Quantum network applications in theory and practice

Theorists and experimenters around the world are working towards the development of quantum networks and ultimately a quantum internet, which would link quantum computers at nodes by reliable quantum communication channels, effectively creating one large quantum computer. This is several steps away from what we presently have: small localized programmable quantum computers together with (but not linked to) long-distance secure quantum key distribution fibre optic and free space links. There are some clear applications of quantum networks for computing and cryptography. They would allow quantum communication links between any pair of network points, whose security can be guaranteed without needing to trust the intermediate nodes. They would also allow users to run quantum programs on remote large quantum computers made available by service providers. Many other applications, including potentially nearer-term ones, are being proposed, but key questions are yet unresolved. We do not generally know the most efficient ways of solving even basic problems in network communication and programming: for example, how to use a quantum network capacity efficiently, how quickly quantum states can be routed in response to incoming data, or how best to use quantum entanglement and other resources to implement given cryptographic or computing tasks. Nor is it known how some interesting proposed applications, such as various schemes for quantum money (secure quantum tokens) and quantum position verification, can be best and most efficiently implemented. These questions raise fundamental and interesting problems that can be addressed by quantum information theory (developing quantum algorithms and quantum communications protocols and suitably secure quantum cryptographic schemes). But they also involve important questions of quantum technology and engineering. For any nearer-term applications, it is crucial to understand in detail what can realistically be achieved with current and foreseeable technology. Also, it is crucial to understand current and foreseeable costs of various types of resources – for example, comparing the cost of generating different types of quantum entanglement, or of creating quantum channels with different fidelities – to understand trade-offs and hence decide what real-world solutions are or may foreseeably be most efficient. A key part of this is comparing alternative implementations that do not require fully quantum networks, and so can be implemented sooner, albeit perhaps with trade-offs. It is crucial to understand all these issues fully to guide the development of quantum networks and to characterise the applications for which they offer truly decisive advantages. Our team includes theorists who have invented and developed several of the relevant quantum computing algorithms and cryptographic protocols, together with Thomas Jennewein, who has led the development of QEYSSat, a Canadian quantum network linking ground stations and satellites, and the roadmap for QEYSSat2.0, a proposal for a future quantum internet in space. We will collaborate closely to analyse the feasibility of network applications, and study their performance via simulations, focusing particularly on networks with satellite nodes. This will allow resource cost- benefit models based firmly on current and foreseeable quantum technologies. We aim to identify novel valuable applications of quantum networks and a future quantum internet, while also investigating and comparing possible alternative hybrid network implementations that require fewer quantum resources, and thus to characterise much better their societal value and help guide their development path. Our work will also contribute significantly to fundamental research on quantum information and quantum communication theory.