  We consider the possibility to measure the quantum decoherence using
gravitational wave interferometers. Gravitational wave interferometers create
the superposition state of photons and measure the interference of the photon
state. If the decoherence occurs, the interference of the photon state vanishes
and it can be measured by the interferometers. As examples of decoherence
mechanisms, we consider 1) decoherence by spontaneous localization, 2)
gravitational decoherence and 3) decoherence by extra-dimensional gravity.
