## 7 Conclusions  
The AirQKD project was ambitious, aiming to encourage UK organisations to increase their capabilities in quantum communications, from single-photon components to networked systems secured with QKD. The race for the next 6G technologies is underway and nations need to work in the areas of quantum- and photonic-based technologies for future economic benefits. The AirQKD project has contributed to the growth of the UK's quantum technology landscape. This work primarily covered the part of the project that completed a connected vehicle pilot demonstration of quantum-secured V2I communication, i.e., a practical implementation of the QKD security developed within the project. We acknowledge the limitations of the presented results due to the scope and size of the project. We provided an overview of QKD, the AirQKD project, and V2I technology in Section 3 to provide the reader with a foundation for the test design. Section 4 describes the AirQKD V2I FSO-QKD system and experimental design. The design incorporates various aspects of the unique UK-designed technologies and IP, developed by the multiple project partners listed in Table 1, including:  
* A new single photon source and detector. * A new FSO system and its control hardware to enable FSO-QKD. * A new QRNG as a source of randomness for the FSO-QKD system. * A new QKD system control and KMM software. * A patented cryptographic key amplification protocol. * A new hardware security module based on a PUF. * A zero-trust data encryption protocol, its verification, and software implementation. * Using the AirQKD-developed technology in a macro V2I application. The organisations involved in the AirQKD project had to improve their skills and abilities to deliver the above-mentioned IP and technology. Therefore, the aim of AirQKD in regard to growing the UK quantum communication industry has been met. The results from the execution of the experiment, presented in Section 5, confirmed that a QKD system generating symmetric keys for a zero-trust security scheme can be functional for V2I applications. This could remove the need for a certificate-based PKI security system; PKI schemes can be considered as possessing lower overall security because of some of theknown PKI issues around certificate management and encryption/decryption performance (as described in the introduction; see Section 1). Advancing the subsequent work from this project will bolster the future 6G communications needed to support our increasingly hyperconnected society. Future endeavours will continue on the path toward commercialising AirQKD-developed technologies. Further, it is important to examine the boundaries of the performance of zero-trust protocols for IoT applications, including their use in the C-V2X systems that are now being tested. This is linked to the need to examine other security and performance aspects of such QKD-supported zero-trust implementations, including maintaining the security of the entire system as a whole and analysing the performance of symmetric key security systems for V2I, for example, the impact on V2I message processing and transmission rates. The AirQKD project has demonstrated that quantum-generated symmetric keys could be the building blocks for future secure 6G-connected systems that implement the zero-trust principle. This work provides an example of applying zero-trust symmetric key security in the realm of IoT applications, which are increasingly appearing in our increasingly evolving hyperconnected society. Such applications may not be able to rely on the long-established PKI-based security due to management and performance constraints. Project administration, validation, writing--original draft preparation, writing--review and editing, D.S.F. ; conceptualization, funding acquisition, methodology, writing--review and editing, and supervision, C.M. ; writing--review and editing and supervision, G.E. All authors have read and agreed to the published version of the manuscript. This research was funded by Innovate UK, project AirQKD, grant number 45364.