  The control of discrete quantum states in solids and their use for quantum
information processing is complicated by the lack of a detailed understanding
of the mechanisms responsible for qubit decoherences. For spin qubits in
semiconductor quantum dots, phenomenological models of decoherence currently
recognize two Basic stages; fast ensemble dephasing due to the coherent
precession of spin qubits around nearly static but randomly distributed
hyperfine fields and a much slower process of irreversible relaxation of spin
qubit polarization due to dynamics of the nuclear spin bath induced by complex
many-body interaction effects. We unambiguosly demonstrate that such a view on
decoherence is greatly oversimplified; the relaxation of a spin qubit state is
determined by three rather than two basic stages. The additional stage
corresponds to the effect of coherent dephasing processes that occur in the
nuclear spin bath that manifests itself by a relatively fast but incomplete
non-monotonous relaxation of the central spin polarization at intermediate
timescales. This observation changes our understanding of the electron spin
qubit decoherence mechanisms in solid state systems.
