Mitigating charge noise in exchange gate operations between spin qubits

Spin qubits in semiconductor quantum dots (QDs) are one of the leading candidates for implementation of quantum computing devices, mainly due to their long coherence times, scalability, and compatibility with existing semiconductor technology. Exchange interactions between spins provide a fast and efficient way of controlling spin-spin interactions and two-qubit gate operations. While electrical control of the spin degree of freedom is promising, the exchange interaction reintroduces charge noise to the spin system.
Recent experiments showed that optimizing the control of exchange interaction can significantly improve the coherence of the operations; they make use of the symmetric operation, which directly controls the tunnel barrier at a sweet spot where the exchange interaction is insensitive to fluctuations in the detuning between QD energy levels up to the first order. This is in contrast to the conventional method of exchange control by tilting the QD energy levels in the detuning regime.
We provide a theoretical study on these two different types of exchange control operations between localized spins – barrier or tilt control. We use two different theoretical models: Hubbard model and confining potential model. The Hubbard model is a very simple and intuitive model with parameters for the QD orbital energies, tunneling, and direct Coulomb interactions. But it is not clear a priori how the parameters in the Hubbard model are affected by charge noise, and it is not straightforward to connect parameters of the Hubbard model with experimental voltage controls on metallic gates used for defining QDs and control electric signals.
A more general theoretical framework is necessary and for this we use a microscopic confining-potential model where each QD is defined by its confining potential and the barrier potential between QDs. Particularly, barrier vs tilt control of the exchange interaction are systematically studied showing that the insensitivity on the gate charge noise for barrier control in symmetric regime can potentially lead to efficient implementation of quantum gates with higher fidelities than the conventional tilting gates in the detuning regime.