Measuring axion gradients with photon interferometry

Abstract

We propose a novel search technique for axions with a CP-violating monopole coupling g˜Q to bulk Standard Model charges Q∈{B,L,B−L}. Gradients in the static axion field configurations sourced by matter induce achromatic circular photon birefringence via the axion-photon coupling gϕγ. Circularly polarized light fed into an optical or (open) radio-frequency (RF) Fabry-Pérot (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 dark-matter 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˜Qgϕγ for axion masses mϕ≲10−5 eV. If shot-noise-limited sensitivity can be achieved, an experiment using high-finesse RF FP cavities could reach a factor of ∼105 into new parameter space for g˜Qgϕγ for masses mϕ≲4×10−11 eV. Published by the American Physical Society 2024