NIST-F1 is a cesium fountain clock, a type of

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwe ...

, in the

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...

(NIST) in

Boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In ...

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of the ...

, and serves as the United States' primary

time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

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. 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 NIST-F2 is a cesium 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 cesium fountain atomic clock used since 1999, ...

, announced in April 2014. The NIST-F2 standard aims to be about three times more accurate than the NIST-F1 standard, and there are plans to operate it simultaneously with the NIST-F1 clock. The most recent contribution of NIST-F1 to BIPM TAI was in March 2016.

Frequency measurement

The apparatus consists of an

optical molasses Optical molasses is a laser cooling technique that can cool neutral atoms to temperatures lower than a magneto-optical trap (MOT). An optical molasses consists of 3 pairs of counter-propagating circularly polarized laser beams intersecting in the ...

made of counter-propagating lasers which cool and trap a gas of cesium atoms. Once trapped, the atoms are propelled upward by two vertical lasers 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 Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

(emit photons). The microwave frequency which produces maximum fluorescence is used to define the second. Similar atomic fountain clocks, with comparable accuracy, are operated by other time and frequency laboratories, such as the

Paris Observatory The Paris Observatory (french: Observatoire de Paris ), a research institution of the Paris Sciences et Lettres University, is the foremost astronomical observatory of France, and one of the largest astronomical centers in the world. Its histo ...

, the National Physical Laboratory (NPL) in the United Kingdom and the

Physikalisch-Technische Bundesanstalt The Physikalisch-Technische Bundesanstalt (PTB) is the national metrology institute of the Federal Republic of Germany, with scientific and technical service tasks. It is a higher federal authority and a public-law institution directly under fed ...

in Germany.

Accuracy

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.

Evaluated accuracy

The evaluated accuracy ''u''_''B'' reports of various primary frequency and time standards ar
published online
by the

International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry ...

(BIPM).
In May 2013 the NIST-F1 cesium fountain clock reported a ''u''_''B'' of . However, that BIPM report and the other recent reports are based on an evaluation that dates to 2005.T.P. Heavner, S.R. Jefferts, E.A. Donley, J.H. Shirley, and T.E. Parker, “NIST-F1: Recent Improvements and Accuracy Evaluations,” Metologia, vol. 42, pp 411-422, 2005 It used a model developed by NIST S.R. Jefferts, J.H. Shirley, N. Ashby, E.A. Burt, and G.J. Dick, “Power Dependence of Distributed Cavity Phase-Induced Frequency Biases in Atomic Fountain Frequency Standards,” IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 52, pp 2314-2321, 2004. to evaluate Doppler frequency shifts, known as distributed cavity phase, which was shown to be incorrect."Comment on ‘Accurate rubidium atomic fountain frequency standard’," Ruoxin Li and Kurt Gibble, Metrologia 48, 446-447 (2011). The recent evaluation of NIST-F2 did not use the NIST model of distributed cavity phase used for NIST-F1 and, while NIST-F2 instead used an approach more aligned with other standards, that evaluation of distributed cavity phase was shown to have other shortcomings.Comment on 'First accuracy evaluation of NIST-F2,' Kurt Gibble, Metrologia 52, 163, January 2015
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References

External links

U.S. Atomic Time

NIST-F1 Cesium Fountain Clock
* {{cite journal , author1=S R Jefferts , author2=J Shirley , author3=T E Parker , author4=T P Heavner , author5=D M Meekhof , author6=C Nelson , author7=F Levi , author8=G Costanzo , author9=A De Marchi , author10=R Drullinger , author11=L Hollberg , author12=W D Lee , author13=F L Walls , url=http://www.boulder.nist.gov/timefreq/general/pdf/1823.pdf , title=Accuracy evaluation of NIST F-1 , journal=Metrologia , volume=39 , pages=321–336 , doi=10.1088/0026-1394/39/4/1 , year=2002 , bibcode=2002Metro..39..321J , issue=4 , s2cid=250794497 , access-date=2007-11-25 , archive-url=https://web.archive.org/web/20050318133755/http://www.boulder.nist.gov/timefreq/general/pdf/1823.pdf , archive-date=2005-03-18 , url-status=dead National Institute of Standards and Technology Atomic clocks