We propose a cryptographic scheme that is deterministic: Alice sends singlephotons to Bob, and each and every photon detected supplies one key bit -- nophoton is wasted. This is in marked contrast to other schemes in which a randomprocess 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 photonpolarization, the other a spatial alternative. Further, we show how one canexploit the deterministic nature for direct secure communication, that is:without the need for establishing a shared key first.
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