Optimized QKD protocol for mitigating afterpulse effects in single-photon detectors

In this paper, we introduce a new Quantum Key Distribution (QKD) protocol designed to reduce afterpulsing by at least 55%. We provide both theoretical analysis and simulation results for a single-photon QKD system using SPADs that exhibit less-than-ideal afterpulse behavior. Our findings show that the afterpulse probability (P AP ) in SPADs does not set a strict lower limit on the error rate of sifted keys in single-photon QKD systems. The system uses a simplified two-state version of the BB84 protocol, with quantum states encoded in different polarizations over fiber or free-space channels. The protocol ensures that consecutive photons have distinct polarizations. In the enhanced protocol, four detectors measure two polarization bases, yielding four possible detection outcomes. The results demonstrate the practical potential of this approach for both fiber-optic and free-space quantum communication, making it especially suitable for real-world deployment. This work paves the way for developing new protocols in quantum communications. In the new method, by avoiding the transmission of two consecutive photons with the same polarization angle, the security level of the optimal BB84 protocol against attacks will be reduced to that of the B92 protocol. Simulations conducted in MATLAB and OptiSystem, under realistic conditions (1550 nm wavelength, InGaAs SPAD parameters: η = 30%, DCR = 10%, clock rate = 100 MHz), demonstrate a drop in afterpulse probability from 53% to 23.2%. The main advantages of the new protocol method over hardware-based methods are lower cost and no need for extensive hardware modifications. Additionally, using the optimized protocol alongside other methods to reduce afterpulse probability further decreases its occurrence.