Fig. 1. Polarization-independent phase-modulated polarization encoding module. LD, laser diode; ISO, isolator; PC, polarization controller; CIR, circulator; PBS, polarization beam splitter; DL, delay line; PM, phase modulator; FM, Faraday mirror.

Fig. 2. Point-to-point QKD based on encoding module. LD, laser diode; ISO, isolator; PC, polarization controller; CIR, circulator; PBS, polarization beam splitter; PM, phase modulator; FM, Faraday mirror; VOA, variable optical attenuator; DG, digital generator; EPG, electrical pulse generator; DL, delay line; RFA, radio-frequency amplifier; QC: quantum channel; BS, optical beam splitter; SPD, single-photon detector.

Fig. 3. Multi-user QKD networks based on point-to-point structure using DWDM. MWLD, multi-wavelength laser diode; ISO, isolator; PC, polarization controller; CIR, circulator; PBS, polarization beam splitter; DWDM, dense wavelength division multiplexer; PM: phase modulator; PD, photo diode; FM: Faraday mirror; VOA, variable optical attenuator; DL, delay line; QC: quantum channel; BS, beam splitter; SPD, single-photon detector.

Fig. 4. QBER of the four polarization states tested under polarization-independent phase modulation over 100 min.

Fig. 5. (a) QBER of CH34 over 100 min at 1 km; (b) QBER of CH35 over 100 min at 1 km; (c) QBER of CH34 over 100 min at 5 km; (d) QBER of CH35 over 100 min at 5 km.