Mediated semi-quantum key distribution protocol mixing single-state and entangled-state

Abstract

Mediated semi-quantum key distribution (MSQKD) is a popular topic in quantum cryptography, which requires less quantum abilities from legitimate users, Alice and Bob, and has the advantage of high noise tolerance. In this paper, we propose a new MSQKD protocol where the third-party quantum server M can randomly send single-state and entangled-state without informing Alice and Bob of the type of qubit. In addition, in the case where M has an infinite quantum source, we derive the key rate and noise tolerance of the protocol, which proves the unconditional security of the protocol. Furthermore, as M changes the ratio of single-state and entangled-state, the noise tolerance also changes. In particular, when M sends only single-state or entangled-state, our protocol degenerates into the previous MSQKD protocols, and we derive a higher noise tolerance by analysing the mismatch terms.