QUROPE Quantum Information Processing and Communication in Europe

Objectives: Quantum communication is the art of transferring a quantum state from one location to another; in this way information, or resources such as entanglement, can be distributed among different locations. From an application point of view, a major interest has been focused on Quantum Key Distribution (QKD), as this offers a provably secure way to establish a confidential key between distributed partners. This has the potential to solve long-standing and central security issues in our information based society as well as emerging problems associated with long term secure storage (e.g. for health records and infrastructure) and will be critical for the secure operation of applications involving the Internet of Things (IoT) and cloud networking.

State of the art: In the last years the field has seen enormous progress, as QKD systems have gone from table-top experiments to compact and autonomous systems and now a growing commercial market. More generally there has been an explosion in the number of groups active in the field working on increasingly diverse physical systems. Quantum memories and interfaces have moved from theory to a wide range of proof-of-principle demonstrations with encouraging results for the future. Conceptually, the idea of device independent quantum information processing made its appearance and has already started to find experimentally feasible applications. While the realisation of basic quantum communication technologies is becoming more routine in the laboratory, non- trivial problems emerge in high-bit-rate systems and long-distance applications as we interface the different technologies and as the network complexity increases.

Future directions: One of the emerging areas of interest for quantum communication schemes is in connecting the nodes within quantum simulators, which can either be all located in the one lab, or more interestingly, in distributed scenarios - the tools from quantum communication playing the role of wiring circuits for these quantum computers. A particular application is a network of entangled clocks providing precise and secure world time reference. While there remain many challenges for proof-of-principle laboratory demonstrations, the transition to deployment in real-world environments defines a new set of challenges in the quantum information domain. The issues of scale, range, reliability, and robustness that are critical in this transition cannot be resolved by incremental improvements, but rather need to be addressed by making them the focal point of the research and technology development agenda as we work towards a quantum internet. To succeed, this needs to target the underlying technologies, ranging from fundamental aspects of engineering quantum systems to integrating quantum and classical, e.g. fast (classical) opto-electrical systems, as well as the end-user applications themselves.

In particular the following need to be addressed:

• Quantum networks, beyond point-to-point, exploring novel protocols, possibly hybrid (continuous-variable and discrete) systems. Quantum repeater concepts will also be critical in the context of computation and simulation, both for short distance scales (local) or large (distributed) processing systems.
• Deterministic and scalable technologies involving on-demand photonic sources, or heralded sources with quantum memories, including quantum memories with multimode capacity.
• Interfaces allowing for the coherent transduction of quantum states between different physical systems.
• The synchronisation and stabilisation of distributed quantum systems and their characterisation – in particular, their quantification with local measurements.
• Device-independent quantum information processing needs to be further investigated and ways to move from purely theoretical concepts to more practical scenarios will be highly relevant. In particular, addressing both QKD and quantum random number generation and providing a new perspective with the potential to also minimise security assumptions and hence simplify the security of real-world quantum communication.
• Systems that exploit increased complexity, e.g. using integrated quantum photonics, which would allow new functionality and protocols in quantum networking.