High-energy vacuum birefringence and dichroism in an ultrastrong laser field

The interaction between real photons in vacuum is a long-standing prediction of quantum electrodynamics, which has never been observed experimentally. Upcoming 10 PW laser systems like the Extreme Light Infrastructure (ELI) will provide laser pulses with unprecedented intensities [1]. If combined with highly energetic gamma photons -- obtainable via Compton backscattering from laser-wakefield accelerated electron beams -- the QED critical field becomes accessible. In [2] we have derived how a generally polarized probe photon beam is influenced by both vacuum birefringence and dichroism in a strong linearly polarized plane-wave laser field. We put forward an experimental scheme to measure these effects in the nontrivial high-energy regime, where the QED critical field is reached and the Euler-Heisenberg approximation, valid for low-frequency electromagnetic fields, breaks down. Our results suggest the feasibility of verifying/rejecting the QED prediction for vacuum birefringence/dichroism at the $3\sigma$ confidence level on the time scale of a few days at several upcoming laser facilities. \newline\noindent [1] Di Piazza et al., Rev. Mod. Phys. \textbf{84}, 1177 (2012)\newline\noindent [2] S. Bragin, SM, C. H. Keitel, A. Di Piazza, arXiv:1704.05234 (2017)