  The performance of multi-electron spin qubits is examined by comparing
exchange oscillations in coupled single-electron and multi-electron quantum
dots in the same device. Fast (> 1 GHz) exchange oscillations with a quality
factor Q > 15 are found for the multi-electron case, compared to Q ~ 2 for the
single-electron case, the latter consistent with previous experiments. A model
of dephasing that includes voltage and hyperfine noise is developed that is in
good agreement with both single- and multi-electron data, though in both cases
additional exchange-independent dephasing is needed to obtain quantitative
agreement across a broad parameter range.
