Atomic clocks work by exposing an atomic element to radio waves and then measuring the vibration between energy states of the atom's electrons. There are three main types of atomic clocks: cesium, rubidium and hydrogen. A second is defined as 9,192,631,770 cesium vibration cycles.
In the United States the official atomic clocks are hosted by the United States Naval Observatory and the National Institute of Standards and Technology. The first cesium atomic clock was developed by NIST in 1952. As of 2014, research is underway into optical clocks, because studies show these offer reduced systematic uncertainty and greater stability. Scientists at the University of Delaware have proposed ytterbium as an alternative to cesium for its greater precision.