  A characterization of topological order in terms of bi-partite entanglement
was proposed recently [A. Kitaev and J. Preskill, Phys. Rev. Lett. 96, 110404
(2006); M. Levin and X.-G. Wen, ibid, 110405]. It was argued that in a
topological phase there is a universal additive constant in the entanglement
entropy, called the topological entanglement entropy, which reflects the
underlying gauge theory for the topological order. In the present paper, we
evaluate numerically the topological entanglement entropy in the ground-states
of a quantum dimer model on the triangular lattice, which is known to have a
dimer liquid phase with Z_2 topological order. We examine the two original
constructions to measure the topological entropy by combining entropies on
plural areas, and we observe that in the large-area limit they both approach
the value expected for Z_2 topological order. We also consider the entanglement
entropy on a topologically non-trivial ``zigzag'' area and propose to use it as
another way to measure the topological entropy.
