Abstract

At the quantum level, a measurement disturbs the system being measured. In an interferometer the probing laser wave carries quantum fluctuations proportional to the square root of the laser power P, causing a fluctuating radiation pressure and a corresponding displacement noise of the interferometer. Here 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 back action in the measurement of a macroscopic system [1]. The cavity consists of a rigid incoupling mirror and a torsional oscillator fabricated from a highly polished silicon wafer [2]. At cryogenic temperatures the mechanical Q-factor of the oscillator’s torsional mode at 26 kHz exceedes 2·10⁶. The finesse of the cavity is ℱ =15000 and a diode-pumped Nd:YAG laser is employed as laser source. The laser frequency is locked to the cavity using the Pound-Drever technique. The cavity's length change caused by the motion of the torsional oscillator is detected by reading out the error signal of the feed back loop at the oscillator's resonance frequency. The rms displacement Δx of the oscillator’s position amounted to (1.28 ± 0.4) · 10⁻¹³m at 300 K and Δx = (1.33 ± 0.6) · 10⁻¹⁴m at 4.5 K in agreement with the theory of Brownian noise which predicts a dependence $\Delta x~\sqrt{T}$.

© 1998 IEEE