  A relation between entanglement and criticality of spin chains is
established. The entanglement we exploit is shared between auxiliary particles,
which are isolated from each other, but are coupled to the same critical
spin-1/2 chain. We analytically evaluate the reduced density matrix, and
numerically show the entanglement of the auxiliary particles in the proximity
of the critical points of the spin chain. We find that the entanglement induced
by the spin-chain may reach one, and it can signal very well the critical
points of the chain. A physical understanding and experimental realization with
trapped ions are presented.
