  Charge noise is the main hurdle preventing high-fidelity operation, in
particular that of two-qubit gates, of semiconductor-quantum-dot-based spin
qubits. While certain sweet spots where charge noise is substantially
suppressed have been demonstrated in several types of spin qubits, the
existence of one for coupled singlet-triplet qubits is unclear. We
theoretically demonstrate, using full configuration-interaction calculations,
that a range of nearly sweet spots appear in the coupled singlet-triplet qubit
system when a strong enough magnetic field is applied externally. We further
demonstrate that ramping to and from the judiciously chosen nearly sweet spot
using sequences based on the shortcut to adiabaticity offers maximal gate
fidelities under charge noise and phonon-induced decoherence. These results
should facilitate realization of high-fidelity two-qubit gates in
singlet-triplet qubit systems.
