  Vacuum birefringence is governed by the amplitude for a photon to flip
helicity or polarisation state in an external field. Here we calculate the flip
and non-flip amplitudes in arbitrary plane wave backgrounds, along with the
induced spacetime-dependent refractive indices of the vacuum. We compare the
behaviour of the amplitudes in the low energy and high energy regimes, and
analyse the impact of pulse shape and energy. We also provide the first
lightfront-QED derivation of the coefficients in the Heisenberg-Euler effective
action.
