Electromagnetic duality anomaly in curved spacetimes
  The source-free Maxwell action is invariant under electric-magnetic duality rotations in arbitrary spacetimes. This leads to a conserved classical Noether charge. We show that this conservation law is broken at the quantum level in presence of a background classical gravitational field with a non-trivial Chern-Pontryagin invariant, in a parallel way to the chiral anomaly for massless Dirac fermions. Among the physical consequences, the net polarization of the quantum electromagnetic field is not conserved. 
I. Agullo, A. del Rio, J. Navarro-Salas
gr-qc hep-th math-ph math.MP quant-ph