Finite resource performance of small satellite-based quantum key distribution missions

In satellite-based quantum key distribution (QKD), the number of secret bitsthat can be generated in a single satellite pass over the ground station isseverely restricted by the pass duration and the free-space optical channelloss. High channel loss may decrease the signal-to-noise ratio due tobackground noise, reduce the number of generated raw key bits, and increase thequantum bit error rate (QBER), all of which have detrimental effects on theoutput secret key length. Under finite-size security analysis, higher QBERincreases the minimum raw key length necessary for non-zero secret key lengthextraction due to less efficient reconciliation and post-processing overheads.We show that recent developments in finite key analysis allow three differentsmall-satellite-based QKD projects CQT-Sat, UK-QUARC-ROKS, and QEYSSat toproduce secret keys even under very high loss conditions, improving onestimates based on previous finite key bounds. This suggests that satellites inlow Earth orbit can satisfy finite-size security requirements, but remainschallenging for satellites further from Earth. We analyse the performance ofeach mission to provide an informed route toward improving the performance ofsmall-satellite QKD missions. We highlight the short and long-term perspectiveson the challenges and potential future developments in small-satellite-basedQKD and quantum networks. In particular, we discuss some of the experimentaland theoretical bottlenecks, and improvements necessary to achieve QKD andwider quantum networking capabilities in daylight and at different altitudes.