  It is currently believed that there is no experimental evidence on possibly
quantum features of gravity or gravity-motivated modifications of quantum
mechanics. Here we show that single-atom interference experi- ments achieving
large spatial superpositions can rule out a framework where the Newtonian
gravitational inter- action is fundamentally classical in the
information-theoretic sense: it cannot convey entanglement. Specifically, in
this framework gravity acts pairwise between massive particles as classical
channels, which effectively induce approximately Newtonian forces between the
masses. The experiments indicate that if gravity does reduce to the pairwise
Newtonian interaction between atoms at the low energies, this interaction
cannot arise from the exchange of just classical information, and in principle
has the capacity to create entanglement. We clarify that, contrary to current
belief, the classical-channel description of gravity differs from the model of
Diosi and Penrose, which is not constrained by the same data.
