21 research outputs found
Formation of Two Component Bose Condensate During the Chemical Potential Curve Crossing
In this article we study the formation of the two modes Bose-Einstein
condensate and the correlation between them. We show that beyond the mean field
approximation the dissociation of a molecular condensate due to the chemical
potential curve crossing leads to the formation of two modes condensate. We
also show that these two modes are correlated in a two mode squeezed state.Comment: 10 page
Formation of a molecular Bose-Einstein condensate and an entangled atomic gas by Feshbach resonance
Processes of association in an atomic Bose-Einstein condensate, and
dissociation of the resulting molecular condensate, due to Feshbach resonance
in a time-dependent magnetic field, are analyzed incorporating non-mean-field
quantum corrections and inelastic collisions. Calculations for the Na atomic
condensate demonstrate that there exist optimal conditions under which about
80% of the atomic population can be converted to a relatively long-lived
molecular condensate (with lifetimes of 10 ms and more). Entangled atoms in
two-mode squeezed states (with noise reduction of about 30 dB) may also be
formed by molecular dissociation. A gas of atoms in squeezed or entangled
states can have applications in quantum computing, communications, and
measurements.Comment: LaTeX, 5 pages with 4 figures, uses REVTeX
Atom loss and the formation of a molecular Bose-Einstein condensate by Feshbach resonance
In experiments conducted recently at MIT on Na Bose-Einstein condensates [S.
Inouye et al, Nature 392, 151 (1998); J. Stenger et al, Phys. Rev. Lett. 82,
2422 (1999)], large loss rates were observed when a time-varying magnetic field
was used to tune a molecular Feshbach resonance state near the state of a pair
of atoms in the condensate. A collisional deactivation mechanism affecting a
temporarily formed molecular condensate [see V. A. Yurovsky, A. Ben-Reuven, P.
S. Julienne and C. J. Williams, Phys. Rev. A 60, R765 (1999)], studied here in
more detail, accounts for the results of the slow-sweep experiments. A best fit
to the MIT data yields a rate coefficient for deactivating atom-molecule
collisions of 1.6e-10 cm**3/s. In the case of the fast sweep experiment, a
study is carried out of the combined effect of two competing mechanisms, the
three-atom (atom-molecule) or four-atom (molecule-molecule) collisional
deactivation vs. a process of two-atom trap-state excitation by curve crossing
[F. H. Mies, P. S. Julienne, and E. Tiesinga, Phys. Rev. A 61, 022721 (2000)].
It is shown that both mechanisms contribute to the loss comparably and
nonadditively.Comment: LaTeX, 14 pages, 12 PostScript figures, uses REVTeX and psfig,
submitted to Physical Review
Heating and atom loss during upward ramps of Feshbach resonance levels in Bose-Einstein condensates
The production of pairs of fast atoms leads to a pronounced loss of atoms
during upward ramps of Feshbach resonance levels in dilute Bose-Einstein
condensates. We provide comparative studies on the formation of these bursts of
atoms containing the physical predictions of several theoretical approaches at
different levels of approximation. We show that despite their very different
description of the microscopic binary physics during the passage of a Feshbach
resonance, all approaches lead to virtually the same prediction on the total
loss of condensate atoms, provided that the ramp of the magnetic field strength
is purely linear. We give the reasons for this remarkable insensitivity of the
remnant condensate fraction to the microscopic physical processes and compare
the theoretical predictions with recent Feshbach resonance crossing experiments
on 23Na and 85Rb.Comment: 12 pages, 7 eps figures; final versio
Tree-body loss of of trapped ultracold Rb atoms due to a Feshbach resonance
The loss of ultracold trapped atoms in the vicinity of a Feshbach resonance
is treated as a two-stage reaction, using the Breit-Wigner theory. The first
stage is the formation of a resonant diatomic molecule, and the second one is
its deactivation by inelastic collisions with other atoms. This model is
applied to the analysis of recent experiments on Rb, leading to an
estimated value of cms for the deactivation rate
coefficient.Comment: LaTeX, 4 pages with 1 figures, uses REVTeX4, uses improved
experimental dat
