  In 1957 Feynman suggested that the quantum/classical character of gravity may
be assessed by testing the gravitational interaction due to source masses in
superposition. However, in all proposed experimental realisations using
matter-wave interferometry the extreme weakness of this interaction requires
pure initial states with extreme squeezing to achieve measurable effects of
non-classical interaction for reasonable experiment durations. In practice, the
systems that can be prepared in such nonclassical states are limited to small
masses, which in turn limits the strength of their interaction. Here we address
this key challenge--the weakness of gravitational interaction--by using a
massive body as an amplifying mediator of gravitational interaction between two
test systems. Our analysis shows that this results in an effective interaction
between the two test-systems that grows with the mass of the mediator, is
independent of its initial state and, therefore, its temperature. This greatly
reduces the requirement on the mass and degree of delocalization of the test
systems and, while still highly challenging, brings experiments on
gravitational source masses a step closer to reality.
