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We summarise the discussions at a virtual Community Workshop on Cold Atoms in Space concerning the status of cold atom technologies, the prospective scientific and societal opportunities offered by their deployment in space, and the developments needed before cold atoms could be operated in space. The cold atom technologies discussed include atomic clocks, quantum gravimeters and accelerometers, and atom interferometers. Prospective applications include metrology, geodesy and measurement of terrestrial mass change due to, e.g., climate change, and fundamental science experiments such as tests of the equivalence principle, searches for dark matter, measurements of gravitational waves and tests of quantum mechanics. We review the current status of cold atom technologies and outline the requirements for their space qualification, including the development paths and the corresponding technical milestones, and identifying possible pathfinder missions to pave the way for missions to exploit the full potential of cold atoms in space. Finally, we present a first draft of a possible road-map for achieving these goals, that we propose for discussion by the interested cold atom, Earth Observation, fundamental physics and other prospective scientific user communities, together with the European Space Agency (ESA) and national space and research funding agencies.

Satellite‐based, long‐distance free‐space quantum key distribution has the potential to realise global quantum secure communication networks. Detecting faint quantum optical pulses sent from space requires highly accurate and robust classical timing systems to pick out signals from the noise and allow for reconciliation of sent and received key bits. For such high‐loss applications, a fault‐tolerant synchronisation signal coding and decoding scheme based on de Bruijn sequences is proposed. A representative synchronisation timing system was tested in laboratory conditions and it demonstrated high fault tolerance for the error‐correction algorithm even under high loss. The performance limitations of this solution are also discussed, and the maximum error tolerance of the scheme and the estimated computational overhead are analysed, allowing for the possibility of implementation on a real‐time system‐on‐chip. This solution not only can be used for synchronisation of high‐loss channels such as channels between satellites and ground stations but can also be extended to applications with low loss, high bit error rate, but require reliable synchronisation such as quantum and non‐quantum communications over terrestrial free space or fibre optic channels.

Terrestrial free-space quantum key distribution is ideally suited for deployment in dense urban environments. The transition from laboratory to commercial deployment, however, raises a number of important engineering and deployment issues. Here, we investigate these issues for efficient BB84 using a weak coherent pulse-decoy state protocol. We calculate expected key lengths for different environmental conditions and when the scope for optimisation of protocol parameters is restricted due to practical considerations. In particular, we find that for a fixed receiver basis choice probability, it can be advantageous to allow the transmitter to have a different basis choice probability depending on varying channel loss and background light levels. Finally, we examine the effects of pulse intensity uncertainty finding that they can dramatically reduce the key length. These results can be used to determine the loss budget for the free-space optics of a QKD systems and assist in their design.

Quantum networking on a global scale is an immensely challenging endeavor that is fraught with significant technical and scientific obstacles. While various types of quantum repeaters have been proposed they are typically limited to distances of a few thousand kilometers or require extensive hardware overhead. Recent proposals suggest that space-borne quantum repeaters composed of a small number of satellites carrying on-board quantum memories would be able to cover truly global distances. In this paper, we propose an alternative to such repeater constellations using an ultra-long lived quantum memory in combination with a second memory with a shorter storage time. This combination effectively acts as a time-delayed version of a single quantum repeater node. We investigate the attainable finite key rates and demonstrate an improvement of at least three orders of magnitude over prior single-satellite methods that rely on a single memory, while simultaneously reducing the necessary memory capacity by the same amount. We conclude by suggesting an experimental platform to realize this scheme.