  We extend the recent gravitational decoherence analysis of Pikovski et al. to
an individual mesoscopic system with internal state characterized by a coherent
superposition of energy eigenstates. We express the Pikovski et al. effect
directly in terms of the energy variance, and show that the interferometric
visibility is bounded from below. Hence unlike collisional decoherence, the
visibility does not approach zero at large times, although for a large system
it can become very small.
