  We propose a new method for optomechanical quantum non-demolition (QND)
detection of phonon number, based on a "shelving" style measurement. The scheme
uses a two-mode optomechanical system where the frequency splitting of the two
photonic modes is near-resonant with the mechanical frequency. The combination
of a strong optical drive and the underlying nonlinear optomechanical
interaction gives rise to spin-like dynamics which facilitate the measurement.
This approach allows phonon number measurement to be accomplished
parametrically faster than in other schemes which are restricted to weak
driving. The ultimate power of the scheme is controlled by the size of the
single photon optomechanical cooperativity.
