Photon-photon scattering in collisions of laser pulses
  A scenario for measuring the predicted processes of vacuum elastic and inelastic photon-photon scattering with modern lasers is investigated. Numbers of measurable scattered photons are calculated for the collision of two, Gaussian-focused, pulsed lasers. We show that a single 10PW optical laser beam split into two counter-propagating pulses is sufficient for measuring the elastic process. Moreover, when these pulses are sub-cycle, our results suggest the inelastic process should be measurable too. 
B. King and C. H. Keitel
hep-ph