  Spontaneous wavefunction collapse theories provide the possibility to resolve
the measurement problem of quantum mechanics. However, the best experimental
tests have been limited by thermal fluctuations and have operated at
frequencies far below those conjectured to allow the physical origins of
collapse to be identified. Here we propose to use high-frequency nanomechanical
resonators to surpass these limitations. We consider a specific implementation
that uses a quantum optomechanical system cooled to near its motional ground
state. The scheme combines phonon counting with efficient mitigation of
technical noise, including non-linear photon conversion and photon coincidence
counting. It is capable of resolving the exquisitely small phonon fluxes
required for a conclusive test of collapse models as well as potentially
identifying their physical origin.
