The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. and promoted worldwide by Gernot M. R. Winkler of the United States Naval Observatory.
A caesium atomic fountain used as part of an atomic clock
National Physical Laboratory (United Kingdom)
The National Physical Laboratory (NPL) is the national measurement standards laboratory of the United Kingdom. It sets and maintains physical standards for British industry.
NPL's main entrance on Hampton Road
The Electricity Division of the National Physical Laboratory in 1944
Robert Watson-Watt
Louis Essen at right, with Jack Perry