  A particle diffusing around a point of stable mechanical equilibrium in a
static but non-conservative force field enters into a steady state
characterized by circulation in the probability flux. Circulation in such a
Brownian vortex is not simply a deterministic response to the solenoidal
component of the force, but rather reflects an interplay between force-driven
probability currents and diffusion. As an example of this previously
unrecognized class of stochastic heat engines, we consider a colloidal sphere
stably trapped in a conventional optical tweezer. Rather than coming into
thermodynamic equilibrium with the surrounding heat bath, the particle's
Brownian fluctuations are biased into a toroidal roll. We demonstrate both
theoretically and experimentally that the circulation in this practical
realization of the Brownian vortex can undergo flux reversal.
