Maxwell theory on a compact manifold as a topologically ordered system
We study novel type of contributions to the entropy of the Maxwell system defined on a compact manifold such as torus. These new terms are not related to the physical propagating photons. Rather, these novel contributions emerge as a result of tunnelling events when transitions occur between topologically different but physical identical vacuum winding states. We compute two new (topologically protected) types of contributions to the entropy in this system resulting from this dynamics. First contribution has a negative sign, expressed in terms of the magnetic susceptibility, and it is similar in spirit to topological entanglement entropy discussed in condensed matter systems. The second contribution with a positive sign results from the emergent degeneracy which occurs when the system is placed into a background of external magnetic field. This degeneracy resembles a similar effect which occurs at \theta=\pi in topological insulators. Based on these computations we claim that the Maxwell system defined on 4 torus behaves in many respects as a topologically ordered system.