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 n_b=6.5×10^6 (T_b=295K) down to a phonon occupation of (n)=66±10, representing a mode temperature of T_m≈3mK. 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 (η_t=0.27)