  We propose practical schemes for concentrating entanglement of a pair of
unknown partially entangled Bell states and three-photon W states with
cross-Kerr nonlinearity. In the schemes, utilizing local operations and
classical communication, two separated parties can obtain one maximally
entangled photon pair from two previously shared partially entangled photon
pairs, and three separated parties can obtain one maximally entangled
three-photon W state and a maximally entangled cluster state from two identical
partially entangled three-photon W state with a certain success probability.
Finally, we discuss the influences of sources of errors and decoherence on the
schemes. The proposed setup is very simple, just employing some linear optical
elements and cross-Kerr medium, which greatly simplifies the experimental
realization of the schemes. The schemes are feasible within current
experimental technology
