  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 the PVLAS-Phase II experiment, in which a time-dependent magnetic field is
modulated. 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.
