A high stability frequency standard using narrow linewidth atoms in an optical cavity

In this work we study a spectroscopic feature suitable for use as a frequency reference for laser stabilization that could offer an appealing compromise between frequency stability and experimental complexity. A cold ensemble of $^{88}$Sr atoms interacts with a single mode of a high-finesse optical cavity via the 7.5kHz linewidth, dipole forbidden $^1$S$_0$ to $^3$P$_1$ transition. The strong collective coupling between the atoms and cavity places the system in the vacuum Rabi splitting regime. By applying a magnetic field to split out the Zeeman sublevels of the $^3$P$_1$ state, we observe a small additional feature as a result of interference between the two dressed states. We demonstrate that this spectroscopic feature approaches the atomic transition linewidth and is highly immune to the type of reference cavity length fluctuations that limit current state-of-the-art lasers.