  This paper points out the importance of the assumption of locality of
physical interactions, and the concomitant necessity of propagation of an
entity (in this case, off-shell quanta - virtual gravitons) between two
non-relativistic test masses in unveiling the quantum nature of linearized
gravity through a laboratory experiment. At the outset, we will argue that
observing the quantum nature of a system is not limited to evidencing
$O\left(\hbar\right)$ corrections to a classical theory: it instead hinges upon
verifying tasks that a classical system cannot accomplish. We explain the
background concepts needed from quantum field theory and quantum information
theory to fully appreciate the previously proposed table-top experiments:
namely forces arising through the exchange of virtual (off-shell) quanta, as
well as Local Operations and Classical Communication (LOCC) and entanglement
witnesses. We clarify the key assumption inherent in our evidencing experiment,
namely the locality of physical interactions, which is a generic feature of
interacting systems of quantum fields around us, and naturally incorporates
micro-causality in the description of our experiment. We also present the types
of states the matter field must inhabit, putting the experiment on firm
relativistic quantum field theoretic grounds. At the end we use a non-local
theory of gravity to illustrate how our mechanism may still be used to detect
the qualitatively quantum nature of a force when the scale of non-locality is
finite. We find that the scale of non-locality, including the entanglement
entropy production in local/ non-local gravity, may be revealed from the
results of our experiment.
