Experimental Entanglement Concentration and Universal Bell-state   Synthesizer
  We report a novel Bell-state synthesizer in which an interferometric entanglement concentration scheme is used. An initially mixed polarization state from type-II spontaneous parametric down-conversion becomes entangled after the interferometric entanglement concentrator. This Bell-state synthesizer is universal in the sense that the output polarization state is not affected by spectral filtering, crystal thickness, and, most importantly, the choice of pump source. It is also robust against environmental disturbance and a more general state, partially mixed$-$partially entangled state, can be readily generated as well. 
Yoon-Ho Kim, Sergei P. Kulik, Maria V. Chekhova, Warren P. Grice, and   Yanhua Shih