  We show that a signature of the quantum nature of gravity is the quantum
mechanical squeezing of the differential motion of two identical masses with
respect to their common mode. This is because the gravitational interaction
depends solely on the relative position of the two masses. In principle, this
squeezing is equivalent to quantum entanglement between the masses. In
practice, detecting the squeezing is more feasible than detecting the
entanglement. To that end, we propose an optical interferometric scheme to
falsify hypothetical models of gravity.
