  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, 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.
