Quantum Key Distribution with a Continuous-Wave-Pumped Spontaneous-Parametric-Down-Conversion Heralded Single-Photon Source

(Received July accepted February published 2023) Quantum key distribution (QKD) protocols utilize single photons for key exchanging. Heralded single -photon sources (HSPSs) with spontaneous parametric down-conversion (SPDC) is a practical and effective way of producing single photons of high fidelity, bringing extra performance especially at long distances. A pulsed laser is usually used as the pump for HSPS QKD, but it has disadvantages in operational stability and practicality compared to a cw one. On the other hand, a precise model describing the performance of HSPS with a cw pump is still missing. It is inaccurate and unsafe to apply the key rate model of the pulse -pumped HSPS QKD directly to the cw one, and it also overlooks the timing jitter and dead time of the single-photon detectors. In this paper, we study the cw case in detail, including practical imperfections of the experimental instruments. Our model shows that a cw-pumped setup can reach a much longer transmission distance (31 % longer) than the fastest pulsed HSPS QKD system at present, together with other advantages such as higher operational stability and better spectral characteristics.

Automated summary

Quantum key distribution (QKD) protocols rely on single photons for key exchange. Heralded single-photon sources (HSPSs) utilizing spontaneous parametric down-conversion (SPDC) are effective in producing high fidelity single photons, especially for long distance applications. While a pulsed laser is commonly used as the pump for HSPS QKD, it has disadvantages compared to a continuous wave (cw) laser in terms of operational stability and practicality. This paper presents a detailed study of the cw-pumped case, including practical imperfections in experimental instruments. The model developed shows that a cw-pumped setup can achieve a significantly longer transmission distance (31% longer) than the fastest pulsed HSPS QKD system currently available, and offers advantages such as higher operational stability and better spectral characteristics.