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NIST-F1, source of the official time of the United States

NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took less than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physical Measurement Laboratory.[1]

The clock replaced NIST-7, a cesium beam atomic clock used from 1993 to 1999. NIST-F1 is ten times more accurate than NIST-7. It has been succeeded by a new standard, NIST-F2, announced in April 2014. The NIST-F2 standard is about three times more accurate than the NIST-F1 standard, but will be operated simultaneously with the NIST-F1 clock.[2]

Frequency measurement

The apparatus consists of counter-propagating lasers that cool and trap a gas of cesium atoms. Once trapped, two vertical lasers propel the atoms upward inside a microwave chamber. Depending on the exact frequency of the microwaves, the cesium atoms will reach an excited state. Upon passing through a laser beam, the atoms will fluoresce (emit photons). The microwave frequency which produces maximum fluorescence is used to define the second.[1]

Similar atomic fountain clocks, with comparable accuracy, are operated by other time and frequency laboratories, such as the Paris Observatory, the National Physical Laboratory (NPL) in the United Kingdom and the Physikalisch-Technische Bundesanstalt in Germany.[1]


As of 2013, the clock's uncertainty was about 3.1 × 10−16. It is expected to neither gain nor lose a second in more than 100 million years.[1]

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).
In May 2013 the NIST-F1 caesium fountain clock reported a uB of 3.1 × 10−16.


  1. 1.0 1.1 1.2 1.3 "NIST-F1 Cesium Fountain Atomic Clock: The Primary Time and Frequency Standard for the United States". NIST. August 26, 2009. Retrieved 2 May 2011.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  2. Ost, Laura (April 3, 2014). "NIST Launches a New U.S. Time Standard: NIST-F2 Atomic Clock". Retrieved 2 January 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>

External links