Optomechanical sensor at cryogenic temperatures

Summary form only given.We present an optomechanical system consisting of a Fabry-Perot cavity with a movable mirror, which has the essential features to allow the observation of quantum backaction in the measurement of a macroscopic system. The Fabry-Perot cavity consists of a rigid in-coupling mirror and a torsional oscillator fabricated from a highly polished silicon wafer. At cryogenic temperatures, the mechanical Q factor of the oscillator´s torsional mode at 25 kHz exceeds 2/spl middot/10/sup 6/. The finesse of the cavity is /spl Ffr/=13,000, and a diode-pumped monolithic Nd:YAG laser is employed as laser source, with typical laser powers incident onto the cavity /spl sim/5 mW. The laser frequency is locked to the cavity, using a standard FM technique. The change in the cavity´s length caused by the motion of the torsional oscillator is detected by reading out the error signal of the feedback loop at the oscillator´s resonance frequency. The Brownian random motion of the oscillator at room temperature is shown. At cryogenic temperatures (4.5 K), we measured a rms displacement of /spl Delta/x=1.4/spl middot/10/sup -14/ m, in agreement with the theory of Brownian noise. We demonstrated the operation of the cryogenic high-finesse interferometric force sensor. With planned improvements of the finesse and a reduction of the laser´s amplitude and phase noise, our sensor opens up the possibility of observing, for the first time, the standard quantum limit of position measurement in interferometry.