Optimising Quantum Networks

Quantum communication offers a solution to the threat of quantum computers on cryptographic security and the ability to share quantum information and entanglement on a global scale. The construction of quantum networks and an overarching quantum internet will provide the infrastructure needed to facilitate worldwide quantum communication and enable groundbreaking advances in science, technology and beyond. However, quantum networks face challenges that classical networks conveniently avoid. The laws of quantum mechanics impose a fundamental rate-loss tradeoff which critically limits the ability to achieve high rates over long distances. This introduces an intrinsic difference between classical and quantum communication networks with effects that ripple throughout every facet of network design, implementation and utilisation. In this thesis, we devise new ways to characterise the performance of large-scale quantum networks. Combining expertise from quantum information theory, graph theory and network theory we derive analytical methods with which to inspect the fundamental limits of large-scale quantum networks. We do this through the introduction of an analytic network architecture which exhibits desirable features within high-rate, well-connected topologies. These techniques (and variants thereof) are then used in a multitude of contexts: to compare the limits and resource demands of quantum fibre networks and satellite-based quantum repeaters, to benchmark end-to-end network capacity bounds, and to access valuable benchmarks for free-space quantum networking. Motivated by insight from analytical architectures, we inspect the practicality and criticality of realistic quantum networking. We investigate random quantum network architectures and practical end-to-end routing protocols in order to understand the trade-off between network connectivity, resource consumption and performance guarantees. In doing so, we build new and efficient multi-path routing strategies. These analyses are then extended into the multi-user setting, enlightening properties of a reliable quantum internet that can support many communicators.