A continuously tunable coupler for switching off adjacent qubit coupling in a superconducting circuit

Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. We experimentally realize a simply designed and flux-tunable coupler with continuous tunability that can turn off the adjacent qubit coupling in a superconducting circuit. Based on this coupler, we demonstrate a new scheme for a two-qubit controlled-Z gate: two Xmon qubits, originally far detuned, are first gradually brought into resonance while adjusting the coupler to keep the qubit-qubit coupling off; then the qubit-qubit coupling is turned on by the tunable coupler for the necessary interaction; finally the two qubits are brought out of resonance to the idle points while keeping the coupling off. This scheme not only efficiently suppresses the leakage out of the computational subspace but also allows for a large qubit-qubit interaction strength. We achieve an average controlled-Z gate fidelity of 98.3%, characterized via quantum process tomography and dominantly limited by the system decoherence.