We argue that if the Newtonian gravitational field of a body can mediate
entanglement with another body, then it should also be possible for the body
producing the Newtonian field to entangle directly with on-shell gravitons. Our
arguments are made by revisiting a gedankenexperiment previously analyzed by
Belenchia et al., which showed that a quantum superposition of a massive body
requires both quantized gravitational radiation and local vacuum fluctuations
of the spacetime metric in order to avoid contradictions with complementarity
and causality. We provide a precise and rigorous description of the
entanglement and decoherence effects occurring in this gedankenexperiment,
thereby significantly improving upon the back-of-the-envelope estimates given
in the analysis of Belenchia et al. and also showing that their conclusions are
valid in much more general circumstances. As a by-product of our analysis, we
show that under the protocols of the gedankenexperiment, there is no clear
distinction between entanglement mediated by the Newtonian gravitational field
of a body and entanglement mediated by on-shell gravitons emitted by the body.
This suggests that Newtonian entanglement implies the existence of graviton
entanglement and supports the view that the experimental discovery of Newtonian
entanglement may be viewed as implying the existence of the graviton.
