Comparison of Discrete and Continuous Variable Quantum Key Distribution Protocols over a Thermal-Loss Channel

In a thermal-loss channel, it is uncertain whether a discrete-variable (DV) or a continuous-variable (CV) quantum key distribution (QKD) protocol is more optimal. In this paper, we investigate common DV-QKD and CV-QKD protocols, including the BB84 and squeezed-state protocols, in a thermal-loss setting but with the assumed availability of perfect sources and detectors. We find that in an intermediate-noise regime, the BB84 protocol attains positive key rates higher than any known CV protocol. On the other hand, the squeezed-state protocol can outperform the BB84 protocol in a high thermal noise regime. Our analysis addresses the question of which QKD platform and their respective protocols can perform optimally for different thermal-loss channel parameters assuming the protocols run perfectly.