NIST-F1 is a cesium fountain clock or atomic clock

Atomic clock

An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

 in the National Institute of Standards and Technology

National Institute of Standards and Technology

The National Institute of Standards and Technology , known between 1901 and 1988 as the National Bureau of Standards , is a measurement standards laboratory, otherwise known as a National Metrological Institute , which is a non-regulatory agency of the United States Department of Commerce...

 (NIST) in Boulder

Boulder, Colorado

Boulder is the county seat and most populous city of Boulder County and the 11th most populous city in the U.S. state of Colorado. Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of...

, Colorado

Colorado

Colorado is a U.S. state that encompasses much of the Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains...

, and serves as the United States' primary time

Time

Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 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 replaces NIST-7

NIST-7

NIST-7 was the atomic clock used by the United States from 1993 to 1999. The caesium beam clock served as the nation's primary time and frequency standard during that time period, but it has since been replaced with the more accurate NIST-F1, a caesium fountain atomic clock that neither gains nor...

, a cesium beam atomic clock used from 1993 to 1999. NIST-F1 is ten times more accurate than NIST-7. NIST-F1 will be replaced by the NIST-F2

NIST-F2

NIST-F2 is a caesium fountain atomic clock under construction that will serve as the United States' primary time and frequency standard. The clock will replace NIST-F1, a caesium fountain atomic clock used since 1999...

.

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.

Similar atomic fountain clocks, with comparable accuracy, are operated by other time and frequency laboratories, such as the Paris Observatory

Paris Observatory

The Paris Observatory is the foremost astronomical observatory of France, and one of the largest astronomical centres in the world...

, the National Physical Laboratory (NPL) in the United Kingdom and the Physikalisch-Technische Bundesanstalt

Physikalisch-Technische Bundesanstalt

The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....

in Germany.

Accuracy

As of 2010, the clock's uncertainty was about 3 x 10. It is expected to neither gain nor lose a second in more than 100 million years.