# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 1 Introduction  
The 5G networks are rapidly adopting various developing technologies to provide high-speed data delivery. The 5G technology is being used in a variety of applications such as smart city, Industrial Internet of Things (IIoT), and smart grid to meet the user needs and provide faster connection speed with high reliability and low latency for remote access to routine services. The 5G architecture intends to utilize various nodes to offer secure and on-demand connectivity to other devices. The 5 GPP mobile networks prefer to use high-frequency technology to strengthen many devices faster with fewer interruptions. With applications involving a phenomenally vast number of IoT devices and BSNs, improving the network's security at all layers becomes a major goal. When many devices are running simultaneously, it becomes difficult to detect the security threats and adversaries who can obscure the vast amounts of data transmitting across the network. Security enhancement is needed since larger spectrum space is accessible on 5G networks that can support heavy bits without reducing transmission range. Consequently, the security improvement demands an advanced set of protocols that consider the operation and architecture of 5G networks. Moreover, 5G communication systems are prone to security vulnerabilities, and various existing studies show the types of attacks that can significantly affect network performances. Furthermore, since these systems must operate instantly, the presence of contaminated codes, malware, wormhole, Trojans, and viruses in 5G devices can disrupt the entire network. Different technology enablers that support the 5G networks are software-defined networks (SDNs), network function virtualizations (NFV), multiple-input multiple-output (MIMO), non-orthogonal multiple access (NOMA), radio access networks (RAN), Massive-MIMO, device-to-device communications (D2D), simultaneous wireless information and power transfer (SWIPT), low-power wide-area networks (LPWAN), network slicing, orthogonal frequency division multiple access (ODFMA), etc. In the information security areas, cryptography is not considered a new field. One of the most classical concepts in cryptography is to combine a message with another piece of information when converting a bit of information. The cryptography techniques can be categorized as classical cryptography and quantum cryptography. The classical techniques can be divided into two categories: Symmetric and Asymmetric cryptographic, where the suitable categories rely on the required number of keys for encryption. However, both types are determined by the complexity of the mathematical factorization. With high technological computing such as quantum computing, any complex problems can be solved within a few minutes. Furthermore, there is a new cryptography category derived from quantum physics, which is called Quantum cryptography. As part of its physical characteristics, the researchers utilized it in cryptographic development to guarantee information security. Quantum cryptography is an upgrowing technology that uses the quantum mechanical system to process, secure, and transmit information instead of using a classical computational method. This method relies on mathematical algorithms that encrypt, secure communication, and transmit information between different parties using symmetric or asymmetric key distribution [1]. However, some advanced mathematical constraints and the massive technology growth are threatening the traditional cryptography protocols. Nowadays, as the intensity of software applications, websites and web browser is growing, it will introduce cybersecurity and cryptography implications [2]. However, when using QKD, the needs for communication and requirements of security collide physically. Furthermore, the engineering that is necessary to balance these challenges has a very narrow margin for error. As a result, rather than being guaranteed by physical laws, the security of QKD is largely implementation-dependent.