  We demonstrate that the system composed of a gyrotropic particle out of
thermal equilibrium with vacuum can be regarded as a heat engine. Such a
particle, initially at rest, will experience a fluctuation-induced torque and
start to rotate, producing mechanical work out from the temperature difference
of the particle and its environment. We rigorously prove that the efficiency of
the heat engine is tightly bound by the Carnot efficiency. We also predict
that, such an engine can be constructed using a heavily-doped semiconductor
nanoparticle under magnetic field, and moreover the particle can reach at a
steady-state rotating frequency in the order of terahertz solely due to thermal
fluctuations.
