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. We present measurements of the spin relaxation rate W in a gate defined single-electron GaAs quantum dot as a function of direction and strength of magnetic field, spanning an unprecedented range from 0.6 T to 14 T applied in the plane of the 2DEG. At high fields, the spin relaxation relies on phonon emission and spin-orbit (SO) leading to a characteristic dependence W~B⁵ and a pronounced B-field anisotropy, due to the interplay of the Rashba and Dresselhaus SO contributions. Along the axis with weak SO, a T₁ of 57 ± 15 sec is obtained at 0.6 T - setting a new record for the spin lifetime in a nanostructure. Surprisingly, this is more than one order of magnitude shorter than the expected value based on SO mediated spin relaxation. Also, W shows a B³ dependence and becomes isotropic at low magnetic fields. These observations indicate hyperfine interaction mediated spin relaxation via phonons in the low field regime as predicted already 15 years ago. In this process the HF replaces the SO interaction as the source of admixture of the spin and orbital degree of freedom.