  Particle creation by strong and time-varying backgrounds is a robust
prediction of quantum field theory. Another well-established feature of field
theory is that classical symmetries do not always extend to the quantized
theory. When this occurs, we speak of quantum anomalies. In this contribution
we discuss the entwining relationship between both predictions, relating chiral
anomalies with an underlying process of particle creation. Within this context,
we will also argue that the symmetry under electric-magnetic duality rotations
of the source-free Maxwell theory is anomalous in curved spacetime. This is a
quantum effect, and it can be understood as the generalization of the fermion
chiral anomaly to fields of spin one. This implies that the net polarization of
photons propagating in a gravitational field could change in time.
