Exploring the sweet-spot regime of singlet-triplet qubits coupled to a microwave resonator

Singlet-triplet S-T0 qubits are robust against global magnetic noise and, at the symmetric operating point, charge fluctuations [4]. However, strong coupling to a resonator requires hybridizing the (1,1) and (0,2) singlet states, making the qubit more sensitive to charge noise. We find that, in the operating regime where the tunnel coupling is comparable to the magnetic field gradient, sweet spots emerge, that are distinct from the symmetric operating point, but offer interesting opportunities for high-fidelity gate operations.

In this work we explore this sweet-spot regime, which provides both resilience to charge noise and strong dipolar coupling to the resonator. We analyze the trade-offs between strong coupling, relaxation, dephasing, and leakage to the (0,2) singlet state, and we maximize the resulting gate fidelities. We identify a wide window in parameter space over which the qubit-resonator system can achieve two-qubit gate fidelities above 95%.