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

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) D (F = 3, mF = 0) transition in the ground state of Cs¹³³ temporarily for the physical measurement of time. They assigned the value 9,192,631,770 Hz to this transition. Thus the physical basis for time measurement has changed from an astronomical unit to an atomic unit of time [Bureau International des Poids et Mesures, 1964].