We propose a novel search technique for axions with a $CP$-violating monopole coupling $\tilde{ g}_Q$ to bulk Standard Model charges $Q \in \{ B,L,B-L\}$. 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-P\'erot (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 non-zero frequency by altering the cavity orientations with respect to the field gradient. Multi-wavelength 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 $\tilde{ g}_Q g_{ \phi\gamma}$ for axion masses $m_{ \phi} \lesssim 10^{ -5} \mathrm{ eV}$. If shot-noise-limited sensitivity can be achieved, an experiment using high-finesse RF FP cavities could reach a factor of $\sim 10^{ 5}$ into new parameter space for $\tilde{ g}_Q g_{ \phi\gamma}$ for masses $m_\phi \lesssim 4\times 10^{ -11} \mathrm{ eV}$.
Categories: hep-ph physics.ins-det physics.optics