Mediated semi-quantum key distribution with improved efficiency

Abstract Mediated semi-quantum key distribution involves the use of two end-users who have very restricted, almost classical, capabilities, wish to establish a shared secret using help fully-quantum server may be adversarial. In this paper, we introduce new mediated protocol, extending prior work, which has asymptotically perfect efficiency. Though comes at cost decreased noise tolerance, our protocol is backwards compatible with so users easily switch old (normally less efficient) if level high enough justify it. To prove security, show an interesting reduction from scenario entanglement based useful when proving security other multi-user quantum protocols.