  Topological spin liquids are robust quantum states of matter with long-range
entanglement and possess many exotic properties such as the fractional
statistics of the elementary excitations. Yet these states, short of local
parameters like all topological states, are elusive for conventional
experimental probes. In this work, we combine theoretical analysis and quantum
Monte Carlo numerics on a frustrated spin model which hosts a $\mathbb Z_2$
topological spin liquid ground state, and demonstrate that the presence of
symmetry-protected gapless edge modes is a characteristic feature of the state,
originating from the nontrivial symmetry fractionalization of the elementary
excitations. Experimental observation of these modes on the edge would directly
indicate the existence of the topological spin liquids in the bulk, analogous
to the fact that the observation of Dirac edge states confirmed the existence
of topological insulators.
