Thermal states of anyonic systems

A study of the thermal properties of two-dimensional topological lattice models is presented. This work is relevant to assess the usefulness of these systems as a quantum memory. For our purposes, we use the topological mutual information I-topo as a "topological order parameter". For Abelian models, we show how I-topo depends on the thermal topological charge probability distribution. More generally, we present a conjecture that I-topo can (asymptotically) be written as a Kullback-Leitner distance between this probability distribution and that induced by the quantum dimensions of the model at hand. We also explain why I-topo. is more suitable for our purposes than the more familiar entanglement entropy S-topo. A scaling law, encoding the interplay of volume and temperature effects, as well as different limit procedures, are derived in detail. A non-Abelian model is next analyzed and similar results are found. Finally, we also consider, in the case of it one-plaquette toric code, an environment model giving rise to a simulation of thermal effects in time. (C) 2009 Elsevier B.V. All rights reserved.