Measurement-based control of a mechanical oscillator at its thermal decoherence rate

In real-time (Markovian) quantum feedback protocols the outcome of a continuous measurement is used to stabilize a desired quantum state. Extending such protocols to macroscopic systems is a significant challenge, as the measurement must in this case compete with rapid environmental decoherence. We report on the realization of an interferometric sensor that approaches the requirements of quantum feedback for a solid state, 4.3 MHz nanomechanical oscillator [UTF-8?]— namely, the ability to resolve its zero-point motion at a rate comparable to that of its thermal decoherence. The sensor is based on near-field optomechanical coupling and achieves an imprecision ~ 39 dB below that at the standard quantum limit (SQL) for a weak continuous position measurement while maintaining an imprecision-backaction product only ~ 5 times in excess of the Heisenberg uncertainty limit. As a demonstration of its utility, we employ the measurement to feedback cool the oscillator to an occupation of ~ 5 quanta. Our results establish qualitatively new benchmarks for the readout and control of a macroscopic mechanical device, and underscore the potential and the challenge of extending such systems to the quantum regime.