  "Topological ordered" phases such as gapped quantum spin-liquids and
fractional quantum Hall states possess ground state degeneracy on a torus. We
show that the topological nature of this degeneracy has interesting
consequences for the entanglement structure of the degenerate ground states.
This leads to a simple method for detecting topological order, which is
ready-made for numerical schemes such as density matrix renormalization group.
The method is valid for phases that do or do not possess edge states alike. We
demonstrate it by calculating the entanglement spectrum and the topological
entanglement entropy for a chiral spin-liquid state on a 18 site system.
