  It is well known that not every symmetry of a classical field theory is also
a symmetry of its quantum version. When this occurs, we speak of quantum
anomalies. The existence of anomalies imply that some classical Noether charges
are no longer conserved in the quantum theory. In this paper, we discuss a new
example for quantum electromagnetic fields propagating in the presence of
gravity. We argue that the symmetry under electric-magnetic duality rotations
of the source-free Maxwell action is anomalous in curved spacetimes. The
classical Noether charge associated with these transformations accounts for the
net circular polarization or the optical helicity of the electromagnetic field.
Therefore, our results describe the way the spacetime curvature changes the
helicity of photons and opens the possibility of extracting information from
strong gravitational fields through the observation of the polarization of
photons. We also argue that the physical consequences of this anomaly can be
understood in terms of the asymmetric quantum creation of photons by the
gravitational~field.
