Continuous recoil-driven lasing and cavity frequency pinning with laser-cooled 88Sr atoms
Dr. Vera Schäfer (MPI Heidelberg)

Precision measurements of atomic transition frequencies have become a promising path for testing theories for new physics beyond the standard model. To achieve even higher precision more stable and narrow-linewidth laser sources are required.

Superradiant lasers are a candidate for realising a narrow-linewidth, high-bandwidth active frequency reference. 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 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 is the continuous supply of cold atoms into a cavity, while staying in the collective strong coupling regime.

We demonstrate continuous loading and transport of cold 88Sr atoms inside a ring cavity, after several stages of laser cooling and slowing. We further describe the emergence of distinct zones of collective continuous lasing of the atoms on the 7.5kHz transition, 7x narrower than the cavity linewidth, and pumped by the cooling lasers via inversion of the motional states. The lasing is supported by self-regulation of the number of atoms inside the cavity that pins the dressed cavity frequency to a fixed value over >3MHz of raw applied cavity frequency. In the process up to 80% of the original atoms are expelled from the cavity.

I will also present a new project in Heidelberg aiming to use precision spectroscopy of highly charged ions to search for a variation of the fine-structure constant.