NIST Cesium Fountains - Current Status and Future Prospects

S.R. Jefferts, T.P. Heavner, T.E. Parker and J.H. Shirley NIST - Time and Frequency Division, 325 Broadway, Boulder, CO 80305, USA

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Received: 3 09 2007;

We review the current status of the U.S. Primary Frequency Standard, NIST-F1. NIST-F1 is a laser-cooled cesium fountain based frequency standard with an inaccuracy of less than δf/f < 5 × 10-16; limited mainly by the radiation field in the room-temperature fountain (blackbody shift). NIST-F1 is one of the best cesium fountains currently contributing to international atomic time, but has reached a point that it is impractical to improve its accuracy substantially. Therefore we are building a new fountain, imaginatively named NIST-F2, with a cryogenic (77 K) Ramsey interrogation zone that lowers the blackbody shift by several orders of magnitude. NIST-F2 is currently undergoing final assembly, and we will discuss our planned (hoped for) performance, which includes frequency inaccuracy of δf/f < 1 × 10-16.

DOI: 10.12693/APhysPolA.112.759 PACS numbers: 06.20.Fn, 32.30.Bv, 32.30-r, 32.10.Fn, 32.80.Pj