Hyperfine-phonon spin relaxation in a single-electron GaAs quantum dot

Understanding and control of the spin relaxation time T-1 is among the key challenges for spinbased qubits. A larger T-1 is generally favored, setting the fundamental upper limit to the qubit coherence and spin readout fidelity. In GaAs quantum dots at low temperatures and high inplane magnetic fields B, the spin relaxation relies on phonon emission and spin-orbit coupling. The characteristic dependence T-1 alpha B-5 and pronounced B-field anisotropy were already confirmed experimentally. However, it has also been predicted 15 years ago that at low enough fields, the spin-orbit interaction is replaced by the coupling to the nuclear spins, where the relaxation becomes isotropic, and the scaling changes to T-1 alpha B-3. Here, we establish these predictions experimentally, by measuring T-1 over an unprecedented range of magnetic fields-made possible by lower temperature-and report a maximum T-1 = 57 +/- 15 s at the lowest fields, setting a record electron spin lifetime in a nanostructure.