Three-stage decoherence dynamics of an electron spin qubit in an optically active quantum dot

The mechanisms of decoherence in solid-state spin qubits subject to low magnetic fields turn out to be more complex than previously expected as an additional fast relaxation stage has now been identified. The control of solid-state qubits requires a detailed understanding of the decoherence mechanisms. Despite considerable progress in uncovering the qubit dynamics in strong magnetic fields 1 , 2 , 3 , 4 , decoherence at very low magnetic fields remains puzzling, and the role of quadrupole coupling of nuclear spins is poorly understood. For spin qubits in semiconductor quantum dots, phenomenological models of decoherence include two basic types of spin relaxation 5 , 6 , 7 : fast dephasing due to static but randomly distributed hyperfine fields (∼2 ns) 8 , 9 , 10 , 11 and a much slower process (>1 μs) of irreversible monotonic relaxation due either to nuclear spin co-flips or other complex many-body interaction effects 12 . Here we show that this is an oversimplification; the spin qubit relaxation is determined by three rather than two distinct stages. The additional stage corresponds to the effect of coherent precession processes that occur in the nuclear spin bath itself, leading to a relatively fast but incomplete non-monotonic relaxation at intermediate timescales (∼750 ns).