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

Understanding and control of the spin relaxation time T₁ is among the key challenges for spin-based qubits. A larger T₁ 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 in-plane magnetic fields B, the spin relaxation relies on phonon emission and spin-orbit coupling. The characteristic dependence T₁ ∝ 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 T₁ ∝ B^-3. Here, we establish these predictions experimentally, by measuring T₁ over an unprecedented range of magnetic fields-made possible by lower temperature-and report a maximum T₁ = 57 ± 15 s at the lowest fields, setting a record electron spin lifetime in a nanostructure.