Fig. 1. Experimental setup for measuring performance degradation of a polarization-based BB84 QKD system under high DC bias current. An example of aperiodic current signals generated from a FPGA is depicted. FPGA, field-programmable gate array; LD, laser diode; Sync. LD, synchronization laser diode; Sync. PD, synchronization photodiode; DM, dichroic mirror; HWP, half-wave plate; ND filter, neutral density filter; PBS, polarization beam splitter; BS, beam splitter; PD, photodetector; SPD, single photon detector.

Fig. 2. Detection probability distribution of the received signals under IDC=0.95Ith, which was estimated at the FPGA with a timing resolution of 125 ps. CSE represents the photon counts created by the spontaneous emission process.

Fig. 3. Side channel effects and the performance of the QKD system under IDC=0. (a) Side channel effects of temporal disparity and intensity fluctuation. Second pulses out of two consecutive pulses in a 1 s time block are measured for different time intervals from 2.5 to 40 ns. Details of the measurement methods are described in [11]. (b) QBER as a function of the signal window with temporal filtering. The signal window was resized from 2.5 to 0.5 ns, with a resolution of 0.25 ns. (c) Sifted key rate and secure key rate as functions of the signal window with temporal filtering.

Fig. 4. Side channel effects and performance of the QKD system under IDC=0.95Ith. (a) Side channel effects of temporal disparity and intensity fluctuation. Second pulses out of two consecutive pulses in a 1 s time block were measured for different time intervals from 2.5 to 40 ns. Details of the measurement methods are described in [11]. (b) QBER as a function of the signal window with temporal filtering. The signal window was resized from 2.5 to 0.5 ns, with a resolution of 0.25 ns. (c) Sifted key rate and secure key rate as functions of the signal window with temporal filtering.

Fig. 5. Spectral characteristics of photon pulses for different DC bias and AC pulse conditions.