Electromagnetic shift arising from the Heisenberg-Euler dipole
  We show that photons may be redshifted or blueshifted when interacting with the field of an overcritical dipole, which incorporates the one-loop QED corrections coming from vacuum polarization. Using the effective metric, it follows that such effect depends on the polarization of the photon. The shifts, plotted against the azimuthal angle for various values of the magnetic field, may show an intensity comparable to the gravitational redshift for a magnetar. To obtain these results, we have corrected previous literature. 
Luca Bonetti, Santiago E. Perez Bergliaffa, and Alessandro D.A.M.   Spallicci
astro-ph.HE gr-qc