Optimizing Continuous-Wave-Pumped Entanglement-based QKD in Noisy Environments

TL;DR

This work investigates the performance of continuous-wave-pumped entanglement-based QKD under extreme noise, highlighting that detector timing jitter, dead time, and rate-dependent efficiency vary with detection rate and can degrade SKR and raise QBER. It introduces a detector-dependent timing-distortion model, validated experimentally with SPDC-based photon pairs and controlled broadband noise, and extends the analysis to a two-basis QKD framework. A saturating recovery model captures the observed rate-dependent coincidence peak shift and FWHM broadening, enabling reliable optimization of the coincidence window; a BFGS-based procedure demonstrates SKRs on the order of tens of thousands of bits per second and noise tolerance up to ~32 Mcps. The findings offer a practical pathway to deploying entanglement-based QKD in noisy real-world environments and raise security considerations for rate-induced timing shifts that warrant further mitigation strategies.

Abstract

Quantum key distribution (QKD) has emerged as a promising solution to protect current cryptographic systems against the threat of quantum computers. As QKD transitions from laboratories to real-world applications, its implementation under various environmental conditions has become a pressing challenge. Major obstacles to practical QKD implementation are the loss of photons in the transmission media and the presence of extreme noise, which can severely limit long-range transmission. In this paper, we investigate the impact of extreme noise on QKD system parameters, including timing jitter, rate-dependent timing shifts, changes in effective detector dead time, and rate-dependent detection efficiency. Contrary to manufacturers' specifications, which assume these parameters to be constant, we demonstrate that these parameters exhibit significant variations in extreme noise conditions. We show that changes in these parameters play a key role in determining system performance in noisy environments. To address these nonidealities, we develop a model that adapts to detector-dependent timing distortions and recovery effects. In particular, our model is independent of source parameters and can be implemented using data from the detection unit. Our results show that the model enables reliable characterization and optimization of QKD performance under strong noise.

Figures (6)

• Figure 1: Experimental setup for coincidence profile characterization in noisy environments. BP - bandpass filter, LP - long pass filter, SMF - single mode optical fiber, L175 - lens with focal length 175 mm, L75- lens with 75mm.
• Figure 2: Effects of noise on the performance of single-photon detectors.
• Figure 3: Single-detector inter-arrival time distributions at low and high count rates. Inter-arrival time distributions of adjacent photon detection events recorded from a single avalanche photodiode operated at different photon count rates. Panel (a) shows the distribution measured at a low count rate ($\approx$ 42 kcounts/s), while panel (b) shows the distribution measured at a high count rate ($\approx$ 18 Mcounts/s). The horizontal axis represents the time separation $\Delta t$ between successive detection events, and the vertical axis shows the number of events per time bin. Dashed vertical lines indicate the minimum detectable photon separation, corresponding to the effective detector dead time extracted from each dataset. At low count rates, the distribution is approximately uniform beyond the dead time, indicating that the detector fully recovers between detection events. At high count rates, a pronounced accumulation of events near the dead time threshold is observed, demonstrating rate-dependent detector recovery dynamics that lead to increased effective dead time and enhanced detection immediately after recovery.
• Figure 4: Effective dead time as a function of photon count rate.
• Figure 5: Typical experimental setup used to implement BBM92 QKD protocol. NLC - nonlinear crystal, PR - polarization rotator, PBS - polarizing beam splitter.