In this paper, we propose a new efficient mediated semi-quantum key distribution (MSQKD) protocol, facilitating the establishment of a shared secret key between two classical participants with the assistance of an untrusted third party (TP). Unlike existing MSQKD protocols, our approach significantly reduces the quantum requirements for TP, who only needs to prepare and measure qubits in the
X basis. Meanwhile, the classical participants are limited to preparing and measuring qubits in the
Z basis, along with performing Hadamard operations. This reduction in quantum overhead enhances the practicality of our MSQKD protocol without compromising qubit efficiency. Additionally, we demonstrate the security of our protocol against various well-known attacks.
Abstract As quantum communication networks approach practical deployment, mediated semi-quantum key distribution (MSQKD) has attracted attention for enabling classical users to share keys via semi-honest servers. This paper proposes a cross-domain MSQKD protocol based on four-particle GHZ-like states. Two semi-honest servers prepare and transmit quantum sequences with decoy states, while users perform only Bell and single-photon measurements to derive shared keys. The protocol eliminates additional unitary operations, significantly reducing user-side quantum requirements, hardware costs, and implementation complexity. By exploiting entanglement correlations, it achieves a qubit efficiency of 42.86% and resists both external and internal attacks. The feasibility of the protocol is verified through simulations on Qiskit and the IBM quantum platform, demonstrating its practicality under current quantum technology.