A Rugged mHz-Linewidth Superradiant Laser Driven by a Hot Atomic Beam

We propose a continuous-wave, ultracoherent laser based on superradiance from a hot atomic beam traversing an optical cavity. Its predicted minimum linewidth and maximum output power are competitive with the best narrow-linewidth light sources. We show that, because of atomic phase synchronization, the phase of the output light is robust against decoherence arising from atomic motion, such as transit time broadening and Doppler broadening, as well as spontaneous emission and $T_2$ dephasing. Additionally, the system is inherently insensitive to effects that limit the best stable lasers, such as environmental noise and frequency drift. We believe the simplicity and ruggedness of our design will make it well suited for wide laboratory use, operation in challenging environments, and possibly serving as an active atomic clock.