87Rb versus 133Cs in cold atom fountains: acomparison

摘要

We describe the operation of a laser cooled 87Rbnfrequency standard and present a new measurement of the 87Rbnground state hyperfine frequency with a relative accuracy ofn2.4×10-15, by comparison with a Cs fountain primarynstandard. The measured frequency is 6 834 682 610.904 333(17) Hz. Annevaluation of the frequency shift induced by cold collisions givesnΔΝ/ΝRb=(-7.2±20)×10-24nn¯, where n¯ is the average atomic density in cm-3.nWith our present 1 Σ uncertainty of 10-15, thisnmeasurement is still compatible with 0 and about 300 times smaller thannfor 133Cs. We also report a test of a possible variation ofnthe fine structure constant at the level of 2.7×10-14nyr-1, comparing Rb and Cs cold atom fountains