Electromagnetic duality anomaly in curved spacetimes

Understand

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.