  The generation of very narrow linewidth light sources is of great importance
in modern science. One such source is the superradiant laser, which relies on
collectively interacting ultra long lived dipoles driven by incoherent light.
Here we discuss a different way of generating spectrally pure light by
coherently driving such dipoles inside an optical QED cavity. The light exiting
the cavity carries information about the detuning between the driving light and
the atomic transition, but is also affected by the noise originating from all
the decoherence processes that act on the combined atom-cavity system. We
calculate these effects to obtain fundamental limits for frequency estimation
and stabilization across a range of values of input light intensities and
atom-light interaction strengths, estimate these limits in state-of-the-art
cavity experiments with alkaline-earth atoms and identify favorable operating
conditions. We find that the achievable linewidths are comparable to those of
the superradiant laser.

Quantum Physics;Atomic Physics