Superradiant lasing on the millihertz clock transition*

We demonstrate that superradiant laser light emitted from an ultra-narrow transition can serve as a highly accurate and stable active atomic frequency reference. We present frequency comparisons between superradiant light emitted from the 1~mHz linewidth optical clock transition in $^{87}$Sr and a state of the art stable laser system and optical lattice clock [Norcia \textit{et. al.} arXiv:1711.10407]. We characterize the stability and absolute accuracy of the superradiant system and demonstrate insensitivity to key environmental perturbations. Additionally, we present our observations of spin-exchange interactions mediated by the emission and reabsorption of photons inside an optical cavity [Norcia \textit{et. al.} arXiv:1711.03673]. We observe the emergence of a many-body energy gap and signatures of gap protection of the optical coherence against dephasing. Finally, we present future prospects for a continuous superradiant laser.