NIST-F1, Atomic clock facility at National Institute of Standards and Technology, Boulder, United States

NIST-F1 is an atomic clock facility at the National Institute of Standards and Technology in Boulder that measures time by observing cesium atoms in a vacuum chamber. This facility was specifically engineered to provide the most accurate time measurements needed for GPS navigation, telecommunications, and power grid operations.

NIST-F1 replaced NIST-7, a cesium beam clock used from 1993 to 1999, and was built and tested in under four years by Steve Jefferts and Dawn Meekhof of NIST's Time and Frequency Division. It was ten times more accurate than its predecessor. NIST announced its successor, NIST-F2, in April 2014, with plans to run both clocks simultaneously; NIST-F1's last contribution to BIPM's TAI came in March 2016.

NIST-F1 defines the second by pushing laser-cooled cesium atoms upward through a microwave chamber; the microwave frequency that makes them fluoresce most strongly sets the unit of time. Similar fountain clocks run at the Paris Observatory, the UK's National Physical Laboratory and Germany's Physikalisch-Technische Bundesanstalt.

Visitors can only view the facility from limited public areas, as it operates in a controlled laboratory environment not open for general access. Contact NIST directly if interested in tours or educational demonstrations about timekeeping technology.

NIST-F1's microwave cavity was rebuilt starting in 2020 to create NIST-F4, which as of April 2025 was undergoing certification by the BIPM. The original clock's last contribution to BIPM's international atomic time was in March 2016.