Abstract

We have achieved Bose-Einstein condensation (BEC) [1,2] of Na atoms in an anisotropic TOP trap [3]. Our TOP trap has a bias field that rotates (at 20 kHz) in a plane containing the symmetry access of the quadrapole field, resulting in an anisotropic, time-averaged TOP potential with spring constants in the ratio of 1:2:4. A dark spot MOT is loaded from a Zeeman-cooled Na beam and the cloud of atoms are cooled in dark optical molasses. This produces a sample of −10¹⁰ atoms, mainly in the F = 1 hyperfine ground state at a temperature of −100 pK. We transfer about 1/3 of these atoms into a static quadrapole trap, trapping the F = 1, mF=−1 state. The quadrapole trap is stiffened to 1000 G/cm to increase the Na density and evaporated for 3 seconds with a linearly swept rf field. The TOP field is then turned on quickly to 150 G, producing a weak TOP trap. Further evaporation is performed by reducing the TOP field to 20 G, bringing in the "circle of death." This 15 second evaporation period is followed by 10 seconds of rf induced evaporation, with BEC occuring at a temperature of ~1 μK, producing a condensate of ~3×10⁶ atoms. By subsequently adiabatically expanding the trapping potential, we have reduced the temperature to −50 nK.

© 1998 Optical Society of America