Ultimate precise measurement in a gravitational wave detector has been developed with an
optomechanical cavity.  Recently, the sensitivity of the detector has reached almost
quantum level.  Such super sensitive cavities can answer the question; is there a
boundary between classical and quantum worlds?  Quantum mechanics have succeeded in
describing all microscopic physics so far, and mass dependence does not exist in
Schrodinger equation.  On the other hand, superposition states with massive oscillators
have not been observed yet.  One possibility is that it is due to gravity, which might
cause decoherence of the quantumness.  In this talk, I will introduce the motivation of
realizing the macroscopic quantum states, and how to test that experimentally using
optomechanical systems.