Experimental Long-Distance Decoy-State Quantum Key Distribution Based on Polarization Encoding

We demonstrate the decoy-state quantum key distribution (QKD) with one-way quantum communication in polarization space over 102 km. Further, we simplify the experimental setup and use only one detector to implement the one-way decoy-state QKD over 75 km, with the advantage to overcome the security loopholes due to the efficiency mismatch of detectors. Our experimental implementation can really offer the unconditionally secure final keys. We use 3 different intensities of 0, 0.2, and 0.6 for the light sources in our experiment. In order to eliminate the influences of polarization mode dispersion in the long-distance single-mode optical fiber, an automatic polarization compensation system is utilized to implement the active compensation.

Figure 1

Schematic diagram of the experimental setup. Solid lines and dashed lines represent the optical fiber and electric cable, respectively. See the text for the abbreviations.

Figure 2

(a) Test of the APC system with 75 km fiber. In positions 1 and 2, the APC system monitors the visibility changes of polarization states and adjusts the voltages of EPC actively to reach the target visibility. Subsequently, the visibility of polarization states slowly becomes worse when free running. In position 3, artificial disturbance induces drastic change of visibility and the APC system can still work well. The time interval of points is 2 s. (b) Comparison of the final key rate of signal states per pulse between the theoretical calculation and experimental results with four different distance settings $L$ (13.448, 50.524, 75.774, and 102.714 km), where their corresponding total attenuations are (24.9, 32.2, 34.8, and 37.0 dB) including channel losses, all the insertion losses of components, detector efficiencies, etc. The first three settings use the one-detector scheme and the last one uses the two-detector scheme. The repetition frequency of the first two settings is $f=4\mathrm{MHz}$ and the last two is $f=2.5\mathrm{MHz}$.

Figure 3

The final key rate of decoy pulses varies with the vacuum counting rate of $s_0$ in the case of 13.448 km, where $s_0=(1+r_0)S_0$.