Continuous-Variable Quantum Key Distribution - Coherent State Protocols for a Free Space Optical Experiment

Abstract

We test a continuous variable quantum key distribution (CV-QKD) protocol using coherent states modulated on a 21.1 MHz sideband of a 1550nm fibre laser (Koheras C15) and demonstrate positive secret keyrates over a lossy channel in a tabletop experiment. Based on analysis for a GG02 protocol, we estimate a maximum keyrate with introduced loss of 1.03 +/- 0.17 bits/pulse at 4.5 dB loss on the X quadrature with a modulation variance of 5.2 SNU and a modulation bandwidth of 500-800 kHz. We observed positive keyrates at 0dB loss for all modulation bandwidths, with distinct peaks at lower modulation variances of approximately 0-5 SNU and 30 SNU for most modulation bandwidths. With introduced loss, the experiment produced positive keyrates on both quadratures for 500kHz modulation bandwidth at 1.4 dB loss, and on the X quadrature for 800kHz modulation bandwidth at 4.5 dB loss. Negative but near zero keyrates were also observed for 600kHz and 1 MHz modulation bandwidths, which may warrant further investigation using different filtering regimes. Digital filtering yielded significant improvements, producing both higher overall keyrates and a much greater number of positive keyrates for filtered versus unfiltered key data. Additional noise or filtering is needed to consistently produce positive keyrates. We identified significant noise on the P quadrature, which limits the viability of using this experiment for a switching protocol. We determined the P quadrature of the laser was not shot noise limited below 15 MHz, with intermittent noise above this from an unknown source. Without further reducing the noise on the P quadrature, it will remain a significant barrier to testing improvements from adaptive optics on this experiment.