Why are Cesium atomic clocks used?
Since 1967, the International System of Units (SI) has defined the second as the period equal to 9,192,631,770 cycles of the radiation, which corresponds to the transition between two energy levels of the ground state of the Cesium-133 atom. This definition makes the cesium oscillator (sometimes referred to generically as an atomic clock) the primary standard for time and frequency measurements. Other physical quantities, like the volt and meter, also rely on the definition of the second as part of their own definitions.
