The Hyperfine Transition for the Definition of the Second. Issue 5 (14th April 2019) - Arias, Elisa Felicitas
Abstract: The unit of time of the International System of Units (SI), the "atomic second" was defined through a constant of physics in 1967. It is derived from the frequency of the hyperfine transition of the atom of cesium 133. From the astronomical definition of the second until today, the accuracy of the realization of the second has improved by eight orders of magnitude, with a rate that has increased since the development of the cesium frequency standards, to reach parts in 10 16 for the best clocks today. In 2018, when the new SI was adopted, the time metrology community proved that a new generation of frequency standards operating in optical wavelengths has uncertainties at the level of 10 –18, and challenge the implementation of high accurate frequency and time comparison techniques to decide on a revision of the definition of the second. Herein, the progress in the definition and realization of the second from astronomy until today is reviewed, an inventory of the present resources is assembled and a brief view for the future given. Abstract : The second defined in 1967 is derived from the frequency of the hyperfine transition of cesium 133. The best primary frequency standards brought the accuracy of the realization of the second to parts in 10 16 today. The progress in the definition and realization of the second is presented, with an inventory of resources and a view for the future.