65 research outputs found
Properties of quasi two-dimensional condensates in highly anisotropic traps
We theoretically investigate some of the observable properties of quasi
two-dimensional condensates. Using a variational model based on a
Gaussian-parabolic trial wavefunction we calculate chemical potential,
condensate size in time-of-flight, release energy and collective excitation
spectrum for varying trap geometries and atom numbers and find good agreement
with recent published experimental results.Comment: 7 pages, 4 figure
Quasi-2D Confinement of a BEC in a Combined Optical and Magnetic Potential
We have added an optical potential to a conventional Time-averaged Orbiting
Potential (TOP) trap to create a highly anisotropic hybrid trap for ultracold
atoms. Axial confinement is provided by the optical potential; the maximum
frequency currently obtainable in this direction is 2.2 kHz for rubidium. The
radial confinement is independently controlled by the magnetic trap and can be
a factor of 700 times smaller than in the axial direction. This large
anisotropy is more than sufficient to confine condensates with ~10^5 atoms in a
Quasi-2D (Q2D) regime, and we have verified this by measuring a change in the
free expansion of the condensate; our results agree with a variational model.Comment: 11 pages, 10 figur
The Experimental Observation of a Superfluid Gyroscope in a dilute Bose Condensed Gas
We have observed a superfluid gyroscope effect in a dilute gas Bose-Einstein
condensate. A condensate with a vortex possesses a single quantum of angular
momentum and this causes the plane of oscillation of the scissors mode to
precess around the vortex line. We have measured the precession rate of the
scissors oscillation. From this we deduced the angular momentum associated with
the vortex line and found a value close to per particle, as predicted
for a superfluid.Comment: 4 pages 5 fig
Vortex nucleation in Bose-Einstein condensates in an oblate, purely magnetic potential
We have investigated the formation of vortices by rotating the purely
magnetic potential confining a Bose-Einstein condensate. We modified the bias
field of an axially symmetric TOP trap to create an elliptical potential that
rotates in the radial plane. This enabled us to study the conditions for vortex
nucleation over a wide range of eccentricities and rotation rates.Comment: 4 pages 4 figure
Calculation of mode coupling for quadrupole excitations in a Bose-Einstein condensate
In this paper we give a theoretical description of resonant coupling between
two collective excitations of a Bose condensed gas (BEC) on, or close, to a
second harmonic resonance. Using analytic expressions for the quasi-particle
wavefunctions we show that the coupling between quadrupole modes is strong,
leading to a coupling time of a few milliseconds (for a TOP trap with radial
frequency 100 Hz and 10^4 atoms). Using the hydrodynamic approximation, we
derive analytic expression for the coupling matrix element. These can be used
with an effective Hamiltonian (that we also derive) to describe the dynamics of
the coupling process and the associated squeezing effects.Comment: 12 pages, 3 figure
The experimental observation of Beliaev damping in a Bose condensed gas
We report the first experimental observation of Beliaev damping of a
collective excitation in a Bose-condensed gas. Beliaev damping is not predicted
by the Gross-Pitaevskii equation and so this is one of the few experiments that
tests BEC theory beyond the mean field approximation. Measurements of the
amplitude of a high frequency scissors mode, show that the Beliaev process
transfers energy to a lower lying mode and then back and forth between these
modes. These characteristics are quite distinct from those of Landau damping,
which leads to a monotonic decrease in amplitude. To enhance the Beliaev
process we adjusted the geometry of the magnetic trapping potential to give a
frequency ratio of 2 to 1 between two of the scissors modes of the condensate.
The ratios of the trap oscillation frequencies and
were changed independently, so that we could investigate
the resonant coupling over a range of conditions.Comment: 4 pages including 5 fig
Collective Sideband Cooling in an Optical Ring Cavity
We propose a cavity based laser cooling and trapping scheme, providing tight
confinement and cooling to very low temperatures, without degradation at high
particle densities. A bidirectionally pumped ring cavity builds up a resonantly
enhanced optical standing wave which acts to confine polarizable particles in
deep potential wells. The particle localization yields a coupling of the
degenerate travelling wave modes via coherent photon redistribution. This
induces a splitting of the cavity resonances with a high frequency component,
that is tuned to the anti-Stokes Raman sideband of the particles oscillating in
the potential wells, yielding cooling due to excess anti-Stokes scattering.
Tight confinement in the optical lattice together with the prediction, that
more than 50% of the trapped particles can be cooled into the motional ground
state, promise high phase space densities.Comment: 4 pages, 1 figur
Bose-Einstein condensation in a stiff TOP trap with adjustable geometry
We report on the realisation of a stiff magnetic trap with independently
adjustable trap frequencies, and , in the axial and radial
directions respectively. This has been achieved by applying an axial modulation
to a Time-averaged Orbiting Potential (TOP) trap. The frequency ratio of the
trap, , can be decreased continuously from the original
TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein
condensate (BEC) into this trap and obtained very good agreement between its
observed anisotropic expansion and the hydrodynamic predictions. Our method can
be extended to obtain a spherical trapping potential, which has a geometry of
particular theoretical interest.Comment: 4 pages, 3 figure
On the suppression of the diffusion and the quantum nature of a cavity mode. Optical bistability; forces and friction in driven cavities
A new analytical method is presented here, offering a physical view of driven
cavities where the external field cannot be neglected. We introduce a new
dimensionless complex parameter, intrinsically linked to the cooperativity
parameter of optical bistability, and analogous to the scaled Rabbi frequency
for driven systems where the field is classical. Classes of steady states are
iteratively constructed and expressions for the diffusion and friction
coefficients at lowest order also derived. They have in most cases the same
mathematical form as their free-space analog. The method offers a semiclassical
explanation for two recent experiments of one atom trapping in a high Q cavity
where the excited state is significantly saturated. Our results refute both
claims of atom trapping by a quantized cavity mode, single or not. Finally, it
is argued that the parameter newly constructed, as well as the groundwork of
this method, are at least companions of the cooperativity parameter and its
mother theory. In particular, we lay the stress on the apparently more
fundamental role of our structure parameter.Comment: 24 pages, 7 figures. Submitted to J. Phys. B: At. Mol. Opt. Phy
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