Improved semi-quantum key distribution with two almost-classical users

Semi-quantum key distribution (SQKD) protocols attempt to establish a shared secret key between users, secure against computationally unbounded adversaries. Unlike standard quantum key distribution protocols, SQKD protocols contain at least one user who is limited in their quantum abilities and is almost classical in nature. In this paper, we revisit a mediated semi-quantum key distribution protocol, introduced by Massa et al. (Experimental quantum cryptography with classical users, 2019. arXiv preprint arXiv:1908.01780), where users need only the ability to detect a qubit, or reflect a qubit; they do not need to perform any other basis measurement; nor do they need to prepare quantum signals. Users require the services of a quantum server which may be controlled by the adversary. In this paper, we show how this protocol may be extended to improve its efficiency and also its noise tolerance. We discuss an extension which allows more communication rounds to be directly usable; we analyze the keyrate of this extension in the asymptotic scenario for a particular class of attacks and compare with prior work. Finally, we evaluate the protocol's performance in a variety of lossy and noisy channels.

Automated summary

This paper investigates a mediated semi-quantum key distribution protocol and proposes improvements and extensions to enhance the protocol's efficiency and noise tolerance. The protocol enables the sharing of a secret key among users who have limited quantum abilities, using only the capability to detect or reflect qubits, with the assistance of a quantum server controlled by the adversary.