Ultra narrow linewidth frequency reference via measurement and feedback

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