The implementation of solid-state spin qubits for quantum information processing requires a detailed understanding of the decoherence mechanisms. At low magnetic fields, when the magnitude of the Zeeman energy becomes comparable to intrinsic couplings, electron spins confined to quantum dots (QDs) have been shown to feature a characteristic decoherence signature with several distinct stages: The electron spin undergoes fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼2ns). At intermediate timescales (∼750ns) we identify an additional stage which corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath itself induced by quadrupolar coupling of nuclear spins to strain-driven electric field gradients. Finally, a slower process (>1μs) of irreversible relaxation of the spin polarization due to nuclear spin co-flips with the central spin causes the complete loss of coherence .
For hole spins, the mainly p-type Bloch function reduces the contact term of the hyperfine interaction due to the reduced wave function overlap with the nuclei. Consequently, at low magnetic fields we observe a more than two orders of magnitude slower dephasing compared to electrons. Time domain measurements of T2* show faster dephasing rates with increasing external magnetic field. We attribute this to electrical noise, which broadens the distribution of Zeeman frequencies. Strategies to counteract this noise source and measurements of T2 (via spin-echo) are discussed .
 A. Bechtold et al., Nature Physics 11, 1005–1008 (2015)
 T. Simmet et al., in preparation