Caesium clocks. Status and research activities

Thirty years ago, in 1967, the new definition of the second based on a quantum phenomenon, the hyperfine splitting of the groundstate of the 133Cs atom, was passed by the 13th General Conference on Weight and Measure. What followed was a fascinating improvement of the performance of caesium clocks which is still pursued. Their total uncertainty could be reduced, until 1995, by 4 orders of magnitude. With the development of the so-called fountain clocks, another factor of ten is within reach. (orig.)