Atomic Time. These caesium clocks also underpin optical frequency measurements.

Optical clocks are based on forbidden optical transitions in ions or atoms. They have frequencies around , with a natural linewidth of typically 1 Hz, so the Q-factor is about , or even higher. They have better stabilities than microwave clocks, which means that they can facilitate evaluation of lower uncertainties. They also have better time resolution, which means the clock "ticks"