Optical clock

An optical clock uses an atomic transition at an optical (visible-light) frequency instead of the microwave transition of a caesium clock. Because that frequency is tens of thousands of times higher, the clock can divide time far more finely.

The best optical clocks would deviate by less than a second over the age of the universe. They are candidates for a future redefinition of the SI second and measure the gravitational redshift over centimetres of height difference.