140 research outputs found
Spatially selective Bragg scattering: a signature for vortices in Bose-Einstein condensates
We demonstrate that Bragg scattering from a condensate can be sensitive to
the spatial phase distribution of the initial state. This allows preferential
scattering from a selected spatial region, and provides a robust signature for
a vortex state. We develop an analytic model which accurately describes this
phenomenon and we give quantitative predictions for current experimental
conditions.Comment: 5 pages, 3 figure
Bragg spectroscopy of an accelerating condensate with solitary-wave behaviour
We present a theoretical treatment of Bragg spectroscopy of an accelerating
condensate in a solitary-wave state. Our treatment is based on the
Gross-Pitaevskii equation with an optical potential representing the Bragg
pulse and an additional external time-dependent potential generating the
solitary-wave behaviour. By transforming to a frame translating with the
condensate, we derive an approximate set of equations that can be readily
solved to generate approximate Bragg spectra. Our analytic method is accurate
within a well defined parameter regime and provides physical insight into the
structure of the spectra. We illustrate our formalism using the example of
Bragg spectroscopy of a condensate in a time-averaged orbiting potential trap.Comment: 9 pages, 3 figure
Quantum kinetic theory VII: The influence of vapor dynamics on condensate growth
We extend earlier models of the growth of a Bose-Einstein condensate to
include the full dynamical effects of the thermal cloud by numerically solving
a modified quantum Boltzmann equation. We determine the regime in which the
assumptions of the simple model are a reasonable approximation, and compare our
new results with those that were earlier compared with experimental data. We
find good agreement with our earlier modelling, except at higher condensate
fractions, for which a significant speedup is found. We also investigate the
effect of temperature on condensate growth, and find that this has a
surprisingly small effect.
The discrepancy between theory and experiment remains, since the speedup
found in these computations does not occur in the parameter regime specified in
the the experiment.Comment: Fourteen pages, TeX source with 11 figures. Changes : Extended
section on formalism to include a derivation of the ergodic Boltzmann
equation that we use, and a fuller explanation of the numerical methods.
Explained more fully the possible errors with the experimental data. Added
section detailing the source of possible errors in this formulation. Added
comparison of our work with the manuscript cond-mat/0001323, and some
analysis of the fits to the MIT growth curve
Stabilizing an atom laser using spatially selective pumping and feedback
We perform a comprehensive study of stability of a pumped atom laser in the
presence of pumping, damping and outcoupling. We also introduce a realistic
feedback scheme to improve stability by extracting energy from the condensate
and determine its effectiveness. We find that while the feedback scheme is
highly efficient in reducing condensate fluctuations, it usually does not alter
the stability class of a particular set of pumping, damping and outcoupling
parameters.Comment: 7 figure
Solitary-wave description of condensate micro-motion in a time-averaged orbiting potential trap
We present a detailed theoretical analysis of micro-motion in a time-averaged
orbiting potential trap. Our treatment is based on the Gross-Pitaevskii
equation, with the full time dependent behaviour of the trap systematically
approximated to reduce the trapping potential to its dominant terms. We show
that within some well specified approximations, the dynamic trap has
solitary-wave solutions, and we identify a moving frame of reference which
provides the most natural description of the system. In that frame eigenstates
of the time-averaged orbiting potential trap can be found, all of which must be
solitary-wave solutions with identical, circular centre of mass motion in the
lab frame. The validity regime for our treatment is carefully defined, and is
shown to be satisfied by existing experimental systems.Comment: 12 pages, 2 figure
Bragg scattering of Cooper pairs in an ultra-cold Fermi gas
We present a theoretical treatment of Bragg scattering of a degenerate Fermi
gas in the weakly interacting BCS regime. Our numerical calculations predict
correlated scattering of Cooper pairs into a spherical shell in momentum space.
The scattered shell of correlated atoms is centered at half the usual Bragg
momentum transfer, and can be clearly distinguished from atoms scattered by the
usual single-particle Bragg mechanism. We develop an analytic model that
explains key features of the correlated-pair Bragg scattering, and determine
the dependence of this scattering on the initial pair correlations in the gas.Comment: Manuscript substantially revised. Version 2 contains a more detailed
discussion of the collisional interaction used in our theory, and is based on
three-dimensional solution
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