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.[1] NIST-F2 was brought online on 3 April 2014.[1][2]

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.[3] At this planned performance level the NIST-F2 clock will not lose a second in at least 300 million years.[4]

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.[5] 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,[2] 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.[6] In addition, the largest systematic error of NIST-F2 is a microwave amplitude dependent frequency shift,[2] which includes frequency shifts due to the microwave lensing of atomic wave-packets by microwave dipole forces on the atoms[7] and microwave leakage. An independent statistical analysis of the NIST measurements[2] showed that this uncertainty was significantly too small and should be increased from 0.8 × 10−16 to at least 1.5 × 10−16.[6] 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.

References

  1. 1 2 NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock
  2. 1 2 3 4 First Accuracy Evaluation of NIST-F2, T. P. Heavner, S. R. Jefferts, J. H. Shirley, T. E. Parker, E. A. Donley, N. Ashby, S. Barlow, F. Levi, and G. Costanzo, May 2014
  3. ↑ NIST Cesium Fountains — Current Status and Future Prospects (PDF)
  4. ↑ "Time gets an upgrade". New Scientist: 7. 12 April 2014.
  5. ↑ Heavner T P, Donley E A , Levi F, Costanzo G, Parker TE, Shirley J H, Ashby N, Barlow S and Jefferts SR, “First accuracy evaluation of NIST-F2,” 2014 Metrologia 51, 174–182, May 2014
  6. 1 2 Comment on 'First accuracy evaluation of NIST-F2,' Kurt Gibble, Metrologia 52, 163, January 2015
  7. ↑ Improved accuracy of the NPL-CsF2 primary frequency standard: evaluation of distributed cavity phase and microwave lensing frequency shifts, Ruoxin Li, Kurt Gibble, and Krzysztof Szymaniec, August 2011

External links

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