Riviere, R.Deleglise, S.Weis, S.Gavartin, E.Arcizet, O.Schliesser, A.Kippenberg, T. J.2011-12-162011-12-162011-12-16201110.1103/PhysRevA.83.063835https://infoscience.epfl.ch/handle/20.500.14299/73913WOS:000292038000011Cooling 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 He-3 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.Radiation-PressureNoise ReductionResonatorCavityMicromirrorInstabilitySilicaFieldtext::journal::journal article::research article