  The current interest in laboratory detection of entanglement mediated by
gravity was sparked by an information--theoretic argument: entanglement
mediated by a local field certifies that the field is not classical. Previous
derivations of the effect modelled gravity as instantaneous; here we derive it
from linearised quantum general relativity while keeping Lorentz invariance
explicit, using the path integral formalism. In this framework, entanglement is
clearly mediated by a quantum feature of the field. We also point out the
possibility of observing retarded entanglement, which cannot be explained by an
instantaneous interaction. This is a difficult experiment for gravity, but is
plausible for the analogous electromagnetic case.
