Repeated radiation damage and thermal annealing of avalanche photodiodes.

Avalanche photodiodes (APDs) are well-suited for single-photon detection on quantum communication satellites as they are a mature technology with high detection efficiency without requiring cryogenic cooling. They are, however, prone to significantly increased thermal noise caused by in-orbit radiation damage. Previous work demonstrated that a one-time application of thermal annealing reduces radiation-damage-induced APD thermal noise. Here we examine the effect of cyclical proton irradiation and thermal annealing, emulating the realistic operating profile of a satellite in low-Earth-orbit over a two-year life span. We show that repeated thermal annealing is effective in maintaining thermal noise of silicon APDs within a range suitable for quantum key distribution throughout the nominal mission life, and beyond. We examine two strategies---annealing at a fixed period of time, and annealing only when the thermal noise exceeds a pre-defined limit---and find that the latter exhibits lower thermal noise at end-of-life for most samples. We also observe that afterpulsing probability of the detector increases with cumulative proton irradiation. This knowledge helps guide design and tasking decisions for future space-borne quantum communication applications.