Vacuum polarisation effects in impulsive fields

Photons impinging on strong electromagnetic fields can change both momentum and helicity state, due to quantum vacuum polarisation. We investigate these effects in the collision of photons with impulsive PP-waves, which describe e.g. the fields of ultra-boosted charge distributions. We connect our results to vacuum birefringence and quantum reflection in both QED and SUSY QED. We also compare with helicity flip in plane wave backgrounds, exploring how and when known tree-level relations, relating amplitudes in PP-waves and in plane waves, extend to one-loop corrections.