# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 1 Introduction
Currently, sector such as governments, military and healthcare are not implementing QKD solutions in their applications due to these limitations. Consequently, QKD techniques must be implemented in conjunction with cryptographic authentication systems to prevent any security regression. To use quantum cryptography efficiently, researchers proposed a Quantum Key Distributed (QKD), a fundamental technology based on the law of physics that ensures a secure exchange of symmetric encryption keys. Even though the concept of QKD was proposed in early 1980 [3], it took many years to be accepted and implemented. Since then, the progress in QKD shows remarkable improvement. The concept of Quantum Key Distribution is based on the principles of key distribution. The theory is that the sender and the receiver share the public key using a channel to ensure that the public key is not tempered. Consequently, the most crucial part of the encryption process is the key distribution. QKD is based on transmitting secure quantum cryptographic keys between different parties [4]. The main concept of communication in QKD relies on enabling two parties to share random keys based on their private polarization angles, which are used as an encryption and decryption mechanism, as mentioned in Figure 1. A sender will send a stream of photon-based on a private polarization pattern which the receiver will receive a stream of photons and then passing it through his private polarization. After that, the sender and receiver will establish a secure classical communication in which they will compare all the photons, keeping the matched photons and destroying the rest. The matched photons are considered thoroughly accurate and safe. Therefore, QKD is regarded as a secure key for data distribution between parties. Using the same session output, the secure key can establish future secure sessions between the same parties. Moreover, QKD can provide future security proof, which means that even if the cryptographic system crashed for any reason, the prior data communications and session remains secure. The 5 th generation or 5G network is launched in 2019 to enhance and overcome some of the limitations in the previous network generations such as enhances broadband mobile services and wireless access services. It works to improve mobility by providing great capacity and faster data speed that offers more excellent connectivity for people on the move. 5G promises to lower the communication latency, opens the possibility of real-time control of devices, industrial robotics, vehicle to vehicle communications and safety systems, autonomous driving, and safer transport networks. Those new featurescontributed to the possibility to connect more devices than ever before which ensure the opportunity of massive machine-to-machine communications without human intervention. 5G also will be mainly involved in manufacturing, industrial, agricultural revolutions. 5G is expected to bring a drastic change in recent technology. As a result, several reviews and research were conducted in 5G [5; 6; 7]. Although security was one of the main requirements in 5G, however, limited research was proposed in that domain such as [8; 9; 10] are some of few a survey that emphasizes on the security challenges and communications security of 5G. In the 5G security model, the main security domain such as confidentiality, integrity and availability are ensured [11]. Therefore, it is critical to emphasize the security challenges that exist in the 5G communication systems. This paper focuses our research on the 5G security challenges that need immediate security measures. We further examined the quantum key distribution (QKD) as the potential security solution for the threats illustrate in this paper. The main contributions of this paper are:  
* We discuss the challenges in 5G networks in terms of privacy and security. * We provide classification of QKD for 5G networks into three types of categories, including, single photon, multiphoton, and entanglement approach.