An atomic clock that depends on the energy difference between two sublevels of the hyperfine structure of the ground state of the caesium atom when it is in a magnetic field. In one type, atoms of caesium are irradiated with radio-frequency radiation, whose frequency is chosen to correspond to the energy difference between the two sublevels. Some caesium atoms absorb this radiation and are excited to the higher quantum state of the hyperfine structure. These atoms are deflected by a further magnetic field, which causes them to hit a detector. A signal from this detector is fed back to the radio-frequency oscillator to prevent it drifting from the resonant frequency of 9 192 631 770 hertz. In this way the device is locked to this frequency with an accuracy better than 1 part in 10¹³. The caesium clock is used in the SI unit definition of the second.