# Quantum Key Distribution for 5G Networks: A Review, State of Art and Future Directions
## 5 Conclusions and Future Works  
As for the future directions, quantum security can be the solution that connects cryptography and network security areas. The existing signature schemes and public-key encryption could not guarantee secure connection once the adversaries are equipped with quantum level powers. The existing signature schemes and public-key encryption cannot provide a secure communication channel once the adversaries are equipped with quantum level powers. Without limiting any abilities of eavesdroppers, quantum cryptography security could be proven mathematically. Quantum cryptography techniques will generate and distribute longer symmetric keys, ensuring the security of many IoT devices over time. This approach will also reduce the network overhead and extend the device battery life by lowering the handshake frequency in the key establishment process. Moreover, quantum security can also be utilized in device authentication, power-efficient algorithms, certification, qualification, securing identity, and applying policies. Furthermore, MEC capabilities are used to implement the key management entities at the network edge and reduce the communication overhead related to security in the 5G backhaul network. This technique can also be used in other crucial areas such as smart grid, smart city, and IoT applications. For example, authors in [161] introduced integration software that combine IoT communication protocol to increase the security and resiliency of electricity grids. The proposed framework reduced overhead and allows 5G and IoT to be integrated. Quantum cryptography has garnered a lot of attention from different organizations both industrial and academic communities. Over the last few years, significant progress in the quantum cryptography and development of optical equipment is shown by numerous successful research studies and testing of QKD technology. This research and testing produce significant results in quantum cryptography and underline the future works that need to be addressed and resolved. This paper reviews and discusses the existing quantum cryptography protocol that has been identified in the literature to be implemented in the 5G networks. Trusted repeaters are required to extend the distance of secure transmission for quantum channels. Currently, the hot topic in optical research is to integrate the QKD networks into conventional telecommunication networks. The future works or the next breakthrough will be on employing quantum cryptography into the real-world information transfer applications. Since 5G is at the core of the future's heterogeneous network, QKD can also be implemented in the backbone network for a secure connection. Implementing QKD in the 5G backbone network will be the focus of our future works. Currently, the maximum key rate is correlated with the distance of the QKD links. One of the limitations of QKD links is length constraints. However, the development in optical equipment has significantly improved as discussed in this paper. In 2003, the DARPA QKD network achieved a key rate of 1 kbps. In 2007, the key rate in SECOQC or European QKD network are increased to 3 kbps and followed by the Tokyo QKD network in 2011 where the key rate is 300 kbps. This achieved key rate is adequate to initiate a secure video conferencing along with OTP cipher established by QKD. In terms of the correlating distance, the DARPA QKD networks have a connection distance of 29 km between Boston and Harvard Universities using the optical switch. For SECOQC network, the QKD link was between St. Polten and BREIT for maximum distance of 82 km while for the Tokyo QKD network, the connection was between the Koganei-1 and Koganei-2 nodes with maximum distance of 90 km. Meanwhile, real-life application using QKD network that has been demonstrated in Hefei has the maximum distance of 85.1 km using the Hefei-Chaohu-Wuhu (HCW) intercity link. Consequently, there is an expectation for a longer distance and a higher key rate in the upcoming years.