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.