  In this paper we study CP violation in photon self-interactions at low
energy. These interactions, mediated by the effective operator $FFF\tilde{F}$,
where ($\tilde F$) $F$ is the (dual) electromagnetic field strength, have yet
to be directly probed experimentally. Possible sources for such interactions
are weakly coupled light scalars with both scalar and pseudoscalar couplings to
photons (for instance, complex Higgs-portal scalars or the relaxion), or new
light fermions coupled to photons via dipole operators. We propose a method to
isolate the CP-violating contribution to the photon self-interactions using
Superconducting Radio-Frequency cavities and vacuum birefringence experiments.
In addition, we consider several theoretical and experimental indirect bounds
on the scale of new physics associated with the above effective operator, and
present projections for the sensitivity of the proposed experiments to this
scale. We also discuss the implications of these bounds on the CP-violating
couplings of new light particles coupled to photons.
