  Microscopic heat engines are microscale systems that convert energy flows
between heat reservoirs into work or systematic motion. We have experimentally
realized a minimal microscopic heat engine. It consists of a colloidal Brownian
particle optically trapped in an elliptical potential well and simultaneously
coupled to two heat baths at different temperatures acting along perpendicular
directions. For a generic arrangement of the principal directions of the baths
and the potential, the symmetry of the system is broken, such that the heat
flow drives a systematic gyrating motion of the particle around the potential
minimum. Using the experimentally measured trajectories, we quantify the
gyrating motion of the particle, the resulting torque that it exerts on the
potential, and the associated heat flow between the heat baths. We find
excellent agreement between the experimental results and the theoretical
predictions.
