National Physical Laboratory

Caesium Fountain Primary Frequency Standard

A caesium fountain primary frequency standard is a piece of apparatus that realises the SI definition of the second.

Caesium fountain primary frequency standard
Caesium fountain primary frequency

During the last decade, the accuracy of primary frequency standards based on atomic fountains has surpassed that of devices based on thermal beams by more than one order of magnitude. The superior performance of atomic fountains over traditional thermal beam devices originates from the significantly extended interaction time between the caesium atoms and the interrogating microwave field.

The NPL caesium fountain primary frequency standard NPL-CsF1 first went into operation in 2003, and since then has undergone a series of rigorous tests on its stability and accuracy. This fountain is used to contribute to the international timescale TAI and has participated in an international comparison of primary frequency standards. It also serves as the frequency reference for absolute frequency measurements of high accuracy optical frequency standards.

A second caesium fountain is also being developed, with the aim of reducing the uncertainty and improving the stability of the realisation of the SI second.

Last Updated: 23 Sep 2013
Created: 19 Aug 2010


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