  Although two-qubit entangling gates are necessary for universal quantum
computing, they are notoriously difficult to implement with high fidelity.
Recently, tunable couplers have become a key component for realizing
high-fidelity two-qubit gates in superconducting quantum computers. However, it
is still difficult to achieve tunable coupling free of unwanted residual
coupling for highly detuned qubits, which are desirable for mitigating
qubit-frequency crowding or errors due to crosstalk between qubits. We thus
propose a design for this kind of tunable coupler, which we call a
double-transmon coupler, because this is composed of two transmon qubits
coupled through a common loop with an additional Josephson junction.
Controlling the magnetic flux in the loop, we can achieve not only fast
high-fidelity two-qubit gates, but also no residual coupling during idle time,
where computational qubits are highly detuned fixed-frequency transmons. The
proposed coupler is expected to offer an alternative approach to
higher-performance superconducting quantum computers.
