Typical optical clock transitions have ~Hz linewidths, not MHz. The intrinsic frequency stability of almost all lasers is by far not good enough to be able to

Typical optical clock transitions have ~Hz linewidths, not MHz. The intrinsic frequency stability of almost all lasers is by far not good enough to be able to probe such transitions. Therefore, ultra-stable optical cavities are used as a frequency reference, and the laser is constantly steered to stay on the cavity resonance by a fast electronic feedback system. In this way, laser linewidths in the sub-Hz range can be achieved. Then an acousto-optic modulator is used to scan the laser frequency across the clock transition.