Silicon optomechanical crystal resonator at millikelvin temperatures

Optical measurements of a nanoscale silicon optomechanical crystal cavity with a mechanical resonance frequency of $3.6$ GHz are performed at subkelvin temperatures. We infer optical-absorption-induced heating and damping of the mechanical resonator from measurements of phonon occupancy and motional sideband asymmetry. At the lowest probe power and lowest fridge temperature (Tf=10 mK), the localized mechanical resonance is found to couple at a rate of γᵢ/2π=400 Hz (Qₘ=9×10⁶) to a thermal bath of temperature Tb≈270 mK. These measurements indicate that silicon optomechanical crystals cooled to millikelvin temperatures should be suitable for a variety of experiments involving coherent coupling between photons and phonons at the single quanta level.