Towards a cw superradiant laser: Continuous strong coupling and transport of ⁸⁸Sr atoms in a ring cavity

Superradiant lasers are a promising path towards realizing a narrow-linewidth, high-precision and high-bandwidth active frequency reference [1]. They shift the phase memory from the optical cavity, which is subject to technical and thermal vibration noise, to an ultra-narrow optical atomic transition of an ensemble of cold atoms trapped inside the cavity. Our previous demonstration of pulsed superradiance on the mHz transition in ⁸⁷Sr [2,3] achieved a fractional Allan deviation of 6.7·10^-16 at 1s of averaging. Moving towards continuous-wave superradiance promises to further improve the short-term frequency stability by orders of magnitude. A key challenge in realizing a cw superradiant laser is the continuous supply of cold atoms into a cavity, while staying in the collective strong coupling regime. 

We demonstrate continuous loading of cold ⁸⁸Sr atoms into a ring cavity, after several stages of laser cooling and slowing. We characterize controlled transport of the atoms within the ring cavity using an intracavity travelling wave optical lattice, and demonstrate continuous strong collective atom-cavity coupling through continuous measurement of the vacuum Rabi splitting.

[1] D. Meiser et al., Phys. Rev. Lett. 102, 163601 (2009).

[2] M. A. Norcia et al., Science Advances 2, e1601231 (2016)

[3] M. A. Norcia et al., PRX 8, 021036 (2018)