A steady-state superradiant laser with less than one intracavity photon

We have demonstrated the quasi-continuous operation of a Raman laser that operates deep into the superradiant or bad-cavity regime.\footnote{J. G. Bohnet, Z. Chen, J. M. Weiner, D. Meiser, M. J. Holland, J. K. Thompson Nature {\bf 484}, 78-81 (05 April 2012)} In this laser, laser-cooled Rb atoms act as the flywheel for phase information, in place of the photons in a good-cavity laser. The system can operate with as few as 0.2 intracavity photons and with an effective excited state decay linewidth $<$ 1 Hz. This model system demonstrates key physics for future active optical clocks\footnote{D. Meiser, J. Ye, D. R. Carlson, M. J. Holland, Phys. Rev. Lett. {\bf 201}, 163601-163604 (2009)} (similar to masers) that may achieve frequency linewidths approaching 1 mHz due to greatly reduced sensitivity to thermal and environmental mirror noise.