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 nˉb=6.5×106 (Tb=295K)
down to a phonon occupation of ⟨n⟩=66±10, representing a
mode temperature of Tm≈3mK. The five decades of cooling is enabled
by the system's large single-photon cooperativity (C1=4) and high
quantum efficiency of optical motion detection (ηt=0.27).Comment: 13 pages, 13 figure