Efficient Quantum Dialogue Using a Photon in Double Degrees of Freedom

In order to increase the capacity of quantum communication, several quantum dialogue (QD) protocols, including this work, are based on single photons in both polarization and spatial-mode degrees of freedom (PSDF). Some of them use seven or four composite unitary operations to implement a QD protocol, while this work adopts a mere two single unitary operations to do. Most importantly, their information-theoretical efficiencies are generally only 50%, yet that of this work is up to 66.67%. The analyses show that the proposed QD protocol is safe without information leakage. It does not adopt auxiliary photons either and simply needs single photons in PSDF, whose preparation and measurement can be easily implemented with current technology. So, it is not only of great feasibility in practice, but also simple and efficient.