Controlling electron and nuclear spins in double quantum dots*

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$_{2}^{\ast }$ of 10 ns, limited by the contact hyperfine interaction with the GaAs host nuclei [1]. 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 [2]. 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. \newline [1] J. R. Petta et al., Science \textbf{309}, 2180 (2005). \newline [2] J. R. Petta, J. M. Taylor \textit{et al.}, Phys. Rev. Lett. (in press).