. Abstract, Rb frequency standard We present a new measurement of the 87 Rb hyperfine frequency with a 1.3 × 10 ?14 relative accuracy, by comparison with a Cs fountain primary standard. The measured 87 Rb ground-state hyperfine splitting is ?87(9) Hz. This value differs from previously published values (see, Nature, pp.189-298, 1961.

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A. Clairon, P. Laurent, G. Santarelli, S. Ghezali, S. N. Lea et al., The current accuracy of the mercury ion standard of ref. [2] is 3.4 × 10 ?15 . The accuracy of the LPTF cold-cesium fountain FO1 is presently 2 × 10 ?15 , the best value reported so far among all frequency standards [3] Using a cryogenic Sapphire microwave resonator [4] as a local oscillator for the fountain, a short-term stability of 4 × 10 ?14 ? ?1/2 has been measured [5]. This stability is only limited by the atomic quantum projection noise, inversely proportional to the square root of the number of detected atoms [5,6]. At this stage, the most severe limitation of cold cesium standards is the collisional ( * ) E-mail: monica@if.sc.usp.br ( * * ) E-mail: cipriana@alpha1.infim.ro ( * * * ) E-mail: salomon@physique.ens.fr ( * * * ) E-mail: lptf@obspm.fr c EDP Sciences s. bize et al.: high-accuracy measurement of the 87 Rb ground-state etc, Proceedings of the 5th Symposium on Frequency Standards and Metrology, p.49, 1995.

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