Fully Collective Superradiant Lasing with Vanishing Sensitivity to Cavity Length Vibrations

To date, realization of a continuous-wave active atomic clock has been elusive, primarily due to parasitic heating from spontaneous emission while repumping the atoms. Here, we propose a solution to this problem by replacing the random emission with coupling to an auxiliary cavity, making repumping a fully collective process. While it is known that collective two-level models do not possess a generic lasing threshold, we show this restriction is overcome with multilevel atoms, since collective pumping and decay can be performed on distinct transitions. Using relevant atomic parameters, we find this system is capable of producing an O(100  μHz)-linewidth continuous-wave superradiant laser. Our principal result is the potential for an operating regime with cavity length vibration sensitivity below O(10^{-14}/g), including a locus of parameter values where it completely vanishes even at steady state.