  The nuclear spins in nanostructured semiconductors play a central role in
quantum applications. The nuclear spins represent a useful resource for
generating local magnetic fields but nuclear spin noise represents a major
source of dephasing for spin qubits. Controlling the nuclear spins enhances the
resource while suppressing the noise. NMR techniques are challenging: the group
III and V isotopes have large spins with widely different gyromagnetic ratios;
in strained material there are large atom-dependent quadrupole shifts; and
nanoscale NMR is hard to detect. We report NMR on 100,000 nuclear spins of a
quantum dot using chirped radiofrequency pulses. Following polarization, we
demonstrate a reversal of the nuclear spin. We can flip the nuclear spin back
and forth a hundred times.We demonstrate that chirped NMR is a powerful way of
determining the chemical composition, the initial nuclear spin temperatures and
quadrupole frequency distributions for all the main isotopes. The key
observation is a plateau in the NMR signal as a function of sweep rate: we
achieve inversion at the first quantum transition for all isotopes
simultaneously. These experiments represent a generic technique for
manipulating nanoscale inhomogeneous nuclear spin ensembles and open the way to
probe the coherence of such mesoscopic systems.
