Strong and weak pulse generation for the B92 protocol

Abstract

One of the challenges in quantum key distribution is to ensure adequate security against eavesdropping. The B92 protocol achieves this through the transmission of a strong and weak pulse, with key bits coded onto the weak pulse and channel monitoring done using the bit error rate of the strong pulse. However, the protocol also assumes synchronization between transmitter and receiver using a shared or transmitted clock signal. We propose a scheme that uses a mode locked laser for the strong pulse train, and a nonlinear four-wave mixing processes to generate the weak pulse train. The detector relies on dispersion effects in the transmission channel to delay the weak pulse relative to the strong pulse, and then use the strong pulse to gate the detection of the weak pulse. The scheme is thus self-synchronized and offers the option of additional security using four-wave-mixing processes.