ALP Search Using Precessing Light in a Magnetized Fabry Perot Cavity

In this paper we outline an experiment to detect the conversion of photons to axion-like particles (ALPs) in a strong magnetic field. We show that by modulating the polarization of the light passing through a Fabry-Perot cavity so that it effectively precesses at the modulation frequency, a signal is produced that is proportional to the square as opposed to the fourth power of the ALP-photon coupling constant. Assuming shot noise to be the dominant source of noise, we estimate that this approach is sensitive ALP masses less than $10^{-4} eV$ and couplings on the order to $g_{a\gamma}\sim 1.6\times 10^{-11} \text{GeV}^{-1}$ with a 10m, 10 T magnet, and $g_{a\gamma}\sim 1.6\times 10^{-12} \text{GeV}^{-1}$ with a 100 m magnet as envisaged by ALPs-IIc. ALPs with these properties have been invoked to explain the apparent transparency of the extragalactic background light (EBL) to ultra high-energy gamma rays emitted by BLAZARs.