Cooling a mesoscopic mechanical oscillator to its quantum ground state is elementary for the preparation and control of low entropy quantum states of large scale objects. Here, we pre-cool a 70-MHz micromechanical silica oscillator to an occupancy be
low 200 quanta by thermalizing it with a 600-mK cold 3He gas. Two-level system induced damping via structural defect states is shown to be strongly reduced, and simultaneously serves as novel thermometry method to independently quantify excess heating due to the cooling laser. We demonstrate that dynamical backaction sideband cooling can reduce the average occupancy to 9+-1 quanta, implying that the mechanical oscillator can be found (10+- 1)% of the time in its quantum ground state.
