NIST-F2
NIST-F2 is a caesium fountain atomic clock that, along with NIST-F1, serves as the United States' primary time and frequency standard. NIST-F2 was brought online on 3 April 2014.
Accuracy
NIST-F1, a caesium fountain atomic clock used since 1999, has a fractional inaccuracy of less than δf/f < 5 × 10−16.
The planned performance of NIST-F2 is δf/f < 1 × 10−16. At this planned performance level the NIST-F2 clock will not lose a second in at least 300 million years.
Evaluated accuracy
The evaluated accuracy uB reports of various primary frequency and time standards are published online by the International Bureau of Weights and Measures (BIPM). The first in-house accuracy evaluation of NIST-F2 reported a uB of 1.1 × 10−16. In March 2014 and March 2015 the NIST-F2 caesium fountain clock reported a uB of 1.5 × 10−16 in the BIPM reports of evaluation of primary frequency standards. However, the uncertainty evaluation of NIST-F2's frequency shift due to distributed cavity phase, which currently limits many other accurate atomic fountain clocks, had a shortcoming that makes it difficult to assign an accuracy until this error is reevaluated. In addition, the largest systematic error of NIST-F2 is a microwave amplitude dependent frequency shift, which includes frequency shifts due to the microwave lensing of atomic wave-packets by microwave dipole forces on the atoms and microwave leakage. An independent statistical analysis of the NIST measurements showed that this uncertainty was significantly too small and should be increased from 0.8 × 10−16 to at least 1.5 × 10−16. As a result, the quoted uncertainty of NIST-F2 would increase to approximately 1.7 × 10−16, if the reevaluation of the uncertainty due to distributed cavity phase shows that it is small.
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