Abstarct: The future prospects for quantum key distribution protocol, received a tremendous boost from the discovery quantum cryptography is realy possible. To carry out a quantum key distribution protocol, we must first encode the quantum information we want to exchange, and then repeatedly perform recovery operations that modify the errors that accumulate. In this thesis, we consider the advantage of the six- state quantum key distribution protocol with respect to four-state (BB84) quantum key distribution protocol. It is proved in BB84 quantum key distribution protocol using one- way classical communications, the bit- flip and phase- flip errors are uncorrelated. We show that in six – state quantum key distribution protocol using one- way classical communications, the bit- flip and phase- flip errors are correlated. This correlation, reduces the conditional entropy and in other words, increases the secret quantum key generation rate