Theoretical and Practical Aspects of Communication in Quantum Networks

The development of large scale quantum networks marks a logical next step in the evolving field of quantum technologies. By connecting multiple quantum devices, quantum networks enable distributed quantum information processing and large scale secure communication, safe against any attack possible within the laws of quantum physics. However, point-to-point quantum communications are inherently distance limited. It is therefore vital to maximise the distance scaling of quantum network architectures and to make efficient and scalable use of network resources. This thesis presents a number of approaches to these problems across various areas of quantum networking. We introduce a multi-party measurement device independent (MDI) quantum key distribution (QKD) protocol, boosting the distance at which multiple users can securely communicate in the MDI setting. We calculate the rate of the protocol under a collective pure-loss attack, demonstrating improved rate-distance performance compared to the equivalent non-post-selected protocol. We also consider the near-term solution of using trusted nodes in a QKD network and we demonstrate how the use of a flooding protocol may improve the performance and security of such networks. In particular we show that such protocols may be used to increase the end-to-end key rate between two users even in the case they are not directly connected. Additionally we demonstrate how such a protocol can reduce the downside of trusted nodes by increasing the security of end-to-end communication against corrupted network nodes. Finally, we characterise the end-to-end performance of free-space quantum networks operating through free-space. We thus illustrate fundamental limits to the performances of such networks and further demonstrate key principles in the optimal design of assistive backbone structures to improve the performance of free-space quantum networks.