Optomechanical sideband cooling of a micromechanical oscillator close to the quantum ground state

Cooling a mesoscopic mechanical oscillator to its quantum ground state is elementary for the preparation and control of quantum states of mechanical objects. Here, we pre-cool a 70-MHz micromechanical silica oscillator to an occupancy below 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 a thermometry method to independently quantify excess heating due to the cooling laser. We demonstrate that dynamical back action optical 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.