Control and Detection of Singlet-Triplet Mixing in a Random Nuclear   Field
  We observe mixing between two-electron singlet and triplet states in a double quantum dot, caused by interactions with nuclear spins in the host semiconductor. This mixing is suppressed by applying a small magnetic field, or by increasing the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarizes the nuclei, resulting in striking bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T2* of 25 ns. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins. 
F.H.L. Koppens, J.A. Folk, J.M. Elzerman, R.Hanson, L.H. Willems van   Beveren, I.T. Vink, H.P. Tranitz, W. Wegscheider, L.P...