Cooling and measurement of a micromechanical oscillator close to the quantum limit

Exploring quantum effects in mesoscopic mechanical oscillators as palpable as a pendulum or a cantilever has been a subject of long-standing interest in Quantum Physics and in the context of gravitational wave detection, and has recently - within the setting of cavity optomechanics - received significant interest. Studying non-classical aspects however requires the ability to both prepare and probe the mechanical degree of freedom at the quantum level. Classical thermal noise, and the tiny displacements associated with quantum signatures, pose two of the main experimental challenges. Up to now, they have been thought to be most easily addressed with nanomechanical oscillators cooled in dilution refrigerators, and probed with nanoelectronic motion transducers. This paper demonstrates that cooling and monitoring of a mechanical device with only a few thermal quanta is possible with objects discernable to the bare eye, in the present case given by a high-quality on-chip silica oscillator. These advances are afforded by two key properties of cavity optomechanics: optical interferometric monitoring of mechanical displacement with sensitivity at the attometer level, and resolved sideband laser cooling of a mechanical mode in addition to cryogenic pre-cooling of the optomechanical system. To probe the fluctuations of the mechanical radial breathing mode (RBM) in the silica oscillator, a high Q mode is excited using a continuous-wave Tksapphire laser. Fluctuations of the cavity radius (as induced by thermal excitation of the RBM) thus induce fluctuations in the WGM resonance frequency. A homodyne detection scheme allows measuring the mechanical fluctuations at a sensitivity of ca. 10^-18 m/Hz^1/2. From the measured phase fluctuation spectrum, resonance frequency, damping rate and occupation number of the RBM can be derived.