Observation of Motion Dependent Nonlinear Dispersion with Narrow Linewidth Atoms in an Optical Cavity

TL;DR

This paper demonstrates the observation of motion-dependent nonlinear dispersion effects in a system of narrow linewidth strontium-88 atoms coupled to an optical cavity, revealing potential for advanced laser stabilization and atomic clock applications.

Contribution

It reports the first systematic study of velocity-dependent nonlinear phase shifts caused by Dopplerons in a cavity-atom system with narrow linewidth atoms at relatively high temperatures.

Findings

Nonlinear phase shifts depend on atom number and intra-cavity power.

Doppleron effects significantly influence cavity transmission and phase.

System demonstrates potential for sub-100 mHz laser stabilization.

Abstract

As an alternative to state-of-the-art laser frequency stabilisation using ultra-stable cavities, it has been proposed to exploit the non-linear effects from coupling of atoms with a narrow transition to an optical cavity. Here we have constructed such a system and observed non-linear phase shifts of a narrow optical line by strong coupling of a sample of strontium-88 atoms to an optical cavity. The sample temperature of a few mK provides a domain where the Doppler energy scale is several orders of magnitude larger than the narrow linewidth of the optical transition. This makes the system sensitive to velocity dependent multi-photon scattering events (Dopplerons) that affect the cavity field transmission and phase. By varying the number of atoms and the intra-cavity power we systematically study this non-linear phase signature which displays roughly the same features as for much lower temperature samples. This demonstration in a relatively simple system opens new possibilities for alternative routes to laser stabilization at the sub 100 mHz level and superradiant laser sources involving narrow line atoms. The understanding of relevant motional effects obtained here has direct implications for other atomic clocks when used in relation with ultranarrow clock transitions.