We study the entanglement of thermal and ground states in Heisernberg XX
qubit rings with a magnetic field. A general result is found that for
even-number rings pairwise entanglement between nearest-neighbor qubits is
independent on both the sign of exchange interaction constants and the sign of
magnetic fields. As an example we study the entanglement in the four-qubit
model and find that the ground state of this model without magnetic fields is
shown to be a four-body maximally entangled state measured by the N-tangle.Comment: Four pages and one figure, small change