Nonlinear quantum electrodynamics in ultra-high intensity laser-plasma interactions

An in-depth analysis of two recent studies on nonlinear QED effects relevant to large-scale numerical simulations of laser-plasma interactions is presented. We demonstrate how accounting for a plasma as a background different from vacuum affects fundamental effects of nonlinear QED, quantify hitherto neglected coherences and propose an improved approach to better account for such effects in simulations. In particular, we show how the background plasma can be included in the calculation of nonlinear QED amplitudes on the example of the emission of a single high-energy photon by a laser-driven electron with the laser experiencing a non-trivial dispersion relation due to its propagation through a background plasma. Second, we discuss the failure of the so-called local-constant-field approximation, which is employed in all state-of-the-art numerical codes implementing QED effects both in nonlinear Compton scattering and in laser-assisted pair production. Finally, we show how in laser-assisted pair production by a high-energy photon emitted from a laser-driven electron the usually employed incoherent assumption, i.e., that the two QED processes of photon emission and pair production occur at separate points in space-time, can become invalid.