Synchronous concentration and purification schemes of arbitrary unknown   hyperentangled mixed states
  We present two efficient schemes which can simultaneously accomplish hyperentanglement concentration and purification for two-photon four-qubit systems in an unknown partially hyperentangled mixed states. The first can correct errors in the polarization entanglement and extract maximal hyperentanglement in polarization and spatial mode with additional partial frequency entanglement. The second uses additional maximal frequency entanglement to purify and concentrate hyperentanglement in polarization and spatial mode deterministically. Both of the two schemes are only based on existing optical devices and cross-Kerr nonlinearities. 
Kun Du, Qiu-Cheng Song and Cong-Feng Qiao
quant-ph