Control and Detection of Singlet-Triplet Mixing in a Random Nuclear Field
F. H. L. Koppens,1*
J. A. Folk,1*
J. M. Elzerman,1
R. Hanson,1
L. H. Willems van Beveren,1
I. T. Vink,1
H. P. Tranitz,2
W. Wegscheider,2
L. P. Kouwenhoven,1
L. M. K. Vandersypen1
We observed 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 was suppressed when we applied a small magnetic field or increased the interdot tunnel coupling and thereby the singlet-triplet splitting. Electron transport involving transitions between triplets and singlets in turn polarized the nuclei, resulting in marked bistabilities. We extract from the fluctuating nuclear field a limitation on the time-averaged spin coherence time T 
of 25 nanoseconds. Control of the electron-nuclear interaction will therefore be crucial for the coherent manipulation of individual electron spins.
1 Kavli Institute of Nanoscience, Delft University of Technology, Post Office Box 5046, 2600 GA Delft, Netherlands.
2 Institut für Angewandte und Experimentelle Physik, Universität Regensburg, Regensburg, Germany.
* These authors contributed equally to this work.
To whom correspondence should be addressed. E-mail: lieven{at}qt.tn.tudelft.nl
