During
the evening of 12 February 2001, one of the Cold Quantum Gases network
partners, the atom optics group of the Laboratoire Charles Fabry de l’Institut
d’Optique in Orsay, observed a BEC of He* (in the 23S1
state). The apparatus uses a cloverleaf-type magnetic trap with coils placed
in re-entrant vacuum flanges. This design allows us to use a microchannel
plate (MCP) 5 cm below the trap to detect the atoms after releasing them
from the trap. Figure 1 shows a diagram of the apparatus.
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Figure
1: Schematic diagram of the apparatus. The magnetic field coils are in
red (the arrows show the direction of the currents), the MCP is the gray
disc at the bottom (not to scale), and the cold atoms are in blue (also
not to scale).
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Figure
2: shows an example of a single-shot time-of-flight spectrum detected by
the MCP is shown in the 2nd figure. The horizontal axis is the arrival
time of the atoms, but the distribution closely corresponds to the spatial
profile of the atoms along one of the strong axes of the trap.The
red curve shows a fit to the wings of the distribution giving a temperature
of 0.7 mK.
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Some
experimental data from the MCP are shown in the second figure.The
condensate peak shown contains about 4000 atoms, but we have determined
that in this experiment we only detected about 10% of the atoms in the
trap. The largest condensate we have observed contained about 105
atoms.
Some
people, including some of us, had been skeptical about whether BEC would
be attainable with He* fearing that Penning ionization would limit the
density to below that which was necessary. Our condensate lifetime however,
is about 1s and this gives an upper limit of 10-13 cm3s-1
for the Penning ionization rate constant. From our data we also estimate
the s-wave scattering length to be 20±10 nm.
For
more details, see A. Robert et al. Sciencexpress/ www.sciencexpress.org/
22 March 2001/ page 1/ 10.1126/science.1060622, and A. Browaeys et al.
http://arXiv/abs/physics/0102068,
as well as the publication by the ENS group, F. Pereira dos Santos et al.,
Phys. Rev. Lett., 86, 3459 (2001).
The
current team members are :
Alice
Robert, Olivier Sirjean, Denis Boiron, Chris Westbrook, Alain Aspect
and
some of the recent ‘graduates’, who made essential contributions are :
Antoine
Browaeys, Julie Poupard, Stephan Nowak.
Orsay,
9 April 2001