Figure2

Figure 2. The schematic description of COW-4 protocol. Single-photon-level pulses with bit values of zeros, ones, decoy and vacuum states are sent from Alice to Bob via the quantum channel. At Bob, the bit-generation detector D_bit generates the QKD keys, while the monitoring interferometer measures the coherence between adjacent pulses at detector D_mon. The key distillation process commences thereafter and the sifting, error correction, privacy amplification and key management process happen over the classical channel (indicated by the blue dashed arrow) to generate the secret keys. COW: Coherent one-way; QKD: quantum key distribution.