Past exam of the quantum condensed matter course of the University of Cambridge / 2006 / Section A / A1 / c / Solution

The fluctuation-dissipation theorem relates spontaneous fluctuations in thermal equilibrium to the dissipative part of the linear response to a weak conjugate force. For an overdamped trapped coordinate, the Langevin equation isThe fluctuation-dissipation relation for a Langevin particle fixes the noise strength from the same drag coefficient $\gamma$ that dissipates motion. With a drive proportional to $e^{-i\omega t}$, the linear response function is $\chi (\omega )=1/(K-i\gamma \omega )$. The two-sided angular-frequency power spectral density then obeysIts integral $\int S_{xx}d\omega /(2\pi )=k_{B}T/K$ agrees with the equipartition theorem. Without a restoring force, the same noise-drag relation gives the diffusion-mobility relation $D=k_{B}T/\gamma$.

In thermal calibration of an optical trap, record a bead's spontaneous displacement fluctuations and compare their spectrum with its response to a weak calibrated oscillatory force. The overdamped harmonic-trap power spectrum and the dissipative response must be consistent with the same temperature and drag. The equilibrium and weak-drive conditions are essential: an actively driven bath need not obey this equilibrium relation.