Fig. 1. Physical structure of the quantum downstream access network.

Fig. 2. Achievable secret key rate against the number of QNUs access in the network and transmission distance. Parameters are set as η=0.481, vel=0.02, β=0.95 [52], VA=4 SNU. (a) Secret key rate against the number of QNUs and transmission distance. (b) Secret key rate as a function of the number of QNUs. (c) Secret key rate as a function of transmission distance.

Fig. 3. Four-end-users downstream quantum access network of the GMCS LLO CV-QKD experimental scheme. DAC, digital-to-analog converter; IQ modulator, in-phase/quadrature modulator; MPC, manual polarization controller; BS, beam splitter; VOA, optical variable attenuator; BHD, balanced homodyne detector.

Fig. 4. Signal processing of four-end-users quantum downstream access network. PF, particle filter; RLS, recursive least squares.

Fig. 5. Time waveform of the BHD output signals collected by the oscilloscope working at 1 GSa/s.

Fig. 6. Frequency spectrum of the BHD output signals.

Fig. 7. Comparison of phase drift estimation before and after PF. The blue circle represents phase drift estimation before PF, the red circle represents phase drift estimation after PF.

Fig. 8. Comparison of experimental secret key rates and excess noise (SNU) levels after PF and RLS. Upper marks are secret key rates; lower marks represent excess noise (SNU) levels. The initial mean excess noise is about 0.013 SNU, the mean excess noise after PF is about 0.007 SNU, the mean excess noise after RLS is about 0.004 SNU.

Fig. 9. Secret key rate curves of experiment as a function of transmission distance for four QNUs. The black solid line represents the PLOB bound in this scheme. The solid line in different colors represents the secret key rate of each QNU respectively in infinite-size scenarios while the dashed line in different colors represents the secret key rate of each QNU respectively under finite-size block of 1×108. The pentagram in different colors represents the experimental secret key rate in infinite-size scenarios respectively. The diamond in different colors represents the experimental secret key rate with block size of 1×108 respectively. The quantum efficiency is η=0.481; the reconciliation efficiency is β=0.95 for all QNUs. The excess noise of QNU1 is 0.004 SNU, with VA=4.008 SNU; the excess noise of QNU2 is 0.003 SNU, with VA=4.006 SNU; the excess noise of QNU3 is 0.005 SNU, with VA=4.0107 SNU; the excess noise of QNU4 is 0.006 SNU, with VA=4.0129 SNU.

Fig. 10. Process of particle filtering.

Fig. 11. Process of recursive least squares.