  Singlet-triplet qubits in lateral quantum dots in semiconductor
heterostructures exhibit high-fidelity single-qubit gates via exchange
interactions and magnetic field gradients. High-fidelity two-qubit entangling
gates are challenging to generate since weak interqubit interactions result in
slow gates that accumulate error in the presence of noise. However, the
interqubit electrostatic interaction also produces a shift in the local double
well detunings, effectively changing the dependence of exchange on the gate
voltages. We consider an operating point where the effective exchange is first
order insensitive to charge fluctuations while maintaining nonzero
interactions. This "sweet spot" exists only in the presence of interactions. We
show that working at the interacting sweet spot can directly produce maximally
entangling gates and we simulate the gate evolution under realistic 1/f noise.
We report theoretical two-qubit gate fidelities above 99% in GaAs and Si
systems.
