# Quantum Key Distribution Networks: Challenges and Future Research Issues in Security
## 2 Background to Quantum Key Distribution (QKD) Networks
### Quantum Link  
A quantum link, in which a logical connection exists between two remote QKD nodes, comprises a quantum channel and an authenticated public classical channel (shown in Figure 2). The quantum channel is used for transmitting qubits, and the public channel implemented by the classical channel and classical cryptography technologies (e.g., universal hash function [49]) is used for post-processing the exchanged information of the QKD protocol (e.g., public discussion, error-correction, and privacy amplification processes).  
Two main methods can be employed to practically implement quantum channels: direct optical fibers and free line-of-sight in a point-to-point (P2P) manner. Although optical fibers can be applied and are common for transmitting qubits, installing dedicated fibers for executing QKD protocols is not practical under all situations. A free space link is sometimes convenient, although it has its drawbacks; for example, appropriate atmospheric conditions, a visible light path, an acceptable signal-to-noise ratio are required, and so on. It is worth noting that [50] proved the feasibility of transmitting the quantum and classical information over the shared optical fibers; that is, the quantum nodes can use the same optical fiber to transmit the classical and quantum information. From a deployment perspective, the technology could significantly reduce the installation costs for additional optical fiber links. In addition, different types of quantum channel correspond to different types of QKD protocol. In other words, discrete-variable-based QKD protocols are usually implemented in the fiber channel, whereas continuous variable-based QKD protocols are implemented in a free space link. The communication distance and key generation rate are the main considerations for implementing quantum links. Although the key generation rate decreases exponentially when the communication distance increases, which is known to all, improving both the performance of the communication distance and key generation rate is still an important research issue in the field of QKD networks. Generally, using fiber channels to implement a QKD network can achieve better performance in terms of both the communication distance and key generation rate than using a free space link; however, the costs (including those of an accurate single-photon detector and of establishing dedicated fibers) are still high. A previous study [51] achieved a breakthrough in the free space link using the free space link to accomplish a satellite-to-ground QKD protocol over a distance of 645 to 1200 km. In addition, [PERSON], [PERSON] et al. [52] proposed the twin-field QKD (TF-QKD) protocol to reach 550 km communication distance using current technology under the standard optical fiber. Some studies [53; 54; 55; 56; 57] have further demonstrated the performance of the TF-QKD protocol in an experimental manner. Therefore, future mainstream technologies still need to be evaluated and observed further.  
Figure 2: Schematic diagram of the quantum link.