U.S.A. National Committee Report, Fifteenth URSI General Assembly, Munich, September 1966: Commission 1, Radio Measurement Methods and Standards: Progress in Radio Measurement Methods and Standards

Abstract

Atomic frequency and time interval standards had advanced to such a high level of performance that in October 1964, the International Committee of Weights and Measures designated the (F = 4, mF = 0) ⇄ (F = 3, mF = 0) transition in the ground state of Cs133 temporarily for the physical measurement of time.