title: Boundary and impurity effects on the entanglement of Heisenberg chains.
abstract: We study entanglement of a pair of qubits and the bipartite entanglement between the pair and the rest within open-ended Heisenberg XXX and XY models. The open boundary condition leads to strong oscillations of entanglements with a two-site period, and the two kinds of entanglements are 180 degrees out of phase with each other. The mean pairwise entanglement and ground-state energy per site in the XXX model are found to be proportional to each other. We study the effects of a single bulk impurity on entanglement, and find that there exist threshold values of the relative coupling strength between the impurity and its nearest neighbors, after which the impurity becomes pairwise entangled with its nearest neighbors.
authors.affiliation: Australian Centre of Excellence for Quantum Computer Technology, Department of Physics, Macquarie University, Sydney, New South Wales 2109, Australia.
authors.forename: Xiaoguang
authors.initials: X
authors.lastname: Wang