BOSE-EINSTEIN CONDENSATION OF LITHIUM. Curtis C. Bradley, Charles A.
Sackett, and Randall G. Hulet, Rice University, Houston, Texas, USA (NIST
address: Building 220, Room B206, NIST, Gaithersburg, MD 20899, 301-975-3768,
Bose-Einstein condensation (BEC) in ultra-cold magnetically-trapped 7Li vapor wasexperimentally observed and quantitative measurements of condensate number were made.
Compared to other BEC experiments, lithium is unique due to its negative s-wave scattering length, corresponding to effectively attractive interactions. Due to this attraction, condensates are expected to undergo mechanical collapse if the condensate number exceeds a critical value. An upper limit of about 1000 condensate atoms was observed, in agreement with theoretical predictions.
In the experiment, atoms are confined by a set of six permanent magnets in the Ioffe configuration. Optical forces are used to slow and guide atoms into the trapping region. Approximately 108 atoms are loaded into the trap and laser-cooled to near 200 µK. Subsequent forced-evaporative cooling produces a million-fold increase in phase-space density, reaching quantum-degenerate conditions with about 105 atoms at temperatures near 300 nK. After cooling, the trapped atom distribution is observed by in situ imaging via an optical probe.
In initial data, as the expected phase transition was approached, the images suddenly became distorted. Fits to the image data suggested as many as 105 condensate atoms, in strong disagreement with theoretical predictions. A model that accounts for imperfections in the imaging optics, shows that the sudden appearance of the distortions is a consequence of BEC, and explains the initial overestimation of condensate number. Improved imaging was obtained using large probe detunings, a novel and flexible Phase-Contrast Polarization Imaging (PCPI) technique, and near-diffraction-limited imaging optics. From the resulting images, quantitative estimates of condensate number are obtained and compared with theory.