  Striving for higher gate fidelity is crucial not only for enhancing existing
noisy intermediate-scale quantum (NISQ) devices but also for unleashing the
potential of fault-tolerant quantum computation through quantum error
correction. A recently proposed theoretical scheme, the double-transmon coupler
(DTC), aims to achieve both suppressed residual interaction and a fast
high-fidelity two-qubit gate simultaneously, particularly for highly detuned
qubits. Harnessing the state-of-the-art fabrication techniques and a model-free
pulse-optimization process based on reinforcement learning, we translate the
theoretical DTC scheme into reality, attaining fidelities of 99.92% for a CZ
gate and 99.98% for single-qubit gates. The performance of the DTC scheme
demonstrates its potential as a competitive building block for superconducting
quantum processors.
