116 research outputs found
Theory of elementary excitations in unstable Bose-Einstein condensates
Like classical fluids, quantum gases may suffer from hydrodynamic instabilities. Our paper develops a quantum version of the classical stability analysis in fluids, the Bogoliubov theory of elementary excitations in unstable Bose-Einstein condensates. In unstable condensates the excitation modes have complex frequencies. We derive the normalization conditions for unstable modes such that they can serve in a mode decomposition of the noncondensed component. Furthermore, we develop approximative techniques to determine the spectrum and the mode functions. Finally, we apply our theory to sonic horizons - sonic black and white holes. For sonic white holes the spectrum of unstable modes turns out to be intrinsically discrete, whereas black holes may be stable
Vortex sorter for Bose-Einstein condensates
We have designed interferometers that sort Bose-Einstein condensates into
their vortex components. The Bose-Einstein condensates in the two arms of the
interferometer are rotated with respect to each other through fixed angles;
different vortex components then exit the interferometer in different
directions. The method we use to rotate the Bose-Einstein condensates involves
asymmetric phase imprinting and is itself new. We have modelled rotation
through fixed angles and sorting into vortex components with even and odd
values of the topological charge of 2-dimensional Bose-Einstein condensates in
a number of states (pure or superposition vortex states for different values of
the scattering length). Our scheme may have applications for quantum
information processing.Comment: 4 pages, high resolution figures can be obtained from the author
Non-Abelian gauge potentials for ultra-cold atoms with degenerate dark states
We show that the adiabatic motion of ultracold, multilevel atoms in spatially varying laser fields can give rise to effective non-Abelian gauge fields if degenerate adiabatic eigenstates of the atom-laser interaction exist. A pair of such degenerate dark states emerges, e.g., if laser fields couple three internal states of an atom to a fourth common one under pairwise two-photon-resonance conditions. For this so-called tripod scheme we derive general conditions for truly non-Abelian gauge potentials and discuss special examples. In particular we show that using orthogonal laser beams with orbital angular momentum an effective magnetic field can be generated that has a monopole component
Finite-temperature properties of quasi-2D Bose-Einstein condensates
Using the finite-temperature path integral Monte Carlo method, we investigate
dilute, trapped Bose gases in a quasi-two dimensional geometry. The quantum
particles have short-range, s-wave interactions described by a hard-sphere
potential whose core radius equals its corresponding scattering length. The
effect of both the temperature and the interparticle interaction on the
equilibrium properties such as the total energy, the density profile, and the
superfluid fraction is discussed. We compare our accurate results with both the
semi-classical approximation and the exact results of an ideal Bose gas. Our
results show that for repulsive interactions, (i) the minimum value of the
aspect ratio, where the system starts to behave quasi-two dimensionally,
increases as the two-body interaction strength increases, (ii) the superfluid
fraction for a quasi-2D Bose gas is distinctly different from that for both a
quasi-1D Bose gas and a true 3D system, i.e., the superfluid fraction for a
quasi-2D Bose gas decreases faster than that for a quasi-1D system and a true
3D system with increasing temperature, and shows a stronger dependence on the
interaction strength, (iii) the superfluid fraction for a quasi-2D Bose gas
lies well below the values calculated from the semi-classical approximation,
and (iv) the Kosterlitz-Thouless transition temperature decreases as the
strength of the interaction increases.Comment: 6 pages, 5 figure
Vortex nucleation in bose-einstein condensates due to effective magnetic fields
We investigate the rotational properties of a Bose-Einstein condensate (BEC) in an effective magnetic field. The corresponding gauge potential is optically generated, and based on the adiabatic motion of the atoms. We demonstrate that the nucleation of vortices is seeded by instabilities in surface excitations and show that this picture also holds when the applied effective magnetic field is not homogeneous. The eventual configuration of vortices in the cloud depends on the geometry of the applied field
Bragg spectroscopy of a cigar shaped Bose condensate in optical lattices
We study properties of excited states of an array of weakly coupled
quasi-two-dimensional Bose condensates by using the hydrodynamic theory. We
calculate multibranch Bogoliubov-Bloch spectrums and its corresponding
eigenfunctions. The spectrum of the axial excited states and its eigenfunctions
strongly depends on the coupling among various discrete radial modes within a
given symmetry. This mode coupling is due to the presence of radial trapping
potential. The multibranch nature of the Bogoliubov-Bloch spectrum and its
dependence on the mode-coupling can be realized by analyzing dynamic structure
factor and momentum transferred to the system in Bragg spectroscopy
experiments. We also study dynamic structure factor and momentum transferred to
the condensate due to the Bragg spectroscopy experiment.Comment: 7 pages, 5 figures, to appear in Journal of Physics B: Atomic,
Molecular & Optical Physic
Excitation of a Dipole Topological Mode in a Strongly Coupled Two-Component Bose-Einstein Condensate
Two internal hyperfine states of a Bose-Einstein condensate in a dilute
magnetically trapped gas of Rb atoms are strongly coupled by an
external field that drives Rabi oscillations between the internal states. Due
to their different magnetic moments and the force of gravity, the trapping
potentials for the two states are offset along the vertical axis, so that the
dynamics of the internal and external degrees of freedom are inseparable. The
rapid cycling between internal atomic states in the displaced traps results in
an adiabatic transfer of population from the condensate ground state to its
first antisymmetric topological mode. This has a pronounced effect on the
internal Rabi oscillations, modulating the fringe visibility in a manner
reminiscent of collapses and revivals. We present a detailed theoretical
description based on zero-temperature mean-field theory.Comment: 10 pages, 8 eps figures included; submitted to PR
Phase separation and vortex states in binary mixture of Bose-Einstein condensates in the trapping potentials with displaced centers
The system of two simultaneously trapped codensates consisting of
atoms in two different hyperfine states is investigated theoretically in the
case when the minima of the trapping potentials are displaced with respect to
each other. It is shown that the small shift of the minima of the trapping
potentials leads to the considerable displacement of the centers of mass of the
condensates, in agreement with the experiment. It is also shown that the
critical angular velocities of the vortex states of the system drastically
depend on the shift and the relative number of particles in the condensates,
and there is a possibility to exchange the vortex states between condensates by
shifting the centers of the trapping potentials.Comment: 4 pages, 2 figure
Dielectric formalism and damping of collective modes in trapped Bose-Einstein condensed gases
We present the general dielectric formalism for Bose-Einstein condensed
systems in external potential at finite temperatures. On the basis of a model
arising within this framework as a first approximation in an intermediate
temperature region for large condensate we calculate the damping of low-energy
excitations in the collisionless regime.Comment: 4 pages, no figures, RevTe
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