Quantum dynamics within solids is usually restricted to low mass particles such as electrons and muons, or single atoms of light elements such as hydrogen. In this letter we report observation of the quantum-assisted motion of self-interstitial atom clusters in tungsten (each of mass 184 Da), travelling distances of several nanometres between trapping points associated with impurity atoms or vacancies. This is the first direct experimental observation of quantum diffusion at low temperatures in a high voltage electron microscope (HVEM). The underlying reasons behind the process are the non-equilibrium effects of electron irradiation, which we exploit to reduce the effective trap depth by several orders of magnitude, and the emergence of the clusters as delocalized quasiparticles that, once escaped, can move almost unimpeded through the crystal. The stochastic cluster motion is driven by quantized atomic vibrations, and involves the collective dynamics of more than the 100 or so atoms forming each cluster. This coherent behaviour leads to low temperature diffusion rates orders of magnitude higher than a naive classical estimate suggests. Our results demonstrate the importance of quantum effects on low temperature defect evolution even in heavy atom systems, and underline the invalidity of the standard Arrhenius form for diffusion rates, under these technologically important conditions. Our analysis shows that this phenomenon is generic to any crystal, and can hence affect low temperature defect transport in almost any material.