  We consider the nonunitary quantum dynamics of neutral massless scalar
particles used to model photons around a massive gravitational lens. The
gravitational interaction between the lensing mass and asymptotically free
particles is described by their second-quantized scattering wavefunctions.
Remarkably, the zero-point spacetime fluctuations can induce significant
decoherence of the scattered states with spontaneous emission of gravitons,
thereby reducing the particles' coherence as well as energy. This new effect
suggests that, when photon polarizations are negligible, such quantum gravity
phenomena could lead to measurable anomalous redshift of recently studied
astrophysical lasers through a gravitational lens in the range of black holes
and galaxy clusters.
