NIST Cesium Fountains - Current Status and Future Prospects

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 thanδ f/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 ofδ f/f<1×$10^{-16}$.