²NU stands for natural unit. This is difference with CV-QKD where the Shot Noise Units were chosen, corresponding to set $\hslash =2$. In natural units, $\hslash =1$, $[\widehat{q},\widehat{p}]=i$ and $\mathrm{\Delta }\widehat{q}\mathrm{\Delta }\widehat{p}\ge 1∕2$.