  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
$m_\phi \lesssim 4\times 10^{-11} \mathrm{eV}$.
