Practical aspects of terahertz wireless quantum key distribution in indoor environments

Abstract

We discuss the feasibility of continuous-variable wireless quantum key distribution (WQKD) with thermal Gaussian states in terahertz (THz) band. We rigorously analyze the secret key rate of practical WQKD system using direct reconciliation and homodyne detection against collective Gaussian attacks. The results show that in the case of low-gain antenna, the free space loss is the dominant limiting factor of secure transmission distance of THz WQKD system. When the antenna diameter increases from 1 to 5 cm, we can get the maximum security distance and its corresponding optimal frequency in the range of 0.1–1 THz. We also obtain that a security distance of 1.95 m can be achieved when using a 5-cm diameter antenna and controlling the excess noise below 0.25 simultaneously. Specifically, we study the finite size effects and show that the transmission distance can reach 78 cm with a 5-cm diameter antenna at the frequency of 300 GHz. This work takes an important step toward short distance wireless QKD system.