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It is well known that not every symmetry of a classical field theory is alsoa symmetry of its quantum version. When this occurs, we speak of quantumanomalies. The existence of anomalies imply that some classical Noether chargesare no longer conserved in the quantum theory. In this paper, we discuss a newexample for quantum electromagnetic fields propagating in the presence ofgravity. We argue that the symmetry under electric-magnetic duality rotationsof the source-free Maxwell action is anomalous in curved spacetimes. Theclassical Noether charge associated with these transformations accounts for thenet circular polarization or the optical helicity of the electromagnetic field.Therefore, our results describe the way the spacetime curvature changes thehelicity of photons and opens the possibility of extracting information fromstrong gravitational fields through the observation of the polarization ofphotons. We also argue that the physical consequences of this anomaly can beunderstood in terms of the asymmetric quantum creation of photons by thegravitational~field.

On the Electric-Magnetic Duality Symmetry: Quantum Anomaly, Optical Helicity, and Particle Creation