In 1967, the SI second was redefined in terms of the frequency supplied by a caesium atomic clock.
From the en.wikipedia.org
This information is sent to the transmitter stations and is used to set a caesium atomic clock at each station.
From the en.wikipedia.org
Alternatively, in a caesium or rubidium clock, the beam or gas absorbs microwaves and the cavity contains an electronic amplifier to make it oscillate.
From the en.wikipedia.org
The revised definition seems to imply that the ideal atomic clock contains a single caesium atom at rest emitting a single frequency.
From the en.wikipedia.org
Eleven physical effects are accounted for in the emissions from the caesium population, which are then controlled for in the NIST-F1 clock.