  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.
