964) [Sov. Phys. JETP 20, 1185 (1965)]) equations are examined analytically for a homogeneous dilute Bose gas and numerically for a gas trapped in a harmonic-oscillator potential. The expansion to one-loop order, and hence the Bogoliubov equation, is shown to be valid for the zero-temperature trapped gas as long as the characteristic length of the trapping potential exceeds the s-wave scattering length.

  • 21.
    Lundin, Joakim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundström, E.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Collier, J.
    Bingham, R.
    Mendonça, J.T.
    Norreys, P.
    Analysis of four-wave mixing of high-power lasers for the detection of elastic photon-photon scattering2006In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, no 4, p. 043821-043831Article in journal (Refereed)
    Abstract [en]

    We derive expressions for the coupling coefficients for electromagnetic four-wave mixing in the nonlinear quantum vacuum. An experimental setup for detection of elastic photon-photon scattering is suggested, where three incoming laser pulses collide and generate a fourth wave with a new frequency and direction of propagation. An expression for the number of scattered photons is derived and, using beam parameters for the Astra Gemini system at the Rutherford Appleton Laboratory, it is found that the signal can reach detectable levels. Problems with shot-to-shot reproducibility are reviewed, and the magnitude of the noise arising from competing scattering processes is estimated. It is found that detection of elastic photon-photon scattering may be achieved.

  • 22.
    Makela, Harri
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Johansson, M.
    Zelan, M.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stability of nonstationary states of spin-1 Bose-Einstein condensates2011In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 84, no 4, p. 043646-Article in journal (Refereed)
    Abstract [en]

    The stability of nonstationary states of homogeneous spin-1 Bose-Einstein condensates is studied by performing Bogoliubov analysis in a frame of reference where the state is stationary. In particular, the effect of an external magnetic field is examined. It is found that a nonzero magnetic field introduces instability in a (23)Na condensate. The wavelengths of this instability can be controlled by tuning the strength of the magnetic field. In a (87)Rb condensate this instability is present already at zero magnetic field. Furthermore, an analytical bound for the size of a stable condensate is found, and a condition for the validity of the single-mode approximation is presented. Realization of the system in a toroidal trap is discussed, and the full time development is simulated.

  • 23. Martikainen, Jani-Petri
    et al.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Paananen, Tomi
    Interband physics in an ultracold fermi gas in an optical lattice2008In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 78, no 2, p. 023607-Article in journal (Refereed)
    Abstract [en]

    We study a gas of strongly polarized cold fermions in an optical lattice when the excited p bands are populated. We derive the relevant Hamiltonian and discuss the expected phase diagram for both repulsive and attractive interactions. In the parameter regime covered here, checkerboard antiferromagnetic ordering is found to be possible for repulsive interactions, while for attractive interactions, transitions between different types of paired phases are predicted.

  • 24.
    Mäkelä, H.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, E.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Excitation spectrum of a toroidal spin-1 Bose-Einstein condensate2013In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 88, no 3, p. Article Number: 033622-Article in journal (Refereed)
    Abstract [en]

    We calculate analytically the Bogoliubov excitation spectrum of a toroidal spin-1 Bose-Einstein condensate that is subjected to a homogeneous magnetic field and contains vortices with arbitrary winding numbers in the m(F) = +/- 1 components of the hyperfine spin. We show that a rotonlike spectrum can be obtained, or an initially stable condensate can be made unstable by adjusting the magnitude of the magnetic field or the trapping frequencies. The structure of the instabilities can be analyzed by measuring the particle densities of the spin components. We confirm the validity of the analytical calculations by numerical simulations.

  • 25.
    Mäkelä, Harri
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stability of nonstationary states of spin-2 Bose-Einstein condensates2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 5, p. 053622-Article in journal (Refereed)
    Abstract [en]

    The dynamical stability of nonstationary states of homogeneous spin-2 rubidium Bose-Einstein condensates is studied. The states considered are such that the spin vector remains parallel to the magnetic field throughout the time evolution, making it possible to study the stability analytically. These states are shown to be stable in the absence of an external magnetic field, but they become unstable when a finite magnetic field is introduced. It is found that the growth rate and wavelength of the instabilities can be controlled by tuning the strength of the magnetic field and the size of the condensate.

  • 26. Nilsen, Halvor M.
    et al.
    Lundh, Emil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Splitting dynamics of doubly quantized vortices in Bose-Einstein condensates2008In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 77, no 013604Article in journal (Refereed)
    Abstract [en]

    We study the nonlinear dynamics of the splitting of a doubly quantized vortex in a trapped Bose-Einstein condensate. The dynamics is studied in detail by solving the Gross-Pitaevskii equation. The main dynamical features are explained in terms of a nonlinear three-level system, to which we find an analytical solution. This result is compared to linear Bogoliubov analysis and to the full numerical time evolution. It is concluded that the time scale for the splitting is mainly determined by the instability of the linearized system, and nonlinear effects contribute logarithmically.

