Two separate experiments in Colorado compare the frequency of emissions from atoms or ions to an uncertainty of 10^-16 or better. Earlier atomic clocks operated by reading out the movements of internal transitions from one quantum state to another in cesium atoms; the light emitted was in the microwave range. With frequency comb techniques (http://www.aip.org/pnu/2008/split/853-1.html) measurement of optical-range frequencies can also be made with high accuracy.
