System stability

We develop two automatic feedback control modules to enhance the stability of the CV-QKD system. (1) The modulator bias control (MBC) module. In our experiment, the light source is a narrow linewidth (1.9 kHz) and low phase noise (2 μrad/rt-Hz 1 m OPD) laser producing CW coherent light at 1,550.12 nm (ITU-34). The CW light is transformed into a 2 MHz clock pulse train by a low jitter (<25 ps) digital square pulse generator and near 65 dB extinction ratio, 10 GHz LiNbO₃ AM. Since the bias voltage of LiNbO₃ modulator drifts with time, it will reduce the effective extinction ratio in the whole system. To stabilize the AM, we employ a MBC module to lock the working point automatically. This calibration process is used during the system initialization to maximize the extinction ratio of optical pulses, and achieved by monitoring the feedback light intensity from the output of modulator. (2) The dynamic polarization control (DPC) module. The polarization-demultiplexing of signal and LO is achieved by a DPC and polarization beam splitter at Bob’s side. The LO pulses split from the LO path at Bob’s side is detected by a PD. The analog electrical signal from the LO is fed back to bring back the State of Polarization (SOP) towards the correct SOP, which guarantees that the LO photons enter into the LO path at Bob’s side. The excess noise induced by the leaked LO photons is detailed in Supplementary S1.