A cold-atom laser with <1 intracavity photon

We demonstrate a cold-atom Raman laser that operates quasi-continuously with as few as 0.2 photons on average inside the cavity and with a single-atom Raman decay rate below 1 Hz. We demonstrate that the laser´s coherence is primarily stored within the atoms, and consider the relationship between the Schawlow-Townes linewidth and the standard quantum limit on phase estimation using unentangled atoms. This proof-of-principle experiment may lead to novel hybrid sensors and guide the development of future ultra-stable lasers with intrinsic insensitivity to currently-limiting thermal and technical sources of cavity mirror vibration.