Direct measurement of thermal fluctuation of high-Q pendulum.

We have achieved a direct measurement of the thermal fluctuation of a pendulum in an off-resonant and wide frequency region using a laser interferometric gravitational-wave detector. These measurements have been well identified for over one decade by an agreement with a theoretical prediction, which is derived by a fluctuation-dissipation theorem. Thermal fluctuation was dominated by the contribution of resistances in coil-magnet actuator circuits. When we tuned these resistances, the noise spectrum also changed according to a theoretical prediction. The measured thermal noise level corresponds to a high quality factor on the order of 10(5) of the pendulum.