The low temperature diffusivity of nanoscale crystal defects, where quantum mechanical fluctuations are known to play a crucial role, are essential to interpret measurements of irradiated microstructures conducted at cryogenic temperatures. Using density functional theory calculations, quantum heat bath molecular dynamics and open quantum systems theory, we provide a first evaluation of the low temperature diffusivity of self interstitial atom clusters in tungsten. Due to an exceptionally low defect migration barrier, our results show that interstitial defects exhibit very high diffusivity of order 103 m2s-1 down to temperatures of 1K.