Title: Localisable entanglement of 1D cluster states

We consider quantum chains in cluster states under the influence of a
variable magnetic field. After reviewing the derivation of the ground
state we compute the localisable entanglement of the two outermost
qubits, after local measurements have been made on the inner ones, for
different chain lengths. The result is mostly as intuitively expected:
the entanglement decreases monotonously with the field strength.