The Fermionic Signature Operator and Hadamard States in the Presence of   a Plane Electromagnetic Wave
  We give a non-perturbative construction of a distinguished state for the quantized Dirac field in Minkowski space in the presence of a time-dependent external field of the form of a plane electromagnetic wave. By explicit computation of the fermionic signature operator, it is shown that the Dirac operator has the strong mass oscillation property. We prove that the resulting fermionic projector state is a Hadamard state. 
Felix Finster and Moritz Reintjes
math-ph hep-th math.MP