Fig. 1. P&P QKD network system architecture for 64 users. The coherent light signals from the server, Bob, are transmitted to 64 users, including Alice, using WDM and polarization-division multiplexing. All users have identical optical systems. BS, beam splitter; PBS, polarization beam splitter; Cir, circulator; APD, avalanche photodiode; DL, delay line; PM, phase modulator; WDM, wavelength-division multiplexing; QC, quantum channel; PD, photodiode; VOA, variable optical attenuator; SL, storage line; IM, intensity modulator; and FM, Faraday mirror.

Fig. 2. Timing control module based on an FPGA for one user. The arrows indicate the flow of the signal for generating laser pulses.

Fig. 3. Flowchart of the control program for laser drivers in the server. After system operation, the control program performs sifting with multiple users. The program corrects the timing parameters using independent monitoring and path length compensation functions.

Fig. 4. Peak wavelengths of the lasers for 64 users.

Fig. 5. Sifted key rate and optimal laser timing of four users in 100 min.

Fig. 6. (a) Map of the 1×4 QKD network and (b) operation scheduling for the server to demonstrate the workings of the QKD network.

Fig. 7. Results of using the 1×4 QKD network over a week in a real-world environment. The points are average key rates and QBERs for one day.

Fig. 8. Variations in optimal laser timings for the real-environment QKD network over 100 h.