NIST-F2

NIST-F2 is a cesium fountain atomic clock developed by the National Institute of Standards and Technology (NIST) in Boulder, Colorado. It served as one of the primary time and frequency standards for the United States from its commissioning in 2014 until its eventual replacement by NIST-F2+. It is used to realize the SI second, contributing to the international definition of time through its accuracy in measuring the frequency of a specific transition in cesium atoms.

NIST-F2 builds upon the principles of its predecessor, NIST-F1, but incorporates significant improvements to achieve higher accuracy. Like other cesium fountain clocks, NIST-F2 operates by cooling cesium atoms to extremely low temperatures (near absolute zero) using lasers, then launching them upwards in a controlled trajectory through a microwave cavity. This process creates a "fountain" of atoms.

As the atoms pass through the microwave cavity twice, they are exposed to a microwave signal precisely tuned to the resonant frequency of the cesium atom's hyperfine transition (9,192,631,770 Hz). By carefully controlling and measuring the microwave frequency that causes the maximum number of atoms to transition between energy states, scientists can accurately determine the SI second.

NIST-F2 achieved an uncertainty of approximately one part in 300 quadrillion. This means the clock would neither gain nor lose a second in about 300 million years. The improved accuracy compared to NIST-F1 came from refinements in the magnetic shielding, temperature control, and other systematic effects that could influence the atomic resonance frequency.

The clock's precise timekeeping is essential for various applications, including calibrating other clocks, providing accurate time signals for communication networks, financial transactions, and scientific research that require precise timing. The data gathered from NIST-F2 was crucial for contributing to the international atomic timescale coordinated by the Bureau International des Poids et Mesures (BIPM).