Microscopic theory of a magnetic-field-tuned sweet spot of exchange interactions in multielectron quantum-dot systems

The exchange interaction in a singlet-triplet qubit defined by two-electron states the double-quantum-dot system ("two-electron qubit") typically varies monotonically with and thus carries no sweet spot. Here we study four-electron ("four-electron qubit"). We demonstrate, using configuration-interaction calculations, that energy as function of detuning can be non-monotonic, suggesting existence spots. further show tuning spot corresponding perpendicular magnetic field related to variation orbital splitting. Our results suggest more than two electrons have advantages realization quantum computing.