  • 27. Ramachandran, Hema
    et al.
    Kastberg, Anders
    Umeå University, Faculty of Science and Technology, Physics.
    Narayanan, Andal
    Srinivasan, R.
    Protocol for quantum-logical operations in an optical lattice using nonresonant absorption of photon pairs2006In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 74, no 4, p. 042314-Article in journal (Refereed)
  • 28.
    Salasnich, Luca
    et al.
    Dipartimento di Fisica “Galileo Galilei,” Università di Padova, Via Marzolo 8, 35131 Padova, Italy.
    Cetoli, Alberto
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Malomed, Boris
    Department of Interdisciplinary Studies, School of Electrical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel.
    Toigo, Flavio
    Dipartimento di Fisica “Galileo Galilei,” Università di Padova, Via Marzolo 8, 35131 Padova, Italy.
    Nearly-one-dimensional self-attractive Bose-Einstein condensates in optical lattices2007In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 75, p. 013623-Article in journal (Refereed)
    Abstract [en]

    Within the framework of a mean-field description, we investigate atomic Bose-Einstein condensates, with attraction between atoms, under the action of a strong transverse confinement and periodic [optical-lattice (OL)] axial potential. Using a combination of the variational approximation, one-dimensional (1D) nonpolynomial Schrödinger equation, and direct numerical solutions of the underlying 3D Gross-Pitaevskii equation, we show that the ground state of the condensate is a soliton belonging to the semi-infinite band gap of the periodic potential. The soliton may be confined to a single cell of the lattice or extended to several cells, depending on the effective self-attraction strength g (which is proportional to the number of atoms bound in the soliton) and depth of the potential, V0, the increase of V0 leading to strong compression of the soliton. We demonstrate that the OL is an effective tool to control the soliton’s shape. It is found that, due to the 3D character of the underlying setting, the ground-state soliton collapses at a critical value of the strength, g=gc, which gradually decreases with the increase of V0; under typical experimental conditions, the corresponding maximum number of 7Li atoms in the soliton, Nmax, ranges between 8000 and 4000. Examples of stable multipeaked solitons are also found in the first finite band gap of the lattice spectrum. The respective critical value gc again slowly decreases with the increase of V0, corresponding to Nmax≃5000.

  • 29.
    Salasnich, Luca
    et al.
    Dipartimento di Fisica “Galileo Galilei,” Università di Padova, Via Marzolo 8, 35131 Padova, Italy.
    Cetoli, Alberto
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Malomed, Boris
    Department of Interdisciplinary Studies, School of Electrical Engineering, Faculty of Engineering, Tel Aviv University, Tel Aviv 69978, Israel.
    Toigo, Flavio
    Dipartimento di Fisica “Galileo Galilei,” Università di Padova, Via Marzolo 8, 35131 Padova, Italy.
    Reatto, Luciano
    Dipartimento di Fisica and CNISM, Università di Milano, Via Celoria 16, 20133 Milano, Italy.
    Bose-Einstein condensates under a spatially modulated transverse confinement2007In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 76, p. 013623-Article in journal (Refereed)
    Abstract [en]

    We derive an effective nonpolynomial Schrödinger equation (NPSE) for self-repulsive or attractive BEC in the nearly one-dimensional cigar-shaped trap, with the transverse confining frequency periodically modulated along the axial direction. In addition to the usual linear cigar-shaped trap, where the periodic modulation emulates the action of an optical lattice (OL), the model may be also relevant to toroidal traps, where an ordinary OL cannot be created. For either sign of the nonlinearity, extended and localized states are found, in the numerical form [using both the effective NPSE and the full three-dimensional (3D) Gross-Pitaevskii equation] and by means of the variational approximation (VA). The latter is applied to construct ground-state solitons and predict the collapse threshold in the case of self-attraction. It is shown that numerical solutions provided by the one-dimensional NPSE are always very close to full 3D solutions, and the VA yields quite reasonable results too. The transition from delocalized states to gap solitons, in the first finite bandgap of the linear spectrum, is examined in detail, for the repulsive and attractive nonlinearities alike.

