  Optomechanical crystals (OMCs) enable coherent interactions between optical
photons and microwave acoustic phonons, and represent a platform for
implementing quantum transduction between microwave and optical signals.
Optical absorption-induced thermal noise at cryogenic (millikelvin)
temperatures is one of the primary limitations of performance for OMC-based
quantum transducers. Here, we address this challenge with a two-dimensional
silicon OMC resonator that is side-coupled to a mechanically detached optical
waveguide, realizing a six-fold reduction in the heating rate of the acoustic
resonator compared to prior state-of-the-art, while operating in a regime of
high optomechanical-backaction and millikelvin base temperature. This reduced
heating translates into a demonstrated phonon-to-photon conversion efficiency
of 93.1 $\pm$ 0.8% at an added noise of 0.25 $\pm$ 0.01 quanta, representing a
significant advance toward quantum-limited microwave-optical frequency
conversion and optically-controlled quantum acoustic memories.
