Classical versus non-classical photon states for detecting vacuum non-linearity
Quantum electrodynamics (QED) predicts that the vacuum should behave as a non-linear medium. For many schemes aimed at verifying this fundamental prediction, the signal consists of one or more photons in a certain mode (determined by polarization $σ$ and wave-number $\boldsymbol{k}$) which would be empty in the absence of a QED vacuum...