  No experimental evidence of the quantum nature of gravity has been observed
yet and a realistic setup with improved sensitivity is eagerly awaited. We find
two effects, which can substantially enhance the signal of gravity-induced
quantum entanglement, by examining an optomechanical system in which two
oscillators gravitationally couple and one composes an optical cavity. The
first effect comes from a higher-order term of the optomechanical interaction
and generates the signal at the first order of the gravitational coupling in
contrast to the second order results in previous works. The second effect is
the resonance between the two oscillators. If their frequencies are close
enough, the weak gravitational coupling effectively strengthens. Combining
these two effects, the signal in the interference visibility could be amplified
by a factor of $10^{24}$ for our optimistic parameters. The two effects would
be useful in seeking feasible experimental setups to probe quantum gravity
signals.
