at least — is 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of a stationary cesium-133 atom at a temperature of 0K.

In 1967, the second was defined as the equivalent of the time it takes for a cesium 133 atom to oscillate 9,192,631,770 times between the two energy states. Cesium atomic clocks define a second ...