  We measure the dynamics of nuclear spins in a self-assembled quantum dot at a
magnetic field of 5 Tesla and identify two distinct mechanisms responsible for
the decay of the Overhauser field. We attribute a temperature-independent decay
which lasts ~100 seconds to intra-dot diffusion induced by hyperfine-mediated
indirect nuclear spin interaction. In addition, we observe a gate-voltage and
temperature dependent decay stemming from co-tunneling mediated nuclear spin
flip processes. By adjusting the gate-voltage and lowering the electron
temperature to ~200 milliKelvin, we prolong the corresponding decay time to ~30
hours. Our measurements indicate possibilities for exploring quantum dynamics
of the central spin model using a single self-assembled quantum dot.
