Fig. 4: Experimental demonstrations of SNS-TF-QKD networks with integrated photonics.

Error rates and secure key rates are measured for 20 QKD client chips {A_i, B_i} through ten quantum channels at different wavelengths \({\lambda }_{i}^{{\prime} }\). a,c, Segmented QBER in the X-basis and their statistical distributions in the 204-km upstream fibre channels. b,d, Segmented QBER in the X-basis and their statistical distributions in the 370-km upstream fibre channels. Each data point represents the error rate measured by sending 150 × 10⁶ bins. The network used short downstream fibres, except for the {A₅, B₅} chips with 120-km downstream fibres. In c and d, the box plots show the mean, quartiles and extreme values. e,f, Measured secure key rates in the 204-km and 370-km fibre channels, respectively. The dotted boxes represent the theoretical value at that distance and the dashed red lines represent the PLOB-repeaterless bound at that distance. In f, it shows an enhancement of secure key rate in comparison with the PLOB-repeaterless bound. In all subplots, coloured points and bars represent different paired QKD client chips.