Reflection-Based Mediated Semiquantum Key Distribution Employing Bell States

Semiquantum key distribution (SQKD) enables a quantum party and a classical party to establish a secret key, where the classical party is restricted to fixed-basis preparations and measurements, qubit reordering, or undisturbed reflection. Mediated SQKD extends this framework by allowing two classical parties to share a secret key with the assistance of an untrusted third party. More recently, reflection-based SQKD has emerged as a promising direction in which the classical party’s reflection operations actively participate in key generation. Motivated by these ideas, in this paper, we propose a reflection-based mediated semiquantum key distribution (RB-MSQKD) protocol employing Bell states. In the proposed protocol, two classical parties establish a secret key with the help of an untrusted third party, while their respective reflection operations contribute directly to the key generation process. We rigorously prove the correctness of the protocol, derive a general expression for the asymptotic key rate, and analyze its performance over depolarizing channels. The proposed Bell-state RB-MSQKD protocol offers a viable alternative to existing single-photon RB-MSQKD for key distribution between two classical parties assisted by an untrusted third party.