Producing the {\it a priori} pure entangled states by type II prametric   downconversion
  We propose a scheme to produce the pure entangled states through type II downconversion. In the scheme, the vacuum states are excluded and the a prori pure entangled states are produced and verified without destroying the state itself. This can help to carry out the many unditional experiments related to quantum entanglement. 
Wang Xiang-Bin