  We propose to explore the quantum nature of gravity using the correlation of
light between two optomechanical cavities, and the quantumness of the
correlation is witnessed by squeezing. As long as the gravity between the end
mirrors of two cavities is quantum in the Newtonian limit, we show that the
squeezing is always nonzero and monotonically increases as the mechanical
property of the mirrors is improved. The proposed scheme provides a new pathway
for testing the quantum nature of gravity systematically with tabletop
experiments.
