Optomechanical Bell test

Photons of a laser beam driving the upper motional sideband of an optomechanical cavity can decay into photon-phonon pairs by means of an optomechanical parametric process. The phononic state can subsequently be mapped to a photonic state by exciting the lower sideband, hence creating photon-photon pairs out of an optomechanical system. We here demonstrate that these pairs can violate a Bell inequality when they are measured with photon counting techniques preceded by small displacement operations in the phase space. Our results show how to detect non-local light-mechanical correlations and open the way for device-independent quantum optomechanics where non-classical light-mechanical correlations can be certified without assumptions on the state structure or on the measurement devices.