  We investigate the entanglement properties of a one dimensional chain of spin
qubits coupled via nearest neighbor interactions. The entanglement measure used
is the n-concurrence, which is distinct from other measures on spin chains such
as bipartite entanglement in that it can quantify "global" entanglement across
the spin chain. Specifically, it computes the overlap of a quantum state with
its time-reversed state. As such this measure is well suited to study ground
states of spin chain Hamiltonians that are intrinsically time reversal
symmetric. We study the robustness of n-concurrence of ground states when the
interaction is subject to a time reversal antisymmetric magnetic field
perturbation. The n-concurrence in the ground state of the isotropic XX model
is computed and it is shown that there is a critical magnetic field strength at
which the entanglement experiences a jump discontinuity from the maximum value
to zero. The n-concurrence for thermal mixed states is derived and a threshold
temperature is computed below which the system has non zero entanglement.
