  We experimentally demonstrate observation of highly pure four-photon GHZ
entanglement produced by parametric down-conversion and a projective
measurement. At the same time this also demonstrates teleportation of
entanglement with very high purity. Not only does the achieved high visibility
enable various novel tests of quantum nonlocality, it also opens the
possibility to experimentally investigate various quantum computation and
communication schemes with linear optics. Our technique can in principle be
used to produce entanglement of arbitrarily high order or, equivalently,
teleportation and entanglement swapping over multiple stages.
