The classical caesium beam frequency standard: fifty years later

The caesium beam frequency standard using the separated oscillatory field technique proposed by Ramsey in 1950 has seen intensive development over the last fifty years. Its practical implementation as a primary time standard made possible the realization of the second with a precision better than that obtained by means of astronomical measurements and provided the basis for its redefinition in terms of atomic properties in 1967. This paper describes the basic principles underlying the operation of such atomic standards, reviews the progress made during the last fifty years since its invention, and provides an update on the state-of-the-art accuracy and frequency stability achieved today in this field.