Semi-Quantum Key Distribution with Single Photons in both Polarization and Spatial-Mode Degrees of Freedom

In this paper, a novel semi-quantum key distribution (SQKD) protocol is designed based on single photons in both polarization and spatial-mode degrees of freedom, which allows to establish a raw key between one quantum communicant and one classical communicant. The proposed SQKD protocol only adopts one kind of quantum state as the initial quantum resource. The detailed security analysis shows that it can resist Eve’s active attacks, such as the intercept-resend attack, the measure-resend attack, the Trojan horse attack and the entangle-measure attack. The proposed SQKD protocol only needs single photons in both polarization and spatial-mode degrees of freedom as quantum resource and employs single-photon measurements. Thus, it has excellent feasibility, since the preparation and the measurement of a single photon in both polarization and spatial-mode degrees of freedom can be easily acheived with present quantum technologies.