The thermal entanglement in the Heisenberg $\mathrm{XYZ}$ chain is investigated in the presence of an external magnetic field B. In the two-qubit system, the critical magnetic field $B_c$ is increased by introducing the interaction of the z component of two neighboring spins $J_z.$ This interaction not only improves the critical temperature $T_c,$ but also enhances the entanglement for a particular fixed B. We also analyze the pairwise entanglement between nearest neighbors in three qubits. The pairwise entanglement, for a fixed T, can become strong by controlling B and $J_z.$

• Received 13 February 2003

DOI:https://doi.org/10.1103/PhysRevA.68.024301