Thermal and ground-state entanglement in Heisenberg XX qubit rings
Xiaoguang Wang · Physical Review A · 2002
We study the entanglement of thermal and ground states in the Heisenberg $\mathrm{XX}$ qubit rings with a magnetic field. A general result is found that for even-number rings, pairwise entanglement between nearest-neighbor qubits is independent of 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 square N-bit concurrence.