Figure 1

Schematic diagram of the polarization compensation principle of phase-mismatch wave plates in a free-space QKD terminal based on the BB84 protocol. Black, red, yellow, and blue arrows indicate H, V, +, and − coding linearly polarized light, respectively. White spheres indicate the evolution of the Poincaré sphere coordinate axes. Solution 1, traditional method; solution 2, supplemented method; R1, R2, phase mismatch 1/4 wave plates; HWP, 1/2 wave plate. After the H, V, +, and − light is degraded by the optical system, it is compensated by R1 and R2 and then transmitted to the receiver. The basis vector deviation is compensated by the HWP of the receiver, and the wrong codes are corrected through classical channels