High-fidelity CPHASE gate in a pair of capacitively coupled few-electron singlet-triplet qubits

Due to their limited coupling to charge noises, spin qubits have been the main candidates for robust quantum information processing in semiconductor quantum dot devices. Among the proposed spin qubits, singlet-triplet qubits stand out due to their all electrical control scheme. Although high-fidelity single-qubit operations have been experimentally demonstrated, the fidelities of two-qubit capacitive gates are limited. In contrast to conventional two-electron singlet-triplet qubits, we propose to host the capacitive gates between a pair of four-electron singlet-triplet qubits, each of which is operated in the detuning regime where the electron occupation is asymmetric. Using full configuration interaction calculations, we show that the non-monotonic behavior of the dipole moment of each qubit leads to an optimal operating point where the capacitive coupling is maximal and the effective exchange energies are first-order insensitive to charge noises. Numerical simulations under realistic charge noises and hyperfine noises show that operating CPHASE gates at the optimal point can achieve fidelities above 99% [1].

[1] G. X. Chan and X. Wang, Phys. Rev. B 106, 075417 (2022)