Decoupling a singlet-triplet qubit from charge noise

Qubits encoded in the spin states of electrons are promising candidates for the implementation of a quantum computer. However, they require the usage of high-frequency (GHz) equipment that is costly and introduces issues in scalability, particularly due to factors like crosstalk. Singlet-triplet qubits offer an alternative qubit encoding based on the states of two interacting spins, that come with characteristic energies of ~MHz and thus are addressable with cheaper instruments and simpler wiring. However, the reliance on exchange interaction makes them particularly sensitive to charge noise, limiting their coherence and gate fidelities. In this work, we introduce an optimal operation point for a singlet-triplet qubit in the presence of a magnetic field gradient. This proposed "sweet spot" takes advantage of the inherent anti-correlation between exchange interaction and the difference in Zeeman energies of the spins in a uniform magnetic gradient. At the sweet spot, the singlet-triplet qubit becomes first order insensitive to charge noise, while remaining addressable via electrical driving. This allows for high fidelity single-qubit operations and long coherence times, limited by hyperfine coupling to nuclear spins. Our findings offer a promising pathway for improving the performance and scalability of quantum processors based on singlet-triplet qubits.