  We present a numerical study of a quantum phase transition from a
spin-polarized to a topologically ordered phase in a system of spin-1/2
particles on a torus. We demonstrate that this non-symmetry-breaking
topological quantum phase transition (TOQPT) is of second order. The transition
is analyzed via the ground state energy and fidelity, block entanglement,
Wilson loops, and the recently proposed topological entropy. Only the
topological entropy distinguishes the TOQPT from a standard QPT, and
remarkably, does so already for small system sizes. Thus the topological
entropy serves as a proper order parameter. We demonstrate that our conclusions
are robust under the addition of random perturbations, not only in the
topological phase, but also in the spin polarized phase and even at the
critical point.
