Brendon Higgins

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

Orbiting satellites can be utilized to go beyond distance limitations of ground-based QKD. We discuss various efforts underway to develop and demonstrate the readiness of technologies that would comprise a satellite acting as a trusted-node receiver. We perform thorough link analysis to optimize the configuration of our apparatus, assess the requirements of optical polarization frame alignment protocol, and demonstrate QKD in a high-loss regime (up to 57 dB). We discuss ongoing work towards demonstrations of QKD with receiver on a moving platform, as well as potential applications of high-loss QKD for convenient short-distance QKD by transmission using polarization-preserving diffusive screens.