Louis Essen and Jack Parry happened to build the first atomic clock in 1955. They used cesium-133 which is its only stable isotope. A special interaction, known as hyperfine splitting, causes the energy level of 6s to split into two different energy levels, and the electron transitions back and forth while releasing a photon each time. Since the frequency of the photon releasing is consistent and there are 9,192,631,770 cycles in one second, it is used for time measurement.