Strongly nonlinear displacement measurement in a nano-optomechanical resonator

Creation of non-classical states of mechanical motion is a long-standing goal of experimental physics. One promising approach to achieve this is measurement-based state preparation where sufficiently strong interaction with a measurement device is used to project the mechanics into an eigenstate of the measured observable. In order to prepare non-classical states, one needs to move away from linear continuous displacement measurements towards non-linear ($x^2$) and possibly also non-continuous (pulsed) measurements. We present here strongly nonlinear measurement of a mechanical resonator in a novel nanophotonic cavity optomechanical system with a record high optomechanical coupling strength. Although the coupling between displacement and optical cavity frequency is itself linear, the high interaction strength in combination with narrow optical linewidth (resulting in a single-photon cooperativity $C_0>1000$) allows us to use the second order working point of a homodyne interferometer to perform a sensitive $x^2$ measurement, with noise floor at $\sim 100$ $x_{zpf}^2/\sqrt{Hz}$. We discuss future potential to reach the quantum regime with such non-linear measurements, as well as the possibilities for pulsed, backaction-evading, measurements.