  A Langevin equation of heavy quarks in high-temperature quark-gluon plasma is
derived. The dynamics of heavy quark color is coupled with the phase space
dynamics and causes a macroscopic superposition state of heavy quark momentum.
Decoherence of the superposition state allows one to use a classical
description. The time scale of decoherence gives an appropriate discretization
time scale $\Delta t\sim \sqrt{M/C_{\rm F}\gamma}$ for the classical Langevin
equation, where $M$ is heavy quark mass and $\gamma$ is heavy quark momentum
diffusion constant.
