First atomic clock

The first atomic clock was built by a team led by Harold Lyons at the US National Bureau of Standards in 1948 and officially announced to the public in January 1949. This clock used the stimulation of ammonia atoms to generate its time signal, rather than the caesium used in later designs. The clock proved that the concept of an atomic clock worked, but it wasn't as accurate as the quartz-based electronic clocks of the time and was switched off soon after its initial trials. The first accurate atomic clock was revealed some six years later, in June 1955, by Louis Essen and Jack Parry at the National Physical Laboratory in the UK using caesium-133 to generate the time signal.

Atomic clocks work by beaming microwaves at a "fountain" of supercooled atoms, typically Caesium-133. The microwave emitter is then tuned to find the exact frequency that energises the greatest number of atoms in the fountain, known as the resonant frequency. At the resonant frequency, which can be very accurately measured, the time taken for the microwave emitter to go through exactly 9,192,631,770 cycles is counted as one single second. It's not possible to just give the correct frequency because any measurement of frequency relies on a standardized definition of a second.