  Wave function collapse models postulate a fundamental breakdown of the
quantum superposition principle at the macroscale. Therefore, experimental
tests of collapse models are also fundamental tests of quantum mechanics. Here,
we compute the upper bounds on the collapse parameters, which can be inferred
by the gravitational wave detectors LIGO, LISA Pathfinder, and AURIGA. We
consider the most widely used collapse model, the continuous spontaneous
localization (CSL) model. We show that these experiments exclude a huge portion
of the CSL parameter space, the strongest bound being set by the recently
launched space mission LISA Pathfinder. We also rule out a proposal for
quantum-gravity-induced decoherence.
