1,079 research outputs found
Experimental evidence for the breakdown of a Hartree-Fock approach in a weakly interacting Bose gas
We study the formation of a quasi-condensate in a nearly one dimensional,
weakly interacting trapped atomic Bose gas. We show that a Hartree Fock
(mean-field) approach fails to explain the presence of the quasi-condensate in
the center of the cloud: the quasi-condensate appears through an
interaction-driven cross-over and not a saturation of the excited states.
Numerical calculations based on Bogoliubov theory give an estimate of the
cross-over density in agreement with experimental results.Comment: submitted to Phys. Rev. Letter
Bose-Einstein Condensation and Spin Mixtures of Optically Trapped Metastable Helium
We report the realization of a BEC of metastable helium-4 atoms (4He*) in an
all optical potential. Up to 10^5 spin polarized 4He* atoms are condensed in an
optical dipole trap formed from a single, focused, vertically propagating far
off-resonance laser beam. The vertical trap geometry is chosen to best match
the resolution characteristics of a delay-line anode micro-channel plate
detector capable of registering single He* atoms. We also confirm the
instability of certain spin state combinations of 4He* to two-body inelastic
processes, which necessarily affects the scope of future experiments using
optically trapped spin mixtures. In order to better quantify this constraint,
we measure spin state resolved two-body inelastic loss rate coefficients in the
optical trap
Hanbury Brown Twiss effect for ultracold quantum gases
We have studied 2-body correlations of atoms in an expanding cloud above and
below the Bose-Einstein condensation threshold. The observed correlation
function for a thermal cloud shows a bunching behavior, while the correlation
is flat for a coherent sample. These quantum correlations are the atomic
analogue of the Hanbury Brown Twiss effect. We observe the effect in three
dimensions and study its dependence on cloud size.Comment: Figure 1 availabl
Solution of Orthopositronium lifetime Puzzle
The intrinsic decay rate of orthopositronium formed in powder
is measured using the direct correction method such that the time
dependence of the pick-off annihilation rate is precisely determined. The decay
rate of orthopositronium is found to be , which is consistent with our previous measurements with
about twice the accuracy. Results agree well with the QED
prediction, and also with a result reported very recently using nanoporous
film
Ionization rates in a Bose-Einstein condensate of metastable Helium
We have studied ionizing collisions in a BEC of He*. Measurements of the ion
production rate combined with measurements of the density and number of atoms
for the same sample allow us to estimate both the 2 and 3-body contributions to
this rate. A comparison with the decay of the number of condensed atoms in our
magnetic trap, in the presence of an rf-shield, indicates that ionizing
collisions are largely or wholly responsible for the loss. Quantum depletion
makes a substantial correction to the 3-body rate constant.Comment: 4 pages, 3 figure
Getting the elastic scattering length by observing inelastic collisions in ultracold metastable helium atoms
We report an experiment measuring simultaneously the temperatureand the flux
of ions produced by a cloud of triplet metastablehelium atoms at the
Bose-Einstein critical temperature. The onsetof condensation is revealed by a
sharp increase of the ion fluxduring evaporative cooling. Combining our
measurements withprevious measurements of ionization in a pure BEC,we extract
an improved value of the scattering length nm. The analysis
includes corrections takinginto accountthe effect of atomic interactions on the
criticaltemperature, and thus an independent measurement of the
scatteringlength would allow a new test of these calculations
Pair correlations of scattered atoms from two colliding Bose-Einstein Condensates: Perturbative Approach
We apply an analytical model for anisotropic, colliding Bose-Einstein
condensates in a spontaneous four wave mixing geometry to evaluate the second
order correlation function of the field of scattered atoms. Our approach uses
quantized scattering modes and the equivalent of a classical, undepleted pump
approximation. Results to lowest order in perturbation theory are compared with
a recent experiment and with other theoretical approaches.Comment: 9 pages, 3 figure
Specular reflection of matter waves from a rough mirror
We have made a high resolution study of the specularity of the atomic reflection from an evanescent wave mirror using velocity selective Raman transitions. We have observed a double structure in the velocity distribution after reflection: a peak consistent with specular reflection and a diffuse reflection pedestal, whose contribution decreases rapidly with increasing detuning. The diffuse reflection is due to two distinct effects: spontaneous emission in the evanescent wave and a roughness in the evanescent wave potential whose amplitude is smaller than the de Broglie wavelength of the reflected atoms
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