Optical read out and feedback cooling of a nanostring optomechanical cavity

Optical measurement of the motion of a 940 kHz mechanical resonance of a silicon nitride nanostring resonator is demonstrated with a read out noise imprecision reaching 37 dB below that of the resonator's zero-point fluctuations. Via intensity modulation of the optical probe laser, radiation pressure feedback is used to cool and damp the mechanical mode from an initial room temperature occupancy of \(\bar{n}_{b} = 6.5 \times 10^6\) (\(T_{b}=295\)K) down to a phonon occupation of \(\langle n \rangle = 66 \pm 10\), representing a mode temperature of \(T_{m} \approx 3\)mK. The five decades of cooling is enabled by the system's large single-photon cooperativity \((C_{1} = 4)\) and high quantum efficiency of optical motion detection (\(\eta_{t} = 0.27\)).