Figure 8

The calculated QBER and SNR as a function of the link losses for two different dark count rates (solid red curve: 100 cps; dotted blue curve: 1000 cps per detector). All other parameters are as in Table 3. No secret key distillation is possible if the QBER exceeds 9.4% (\(\mbox{SNR} > 9.6\)) for the case that the bidirectional error correction efficiency f equals 1.22 (dashed horizontal green line). The corresponding SNR is shown on the right y-axis (solid purple: 100 cps; dashed light blue: 1000 cps dark count rate)