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