Quantum key distribution (QKD) offers future proof security based on fundamental laws of physics. Long-distance QKD spanning regions such as the United Kingdom (UK) may employ a constellation of satellites. Small satellites, CubeSats in particular, in low Earth orbit are a relatively low-cost alternative to traditional, large platforms. They allow the deployment of a large number of spacecrafts, ensuring greater coverage and mitigating some of the risk associated with availability due to cloud cover. We present our mission analysis showing how a constellation comprising 15 low-cost 6U CubeSats can be used to form a secure communication backbone for ground-based and metropolitan networks across the UK. We have estimated the monthly key rates at 43 sites across the UK, incorporating local meteorological data, atmospheric channel modelling and orbital parameters. We have optimized the constellation topology for rapid revisit and thus low-latency key distribution.

We present a miniaturized quantum key distribution system, designed to augment the more mature quantum key distribution systems currently commercially available. Our device is designed for the consumer market, and so size, weight and power are more important than raw performance. To achieve our form factor, the transmitter is handheld and the receiver is a larger fixed terminal. We envisage users would bring their transmitters to centrally located receivers and exchange keys which they could use at a later point. Transmitting qubits at 80 MHz, the peak key rate is in excess of 20 kbps. The transmitter device fits within an envelope of <150 ml, weighs 65 g and consumes 3.15 W of power.

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.