Ultranarrow Superradiant Lasing by Dark Atom-Photon Dressed States.

We show that incoherent pumping of an optical lattice clock system with ultracold strontium-88 atoms produces laser light with a ≃10  Hz linewidth when the atoms are exposed to a magnetic field. This linewidth is orders of magnitude smaller than both the cavity linewidth and the incoherent atomic decay and excitation rates. The narrow lasing is due to an interplay of multiatom superradiant effects and the coupling of bright and dark atom-light dressed states by the magnetic field.