  In 1957 Feynman suggested that the quantum/classical character of gravity can
be assessed by the presence/absence of entanglement between gravitationally
interacting test masses. 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
entanglement for reasonable interaction times. In practice, the systems that
can be prepared in such nonclassical states are limited to small masses, which
in turn limits the rate at which they get entangled. 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 and 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.
