Continuous collective strong coupling between atoms and a high finesse optical cavity

Abstract: We demonstrate continuous loading of strontium atoms into a high finesse ring cavity and observe continuous strong collective coupling in the form of a vacuum Rabi splitting between the atoms and the cavity on the 7.5 kHz transition $^1{\rm S}_0$ to $^3{\rm P}_1$. The atoms are loaded into the cavity from a 3D narrow linewidth molasses, thus avoiding large magnetic field gradients and associated broadening of transition frequencies. The ring cavity allows us to realize a deterministic conveyor belt to transport atoms away from the loading region where the laser cooling beams lead to broadening of the strontium clock transition. We trap up to $10^6$ atoms in an intracavity 813 nm lattice in the Lamb-Dicke regime, and transport the atoms along the cavity axis. This work opens the path to the creation of a continuous wave superradiant laser with millihertz linewidth enabling searches for new physics and the use of high-precision optical frequency references outside of low vibration laboratory environments.