  • 30.
    Satapathy, Nandan
    et al.
    Raman Research Institute, Sadashivnagar, Bangalore 560 080, India.
    Hagman, Henning
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zelan, Martin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kastberg, Anders
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ramachandran, Hema
    Raman Research Institute, Sadashivnagar, Bangalore 560 080, India.
    Theoretical investigation of quantum walks by cold atoms in a double optical lattice2009In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 80, no 1, p. 012302-Article in journal (Refereed)
    Abstract [en]

    We investigate the feasibility of carrying out quantum walks with cold atoms in a double optical lattice. Monte Carlo simulations of time-of-flight (TOF) detection and absorption imaging were carried out, focusing on a specific experimental implementation. These indicate that absorption imaging would be best suited for detection of quantum walks. With typical experimental parameters a few hundred quantum walk steps will be needed for an unambiguous detection of the quantum walk signature. We show that in special cases, few-step quantum walks can also be detected in our system if one measures the relative population of the atoms in their internal states rather than their displacement in space, that is, measurements are made in the space of the coin operator rather than in that of the displacement operator.

  • 31. Schmidt, HT
    et al.
    Jensen, J
    Reinhed, P
    Schuch, R
    Stöchkel, Kristian
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, Stockholm University, S-10691 Stockholm, Sweden.
    Zettergren, H
    Cederquist, H
    Bagge, L
    Danared, H
    Källberg, A
    Schmidt-Böcking, H
    Cocke, CL
    Recoil-ion momentum distributions for transfer ionization in fast proton-He collisions2005In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 72, no 1, article id 012713Article in journal (Refereed)
    Abstract [en]

    We present high-luminosity experimental investigations of the transfer ionization (TI:p+He -> H-0+He2++e(-)) process in collisions between fast protons and neutral helium atoms in the earlier inaccessibly high-energy range 1.4-5.8 MeV. The protons were stored in the heavy-ion storage and cooler ring CRYRING, where they intersected a narrow supersonic helium gas jet. We discuss the longitudinal recoil-ion momentum distribution, as measured by means of cold-target recoil-ion momentum spectroscopy and find that this distribution splits into two completely separated peaks at the high end of our energy range. These separate contributions are discussed in terms of the earlier proposed Thomas TI (TTI) and kinematic TI mechansims. The cross section of the TTI process is found to follow a sigma proportional to v(-b) dependence with b=10.78 +/- 0.27 in accordance with the expected v(-11) asymptotic behavior. Further, we discuss the probability for shake-off accompanying electron transfer and the relation of this TI mechanism to photodouble ionization. Finally the influence of the initial-state electron velocity distribution on the TTI process is discussed.

  • 32. Watanabe, Gentaro
    et al.
    Mäkelä, Harri
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dissipation-induced squeezing2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 2, p. 023604-Article in journal (Refereed)
    Abstract [en]

    We present a method for phase and number squeezing in two-mode Bose systems using dissipation. The effectiveness of this method is demonstrated by considering cold Bose gases trapped in a double-well potential. The extension of our formalism to an optical lattice gives control of the phase boundaries of the steady-state phase diagram, and we discover a phase characterized by a nonzero condensate fraction and thermal-like particle-number statistics. We also show how to perform amplitude squeezing in a single-mode system using dissipation.

  • 33.
    Watanabe, Gentaro
    et al.
    APCTP, Pohang 790784, Gyeongbuk, South Korea and POSTECH, Dept Phys, Pohang 790784, Gyeongbuk, South Korea and RIKEN, Nishina Ctr, Wako, Saitama 3510198, Japan.
    Mäkelä, Harri
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Floquet analysis of the modulated two-mode Bose-Hubbard model2012In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 5, p. 053624-Article in journal (Refereed)
    Abstract [en]

    We study the tunneling dynamics in a time-periodically modulated two-mode Bose-Hubbard model using Floquet theory. We consider situations where the system is in the self-trapping regime and either the tunneling amplitude, the interaction strength, or the energy difference between the modes is modulated. In the former two cases, the tunneling is enhanced in a wide range of modulation frequencies, while in the latter case the resonance is narrow. We explain this difference with the help of Floquet analysis. If the modulation amplitude is weak, the locations of the resonances can be found using the spectrum of the nonmodulated Hamiltonian. Furthermore, we use Floquet analysis to explain the coherent destruction of tunneling (CDT) occurring in a large-amplitude modulated system. Finally, we present two ways to create a NOON state (a superposition of N particles in mode 1 with zero particles in mode 2 and vice versa). One is based on a coherent oscillation caused by detuning from a partial CDT. The other makes use of an adiabatic variation of the modulation frequency. This results in a Landau-Zener type of transition between the ground state and a NOON-like state.

  • 34. Zygelman, B.
    et al.
    Saenz, A.
    Froelich, P.
    Jonsell, Svante
    Umeå University, Faculty of Science and Technology, Physics.
    Cold collisions of atomic hydrogen with antihydrogen atoms: An optical potential approach2004In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 69, no 4, p. 042715-Article in journal (Refereed)
1 - 34 of 34
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