Interaction of photons traversing a slowly varying electromagnetic   background
  When two electromagnetic fields counterpropagate, they are modified due to mutual interaction via the polarised virtual electron-positron states of the vacuum. By studying how photon-photon scattering effects such as birefringence and four-wave mixing evolve as the fields pass through one another, we find a significant increase during overlap. The results have particular relevance for calculations based on a constant field background. 
B. King, P. B\"ohl, H. Ruhl
hep-ph physics.optics