  High-performance two-qubit gates have been reported with superconducting
qubits coupled via a single-transmon coupler (STC). Most of them are
implemented for qubits with a small detuning since reducing residual $ZZ$
coupling for highly detuned qubits by an STC is challenging. In terms of the
frequency crowding and crosstalk, however, highly detuned qubits are desirable.
Here, we numerically demonstrate a high-performance parametric gate for highly
detuned fixed-frequency qubits using a recently proposed tunable coupler called
a double-transmon coupler (DTC). Applying an ac flux pulse, we can perform a
maximally entangling universal gate ($\sqrt{\rm iSWAP}$) with an average
fidelity over 99.99$\%$ and a short gate time of about 24 ns. This speed is
comparable to resonance-based gates for slightly detuned tunable qubits.
Moreover, using a dc flux pulse alternatively, we can achieve another kind of
entangling gate called a CZ gate with an average fidelity over 99.99$\%$ and a
gate time of about 18 ns. Given the frexibility and feasible settings, we can
expect that the DTC will contribute to realizing a high-performance quantum
computer in the near future.
