Realization of high-fidelity CZ gates in extensible superconducting qubits design with a tunable coupler

Abstract: High-fidelity two-qubits gates are essential for the realization of large-scale quantum computation and simulation. Tunable coupler design is used to reduce the problem of parasitic coupling and frequency crowding in many-qubit systems and thus thought to be advantageous. Here we design a extensible 5-qubit system in which center transmon qubit can couple to every four near-neighbor qubit via a capacitive tunable coupler and experimentally demonstrate high-fidelity controlled-phase (CZ) gate by manipulating center qubit and one near-neighbor qubit. Speckle purity benchmarking (SPB) and cross entrophy benchmarking (XEB) are used to assess the purity fidelity and the fidelity of the CZ gate. The average purity fidelity of the CZ gate is 99.69$\pm$0.04\% and the average fidelity of the CZ gate is 99.65$\pm$0.04\% which means the control error is about 0.04\%. Our work will help resovle many chanllenges in the implementation of large scale quantum systems.