CP violation in low-energy photon-photon interactions

CP violation in the loop-mediated photon-photon interactions would modify the birefringence of the quantum vacuum. We discuss the implications of this effect on an experiment in which light propagates in a Fabry-Perot cavity permeated by a time-dependent electric and magnetic field: for some suitable orientation of the electric field, the effect does not cancel out in a round-trip. We show that the violation of CP would imply the existence of characteristic Fourier components, in the intensity spectrum of the outgoing wave. In order to estimate the magnitude of this effect within the standard model, we use chiral perturbation theory to compute the theta-term contribution to the coefficient of the leading CP-odd vertex, in the low-energy effective field theory for photon dynamics.