Single-trapped-ion frequency standards based on a 282 nm transition in 199Hg+ and on a 267 nm transition in
27Al+ have been developed at NIST over the past several years. Their frequencies are measured relative to each
other and to the NIST primary frequency standard, the NIST-F1 cesium fountain, by means of a self-referenced
femtosecond laser frequency comb. Both ion standards have demonstrated instabilities and inaccuracies of less
than 1 × 10-16.
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 δ f / f ≤ 5x10-16 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
δ f / f < 1x10-16
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