/sup 87/Rb versus /sup 133/Cs in cold atom fountains: a comparison

We describe the operation of a laser cooled /sup 87/Rb frequency standard and present a new measurement of the /sup 87/Rb ground state hyperfine frequency with a relative accuracy of 2.4/spl times/10/sup -15/, by comparison with a Cs fountain primary standard. The measured frequency is 6 834 682 610.904 333(17) Hz. An evaluation of the frequency shift induced by cold collisions gives /spl Delta//spl nu///spl nu//sub Rb/=(-7.2/spl plusmn/20)/spl times/10/sup -24/ n~, where n~ is the average atomic density in cm/sup -3/. With our present 1 /spl sigma/ uncertainty of 10/sup -15/, this measurement is still compatible with 0 and about 300 times smaller than for /sup 133/Cs. We also report a test of a possible variation of the fine structure constant at the level of 2.7/spl times/10/sup -14/ yr/sup -1/, comparing Rb and Cs cold atom fountains.