Influence of non-conservative optical forces on the dynamics of   optically trapped colloidal spheres: The fountain of probability
  We demonstrate both experimentally and theoretically that a colloidal sphere trapped in a static optical tweezer does not come to equilibrium, but rather reaches a steady state in which its probability flux traces out a toroidal vortex. This non-equilibrium behavior can be ascribed to a subtle bias of thermal fluctuations by non-conservative optical forces. The circulating sphere therefore acts as a Brownian motor. We briefly discuss ramifications of this effect for studies in which optical tweezers have been treated as potential energy wells. 
Yohai Roichman, Bo Sun, Allan Stolarski and David G. Grier