We recently demonstrated a superradiant or bad-cavity laser that operates with on average as few as 0.2 photons of light inside the cavity. In this laser, the atoms replace the photons as the flywheel for phase information. To observe continuous superradiance, one must follow the Goldilocks principle and repump the atoms “just right” so as to not destroy the quantum coherence stored in the atoms.
The measured linewidth is more than 10000x below the quantum Schawlow-Townes laser linewidth that normally applies to optical lasers. We also showed that our light source is much less sensitive (by >10000) to the cavity mirror positions. Thermal mirror fluctuations limit the most narrow lasers to ~0.1 to 1Hz linewidths, and this approach may sidestep this thermal jiggling and allow lasers with linewidths of 0.001 Hz or less. Such a laser would instantly improve the best atomic clocks, and would extend coherence lengths from earth-moon distances to earth-sun distances. To pursue this goal, we have built a new experiment and demonstrated lasing on a 1 millihertz linewidth optical transition in strontium.