Optomechanical sensor at cryogenic temperatures

Abstract

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.