### Session D4: Spin Qubits in Quantum Dots

2:30 PM–5:30 PM, Monday, March 16, 2009
Room: 306/307

Chair: Jason Petta, Princeton University

Abstract ID: BAPS.2009.MAR.D4.5

### Abstract: D4.00005 : Dynamic nuclear polarization with single electron spins

4:54 PM–5:30 PM

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#### Author:

Jacob Taylor
(Massachusetts Institute of Technology)

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.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.MAR.D4.5