Figure 1

Key rate over channel loss with a 12.5 MHz bandwidth quantum channel. “ideal AN” and “ideal” refer respectively to ideal error correction with and without artificial noise added at the receiver. It can be seen that the addition of artificial noise reduces the key rate in the low to medium loss regime but extends the reach. The curves labelled with “signal-to-noise ratio (SNR) x” are upper bounds for the key rate with the SNR target x of the artificial noise set to 0.0125 (−19.0 dB), 0.0118 (−19.3 dB), and 0.0114 (−19.4 dB), respectively. These curves are calculated with a fixed-rate code and decoding failure probability corresponding to the target SNR. The transmit power and target SNR of the artificial noise are adapted to the channel situation for each measurement individually to maximise the key rate. The measured key rates suffer from a limited processing power available for the post-processing and can have a wide confidence interval