  Coherent two-level systems, or qubits, based on electron spins in GaAs
quantum dots are strongly coupled to the nuclear spins of the host lattice via
the hyperfine interaction. Realizing nuclear spin control would likely improve
electron spin coherence and potentially enable the nuclear environment to be
harnessed for the long-term storage of quantum information. Toward this goal,
we report experimental control of the relaxation of nuclear spin polarization
in a gate-defined two-electron GaAs double quantum dot. A cyclic gate-pulse
sequence transfers the spin of an electron pair to the host nuclear system,
establishing a local nuclear polarization that relaxes on a time scale of
seconds. We find nuclear relaxation depends on magnetic field and
gate-controlled two-electron exchange, consistent with a model of electron
mediated nuclear spin diffusion.
