  We show that binary black hole mergers with precessing evolution can
potentially excite photons from the quantum vacuum in such a way that total
helicity is not preserved in the process. Helicity violation is allowed by
quantum fluctuations that spoil the electric-magnetic duality symmetry of the
classical Maxwell theory without charges. We show here that precessing binary
black hole systems in astrophysics generate a flux of circularly polarized
gravitational waves which, in turn, provides the required helical background
that triggers this quantum effect. Solving the fully non-linear Einstein's
equations with numerical relativity we explore the parameter space of binary
systems and extract the detailed dependence of the quantum effect with the
spins of the two black holes. We also introduce a set of diagrammatic
techniques that allows us to predict when a binary black hole merger can or
cannot emit circularly polarized gravitational radiation, based on
mirror-symmetry considerations. This framework allows to understand and to
interpret correctly the numerical results, and to predict the outcomes in
potentially interesting astrophysical systems.
