Figure 4.

Comparison among physical regions of the UD protocol under both ideal and realistic detection conditions. The red solid line represents the realistic parabolic curve (equivalent to Bob using a realistic homodyne detector with $\eta =0.6,v_{el}=0.1$) and black solid line is the ideal parabolic curve (equivalent to Bob using an ideal homodyne detector with $\eta =1,v_{el}=0$). The red dashed line represents the part where the key rate is zero under realistic detection condition. Here, we set: $\beta =0.99$, $T_x=0.4$ (corresponding to a distance of 20 km fiber), ${\epsilon }_x=0.01$ and $V_M=6.35$.