Encoding control system for twin-field quantum key distribution

The Twin-Field Quantum Key Distribution (TF-QKD) protocol has the potential to realize secure key distribution over extremely long distances, which is an important technique for realizing a global quantum network. Compared to the conventional BB84 protocol, practical TF-QKD and its variant protocols require an accurate phase modulation to at least 16 different values with randomized encoding. In this work, we developed an encoding control system for TF-QKD. Optical pulses with five different intensities and 16 different phases can be modulated with a clock frequency of 100 MHz with a field programmable gate array based arbitrary waveform generator (AWG). With the assistance of DDR4 memory, waveforms exceeding 200 ms in length can be output simultaneously on 4 pairs of differential channels, making the random number pairing between two different encoding systems close to the expected ratio when using cyclic random numbers for experimental demonstration. The AWG boasts a long-term amplitude stability better than 0.03% and supports seamless concatenation and cyclic output of waveforms, demonstrating a strong and sustained performance in long-duration experiments. Sending-or-not-sending TF-QKD was demonstrated with the encoding control system, with a secure key rate of 1.33 × 10−5 per pulse under the total channel loss of ∼32 dB.