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BEC in 87Rb - A new Aspect

[Historical report created from GSU BEC Homepage:]


A new report of BEC in 87Rb has been received from Orsay:

Our Atom Optics group at the Institut d'Optique in Orsay France (unité du CNRS) has achieved BEC in 87Rb. The first indication of successful runaway evaporative cooling to a phase space density larger than 1 came in the early hours of 12 June. Improved imaging later gave pictures showing the appearance of the expected narrow peak over the thermal background, in a time of flight distribution.

An unusual feature of our Ioffe trap is that it uses iron core electromagnets. We are able to produce quadrupole gradients of up to 1.2 kG/cm with a modest electrical power (100 watts). See Eur. Phys. J. D 1, 255 (1998) for a description of a preliminary version of the magnet. The geometry of our pole pieces is such that the bias field is necessarily 100-200 Gauss. Our strong quadrupole confinement allows us nevertheless to achieve sufficiently high collision rates to evaporate in the runaway regime. The large bias however, has so far prevented us from evaporating down to BEC using the |F MF> = |2 2> state. This appears to be because the quadratic Zeeman effect makes the frequencies of the various RF transitions in the F = 2 manifold unequal, so that atoms removed from |2 2> may remain trapped in |2 1> or |2 0> . We observed strong heating which prevented further phase space reduction. BEC was achieved by using the |1 -1> state for which this problem does not occur.

Other characteristics are:

  • A dark MOT is loaded with 109 atoms using a Zeeman cooled atomic beam and a 5 sec loading time. The atoms are then optically pumped and loaded into the magnetic trap with an efficiency of ~60%.
  • The trap is compressed to give oscillation frequencies of 25 Hz and 75 Hz (axial and radial).
  • RF ramp proceeds in 30 sec.
  • The trap is then adiabatically opened to a nearly isotropic geometry (25 Hz oscilliation frequency in all directions), and is then shut off in 1 ms. The time of flight distribution is measured 15 ms after release.
  • number of condensed atoms > 2 x 105

The results were obtained by a team led by Philippe Bouyer, with Bruno Desruelle, Vincent Boyer, Stuart Murdoch, Guillaume Delanoy.

The electromagnet was designed and built at L.E.Si.R. (CNRS and E.N.S. de CACHAN) by Michel Lécrivain, with the contribution of F. Alvès.

A preliminary experiment demonstrating the iron-core electromagnet involved Chris Westbrook and Gerhard Birkl.

Further information will be available soon on our web site.

Alain Aspect"