  We investigate the change of entanglement of photons due to propagation. We
find that post-selected entanglement in general varies by propagation and, as a
consequence, states with maximum bi- and tri-partite entanglement can be
generated from propagation of unentangled photons. We generalize the results to
n photons and show that entangled states with permutation symmetry can be
generated from propagation of unentangled states. Generation of n-photon GHZ
states is discussed as an example of a class of states with the desired
symmetry.
