  The ability to coherently control mechanical systems with optical fields has
made great strides over the past decade, and now includes the use of photon
counting techniques to detect the non-classical nature of mechanical states.
These techniques may soon be used to perform an opto-mechanical Bell test,
hence highlighting the potential of cavity opto-mechanics for
device-independent quantum information processing. Here, we propose a witness
which reveals opto-mechanical entanglement without any constraint on the global
detection efficiencies in a setup allowing one to test a Bell inequality. While
our witness relies on a well-defined description and correct experimental
calibration of the measurements, it does not need a detailed knowledge of the
functioning of the opto-mechanical system. A feasibility study including
dominant sources of noise and loss shows that it can readily be used to reveal
opto-mechanical entanglement in present-day experiments with photonic crystal
nanobeam resonators.
