  Vacuum fluctuations of quantum fields are altered in presence of a strong
gravitational background, with important physical consequences. We argue that a
non-trivial spacetime geometry can act as an optically active medium for
quantum electromagnetic radiation, in such a way that the state of polarization
of radiation changes in time, even in the absence of electromagnetic sources.
This is a quantum effect, and is a consequence of an anomaly related to the
classical invariance under electric-magnetic duality rotations in Maxwell
theory.
