Measuring axion gradients with photon interferometry

We propose a novel search technique for axions with a CP -violating monopole coupling g(Q) to bulk Standard Model charges Q is an element of {B; L; B - L}. Gradients in the static axion field configurations sourced by matter induce achromatic circular photon birefringence via the axion-photon coupling g(phi gamma). Circularly polarized light fed into an optical or (open) radio -frequency (RF) Fabry-Perot (FP) cavity develops a phase shift that accumulates up to the cavity finesse: the fixed axion spatial gradient prevents a cancellation known to occur for an axion darkmatter search. The relative phase shift between two FP cavities fed with opposite circular polarizations can be detected interferometrically. This time -independent signal can be modulated up to nonzero frequency by altering the cavity orientations with respect to the field gradient. Multiwavelength co -metrology techniques can be used to address chromatic measurement systematics and noise sources. With Earth as the axion source, we project reach beyond current constraints on the product of couplings g(Qg phi gamma) for axion masses m(phi) less than or similar to 10(-5) eV. If shot -noise -limited sensitivity can be achieved, an experiment using high -finesse RF FP cavities could reach a factor of similar to 10(5) into new parameter space for g(Q)g(phi gamma) for masses m(phi) less than or similar to 4 x 10(-11) eV.