Dynamic nuclear polarization with single electron spins*

Hyperfine interactions limit electron spin coherence times in GaAs quantum dots. By separating a spin singlet state on a chip, we measure an ensemble averaged spin dephasing time $T_2^*$ of 10 ns, limited by the contact hyperfine interaction with the GaAs host nuclei\footnote{J. R. Petta \textit{et al.}, Science $\bf{309}$, 2180 (2005).}. We use electrical control of the exchange interaction to drive coherent spin rotations. Exchange driven spin rotations are used to implement a ``singlet-triplet spin echo'' pulse sequence, which leads to a spin coherence time, $T_2$, exceeding 1 microsecond. We show that nuclear spins can be polarized by controlling two-electron spin states near the anti-crossing of the singlet (S) and triplet ($T_+$). An initialized S state is cyclically brought into resonance with the $T_+$ state, where hyperfine fields drive rapid rotations between S and $T_+$, ``flipping'' an electron spin and ``flopping'' a nuclear spin\footnote{J. R. Petta, J. M. Taylor \textit{et al.}, Phys. Rev. Lett. $\bf{100}$, 067601 (2008).}. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage.