(Invited Paper) Yvan Sortais, Sebastien Bize, Christophe Nicolas, Giorgio Santarelli, C. Salomon, and A. Clairon
ABSTRACT We describe the operation of a laser cooled 87Rb frequency standard and present a new measurement of the 87Rb ground state hyperfine frequency with a relative accuracy of 2.4 x 10-15, 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 Δν/νRb = (-7.2 ± 20)x 10-24 n, where n is the average atomic density in cm-3. With our present 1σ uncertainty of 10-15, this measurement is still compatible with 0 and about 300 times smaller than for 133Cs. We also report a test of a possible variation of the fine structure constant at the level of 2.7 x 10-14yr-1, comparing Rb and Cs cold atom fountains.
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