Laser Cooling of ⁸⁷Rb to 1.5µK in a Fountain Clock

We report an experiment on the adiabatic cooling of ⁸⁷Rb atoms in an atomic fountain to a temperature as low as 1.5 μK, which is roughly twice the recoil temperature. The atomic fountain has the (1,1,1) optical geometry for cooling and launching of cold atoms. The atoms are first cooled in an optical molasses of 6 beams to 3.4 μK by polarization gradient geometry and then are adiabatically cooled by decreasing the intensity of laser from 1.8I_s per beam to zero in 1 ms during the launching of cold atoms. We also study the dependences of atomic temperature on different laser parameters. The method we used is useful in any cold atom physics experiment.