  We review a scheme for performing a back-action evading measurement of one
mechanical quadrature in an optomechanical setup. The experimental application
of this scheme has been limited by parametric instabilities caused in general
by a slight dependence of the mechanical frequency on the electromagnetic
energy in the cavity. We find that a simple modification to the optical drive
can effectively eliminate the parametric instability even at high intracavity
power, allowing realistic devices to achieve sub-zero-point uncertainties in
the measured quadrature.
