Vacuum refractive indices and helicity flip in strong-field QED

Understand

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.