Secure communication with single-photon two-qubit states
Almut Beige, Berthold‐Georg Englert, Christian Kurtsiefer, Harald Weinfurter · Journal of Physics A Mathematical and General · 2002
We propose a cryptographic scheme that is deterministic: Alice sends single photons to Bob, and each and every photon detected supplies one key bit—no photon is wasted. This is in marked contrast to other schemes in which a random process decides whether the next photon sent will contribute to the key or not. The determinism is achieved by preparing the photons in two-qubit states, rather than the one-qubit states used in conventional schemes. In particular, we consider the realistic situation in which one qubit is the photon polarization and the other a spatial alternative. Further, we show how one can exploit the deterministic nature for direct secure communication, that is without the need for establishing a shared key first.