Dephasing of Si singlet-triplet qubits due to charge and spin defects

We study the effect of charge and spin noise on singlet-triplet qubits in Si quantum dots. We set up a theoretical framework aimed at enabling experiment to efficiently identify the most deleterious defects, and complement it with the knowledge of defects gained in decades of industrial and academic work. We relate the dephasing rates $\Gamma_\phi$ due to various classes of defects to experimentally measurable parameters such as charge dipole moment, spin dipole moment and fluctuator switching times. We find that charge fluctuators are more efficient in causing dephasing than spin fluctuators.