Unconditional security of the Bennett 1992 quantum-key-distribution scheme with a strong reference pulse

We prove the unconditional security of the original Bennett 1992 protocol with strong reference pulse. We show that we may place a projection onto suitably defined qubit spaces before the receiver, which makes the analysis as simple as qubit-based protocols. Unlike the single-photon-based qubits, the qubits identified in this scheme are almost surely detected by the receiver even after a lossy channel. This leads to the key generation rate that is proportional to the channel transmission rate for proper choices of experimental parameters.

Figure 1

(a) An experimental setup for the B92 protocol with strong reference pulse. PM: phase modulator. (b) An equivalent setup for Bob.

Figure 2

The key generation rate. See the main text for the explanations.

Figure 3

The measurement diagram.