En 1949, la première horloge atomique révolutionne le chronométrage. Elle utilise les transitions de la molécule d’ammoniac comme source de fréquence. Le temps atomique international est depuis devenu le standard.
First Atomic Clock, 1948
The first atomic clock, developed near this site by Harold Lyons at the National Bureau of Standards, revolutionized timekeeping by using transitions of the ammonia molecule as its source of frequency. Far more accurate than previous clocks, atomic clocks quickly replaced the Earth’s rotational rate as the reference for world time. Atomic clock accuracy made possible many new technologies, including the Global Positioning System (GPS).
https://ethw.org/Milestones:First_Atomic_Clock,_1948