Robust entangling gate for capacitively coupled few-electron singlet-triplet qubits

The search for a sweet spot, the locus in qubit parameters where quantum control is first-order insensitive to noises, is key to achieve high-fidelity quantum gates. Efforts to search for such a sweet spot in conventional double-quantum-dot singlet-triplet qubits where each dot hosts one electron ( "two-electron singlet-triplet qubit "), especially for two-qubit operations, have been unsuccessful. Here we consider singlet-triplet qubits allowing each dot to host more than one electron, with a total of four electrons in the double quantum dots ( "four-electron singlet-triplet qubit "). We theoretically demonstrate, using configuration interaction calculations, that sweet spots appear in this coupled qubit system. We further demonstrate that, under realistic charge noise and hyperfine noise, a two-qubit operation at the proposed sweet spot could offer gate fidelities (-99%) that are higher than the conventional two-electron singlet-triplet qubit system (-90%). Our results should facilitate realization of high-fidelity two-qubit gates in singlet-triplet qubit systems.