Improvement on controlled-controlled-NOT operation-based entanglement purification protocol

We show that controlled-controlled-NOT(CCN)operation-based entanglement purification protocol can be further improved.CCN protocol requires Bell state measurements after performing the CCN operations.In the original CCN protocol,the measured states are assumed to be destroyed.However,if controlled-NOT gates are used to perform such Bell state measurements,in some unsuccessful situations of the CCN protocol,one can further purify the two mixed entangled states which are to be measured.In this way,the total efficiency of the CCN protocol is further increased.