An important property of entanglement: pairwise entanglement that can only be transferred by an entangled pair

Based on the calculation of all the pairwise entanglements in the n ( n \leq 6)-qubit Heisenberg XX open chain with system impurity, we find an important result: pairwise entanglement can only be transferred by an entangled pair. The non-nearest pairwise entanglements will have the possibility to exist as long as there has been even number of qubits in their middle. This point indicates that we can obtain longer distance entanglement in a solid system.