Preparation-Attack-Immune Quantum Secure Direct Communication Using Two-Fold Photon Degree of Freedom

Quite recently, enhancing security against device-attack vulnerability has been theoretically challenging but also practically important in quantum cryptographic communication. For dealing with this issue in a general and strict scenario, we design a seemingly-new kind of quantum secure direct communication (QSDC) in a linear-optical regime, which we call "preparation-attack-immune QSDC." We address that in our "naive" analysis, it is quite formidable to extract even a piece of information of the secret message, and any malicious eavesdropping attempts will be unsuccessful. The most remarkable feature is that even in the case where a powerful eavesdropper can peep at all preparation device settings, our protocol still keeps a high level of security without invoking any additional resources and physical properties. This novel advantage that we call "preparation-attack immunity" comes from the simultaneous use of the two degrees of freedom involved in a single-photon (polarization and spatial modes), which enables one to faithfully deal with the single-photon quantum superposition nature. Our idea can be generalized to other single-photon based protocols.