  We study the defect dynamics in a colloidal spin ice system realized by
filling a square lattice of topographic double well islands with repulsively
interacting magnetic colloids. We focus on the contraction of defects in the
ground state, and contraction/expansion in a metastable biased state. Combining
real-time experiments with simulations, we prove that these defects behave like
emergent topological monopoles obeying a Coulomb law with an additional line
tension. We further show how to realize a completely resettable "NOR" gate,
which provides guidelines for fabrication of nanoscale logic devices based on
the motion of topological magnetic monopoles.
