ABSTRACT
At the end of the 1980s, atomic frequency standards reached a level of refinement that made it the envy of many other fields of physics. The accuracy of primary caesium (Cs) standards maintained in operation at national institutes reached a level better that 10−13 and the frequency stability of the hydrogen (H) maser in the medium term was better than 10−14. These characteristics made possible the verification to great accuracy of basic physics predictions such as those resulting from the theory of relativity and made possible the maintenance of a timescale to an unsurpassed stability. It also opened the use of such devices in many applications. The time unit, the second, became the most accurate unit of the International System of Units (SI), with consequences for the implementation of other units such as the metre, the volt, and the ohm. On the other hand, Rb standards had reached a level of development that made them an excellent support of digital communication systems with improved reliability and also made them appropriate for navigation systems using satellites requiring medium frequency stability and small size.
