Direct Measurement of Thermal Fluctuation of High-Q Pendulum

TL;DR

This paper reports the first direct measurement of thermal fluctuations in a high-Q pendulum using a laser interferometer, confirming theoretical predictions and highlighting the role of coil resistances in thermal noise.

Contribution

It presents the first direct experimental observation of thermal fluctuations in a high-Q pendulum, validating fluctuation-dissipation theorem predictions.

Findings

Thermal fluctuation measured matches theoretical predictions.

Resistances in coil circuits dominate thermal noise.

Tuning resistances alters the noise spectrum as expected.

Abstract

We achieved for the first time a direct measurement of the thermal fluctuation of a pendulum in an off-resonant 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 was derived by a fluctuation-dissipation theorem. Thermal fluctuation is 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.