Impurity in Pairwise Entanglement of Heisenberg XX Open Chain

We calculate the concurrence of all pairwise entanglement of Heisenberg XX open chain with single system impurity in three-qubit and four-qubit cases, and find that the impurity parameter Ji has great effect on pairwise entanglement. Choosing the proper parameter Ji, we can obtain the maximal pairwise entanglement of the nearest qubits and make the non-nearest qubits entangle.