  We demonstrate that a cycle of three holographic optical trapping patterns
can implement a thermal ratchet for diffusing colloidal spheres, and that the
ratchet-driven transport displays flux reversal as a function of the cycle
frequency and the inter-trap separation. Unlike previously described ratchet
models, the approach we describe involves three equivalent states, each of
which is locally and globally spatially symmetric, with spatiotemporal symmetry
being broken by the sequence of states.
