High-fidelity three-qubit iToffoli gate for fixed-frequency superconducting qubits

The development of noisy intermediate-scale quantum devices has extended the scope of executable quantum circuits with high-fidelity single- and two-qubit gates. Equipping these devices with three-qubit gates will enable the realization of more complex quantum algorithms and efficient quantum error correction protocols with reduced circuit depth. Several three-qubit gates have been implemented for superconducting qubits, but their use in gate synthesis has been limited owing to their low fidelity. Here, using fixed-frequency superconducting qubits, we demonstrate a high-fidelity i Toffoli gate based on two-qubit interactions, the so-called cross-resonance effect. As with the Toffoli gate, this three-qubit gate can be used to perform universal quantum computation. The i Toffoli gate is implemented by simultaneously applying microwave pulses to a linear chain of three qubits, revealing a process fidelity as high as 98.26(2)%. Moreover, we numerically show that our gate scheme can produce additional three-qubit gates that provide more efficient gate synthesis than the Toffoli and i Toffoli gates. Our work not only brings a high-fidelity i Toffoli gate to current superconducting quantum processors but also opens a pathway for developing multi-qubit gates based on two-qubit interactions. The efficiency of running quantum algorithms can be improved by expanding the hardware operations that a quantum computer can perform. A high-fidelity three-qubit i Toffoli gate has now been demonstrated using superconducting qubits.