  We show that the interaction of a pulsed laser light with a mechanical
oscillator through the radiation pressure results in an opto-mechanical
entangled state in which the photon number is correlated with the oscillator
position. Interestingly, the mechanical oscillator can be delocalized over a
large range of positions when driven by an intense laser light. This provides a
simple yet sensitive method to probe hypothetic post-quantum theories including
an explicit wave function collapse model, like the Diosi and Penrose model. We
propose an entanglement witness to reveal the quantum nature of this
opto-mechanical state as well as an optical technique to record the decoherence
of the mechanical oscillator. We also report on a detailed feasibility study
giving the experimental challenges that need to be overcome to confirm or rule
out predictions from explicit wave function collapse models.
