Fig. 4: CV-QKD experimental setup.
From: Machine learning aided carrier recovery in continuous-variable quantum key distribution
a An ensemble of coherent states at 1550 nm was encoded into continuous-wave laser light by electro-optic in-phase/quadrature single sideband modulation driven by an arbitrary waveform generator (AWG). A reference pilot tone was digitally frequency multiplexed with these coherent states, which after suitable attenuation became the ‘quantum signal’. This pilot tone is ~3.2 times the power of the quantum signal. The polarisation of the combined quantum signal and pilot tone, transmitted through a 20-km SMF-28 fibre, was corrected with a manual polarisation controller to match the polarisation of an independent local oscillator (LO). The output of the radio-frequency heterodyne detector was sampled by a digital storage oscilloscope (DSO) at 1 GSamples/s before undergoing various digital-signal-processing (DSP) methods, including the unscented Kalman filter (UKF) assisted phase tracking. b Sample phase profiles extracted by UKF at high SNRpilot for 100 Hz and 10 kHz linewidth transmitter lasers. The receiver used the same ≈100 Hz linewidth laser as the LO for both setups.