Quantum effects on the dynamics of a two-mode atom-molecule Bose-Einstein condensate
We study the system of coupled atomic and molecular condensates within the
two-mode model and beyond mean-field theory (MFT). Large amplitude
atom-molecule coherent oscillations are shown to be damped by the rapid growth
of fluctuations near the dynamically unstable molecular mode. This result
contradicts earlier predictions about the recovery of atom-molecule
oscillations in the two-mode limit. The frequency of the damped oscillation is
also shown to scale as with the total number of atoms ,
rather than the expected pure scaling. Using a linearized model, we
obtain analytical expressions for the initial depletion of the molecular
condensate in the vicinity of the instability, and show that the important
effect neglected by mean field theory is an initially non-exponential
`spontaneous' dissociation into the atomic vacuum. Starting with a small
population in the atomic mode, the initial dissociation rate is sensitive to
the exact atomic amplitudes, with the fastest (super-exponential) rate observed
for the entangled state, formed by spontaneous dissociation.Comment: LaTeX, 5 pages, 3 PostScript figures, uses REVTeX and epsfig,
submitted to Physical Review A, Rapid Communication
Second-quantized Landau-Zener theory for dynamical instabilities
State engineering in nonlinear quantum dynamics sometimes may demand driving
the system through a sequence of dynamically unstable intermediate states. This
very general scenario is especially relevant to dilute Bose-Einstein
condensates, for which ambitious control schemes have been based on the
powerful Gross-Pitaevskii mean field theory. Since this theory breaks down on
logarithmically short time scales in the presence of dynamical instabilities,
an interval of instabilities introduces quantum corrections, which may possibly
derail a control scheme. To provide a widely applicable theory for such quantum
corrections, this paper solves a general problem of time-dependent quantum
mechanical dynamical instability, by modelling it as a second-quantized
analogue of a Landau-Zener avoided crossing: a `twisted crossing'.Comment: 4 pages, 3 figure
Bose-Einstein condensate collapse: a comparison between theory and experiment
We solve the Gross-Pitaevskii equation numerically for the collapse induced
by a switch from positive to negative scattering lengths. We compare our
results with experiments performed at JILA with Bose-Einstein condensates of
Rb-85, in which the scattering length was controlled using a Feshbach
resonance. Building on previous theoretical work we identify quantitative
differences between the predictions of mean-field theory and the results of the
experiments. Besides the previously reported difference between the predicted
and observed critical atom number for collapse, we also find that the predicted
collapse times systematically exceed those observed experimentally. Quantum
field effects, such as fragmentation, that might account for these
discrepancies are discussed.Comment: 4 pages, 2 figure
Quantum correlated twin atomic beams via photo-dissociation of a molecular Bose-Einstein condensate
We study the process of photo-dissociation of a molecular Bose-Einstein
condensate as a potential source of strongly correlated twin atomic beams. We
show that the two beams can possess nearly perfect quantum squeezing in their
relative numbers.Comment: Corrected LaTeX file layou
Integrability breakdown in longitudinaly trapped, one-dimensional bosonic gases
A system of identical bosons with short-range (contact) interactions is
studied. Their motion is confined to one dimension by a tight lateral trapping
potential and, additionally, subject to a weak harmonic confinement in the
longitudinal direction. Finite delay time associated with penetration of
quantum particles through each other in the course of a pairwise
one-dimensional collision in the presence of the longitudinal potential makes
the system non-integrable and, hence, provides a mechanism for relaxation to
thermal equilibrium. To analyse this effect quantitatively in the limit of a
non-degenerate gas, we develop a system of kinetic equations and solve it for
small-amplitude monopole oscillations of the gas. The obtained damping rate is
long enough to be neglected in a realistic cold-atom experiment, and therefore
longitudinal trapping does not hinder integrable dynamics of atomic gases in
the 1D regime