Hadamard States for the Vector Potential on Asymptotically Flat   Spacetimes
  We develop a quantization scheme for the vector potential on globally hyperbolic spacetimes which realizes it as a locally covariant conformal quantum field theory. This result allows us to employ on a large class of backgrounds, which are asymptotically flat at null infinity, a bulk-to-boundary correspondence procedure in order to identify for the underlying field algebra a distinguished ground state which is of Hadamard form. 
Claudio Dappiaggi and Daniel Siemssen
gr-qc hep-th math-ph math.MP