Fig. 2: Theoretical prediction for the dependence of channel capacity on transmission distance with different encoded classical signals.

The transmission distance with improved classical signals is increased compared with that using fixed classical signals at Alice’s station. The black solid and gray dashed curves are obtained with the squeezing (antisqueezing) level of −6 dB (+10 dB), and the blue dotted and orange dot-dashed curves are obtained with the squeezing (antisqueezing) level of −10 dB (+15 dB). Phase fluctuation is not considered here