Security Proof of a Semi-Quantum Key Distribution Protocol: Extended Version

TL;DR

This paper proves the unconditional security of a semi-quantum key distribution protocol, deriving its key rate as a function of channel noise and showing it tolerates noise levels comparable to fully quantum protocols.

Contribution

The paper provides the first security proof for a semi-quantum protocol, including an explicit key rate formula and noise tolerance analysis.

Findings

Unconditional security of the semi-quantum protocol is established.

Derived an explicit key rate expression based on channel noise.

Protocol tolerates noise levels comparable to fully quantum protocols.

Abstract

Semi-quantum key distribution protocols are designed to allow two users to establish a secure secret key when one of the two users is limited to performing certain "classical" operations. There have been several such protocols developed recently, however, due to their reliance on a two-way quantum communication channel (and thus, the attacker's opportunity to interact with the qubit twice), their security analysis is difficult and little is known concerning how secure they are compared to their fully quantum counterparts. In this paper we prove the unconditional security of a particular semi-quantum protocol. We derive an expression for the key rate of this protocol, in the asymptotic scenario, as a function of the quantum channel's noise. Finally, we will show that this semi-quantum protocol can tolerate a maximal noise level comparable to certain fully quantum protocols.