Fabrication and characterization of superconducting multiqubit device with niobium base layer* *Project supported by the National Key R&D Program of China (Grant No. 2016YFA0300601), the National Natural Science Foundation of China (Grant Nos. 11934018 and 11874063), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB28000000), and the Key-Area Research and Development Program of GuangDong Province,

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Superconducting transmon qubits are the leading platform in solid-state quantum computing and quantum simulation applications. In this work, we develop a fabrication process for the transmon multiqubit device with a niobium base layer, shadow-evaporated Josephson junctions, and airbridges across the qubit control lines to suppress crosstalk. Our results show that these multiqubit devices have well-characterized readout resonators, and that the energy relaxation and Ramsey (spin-echo) dephasing times are up to ∼ 40 μs and 14 (47) μs, respectively. We perform single-qubit gate operations that demonstrate a maximum gate fidelity of 99.97%. In addition, two-qubit vacuum Rabi oscillations are measured to evaluate the coupling strength between qubits, and the crosstalk among qubits is found to be less than 1% with the fabricated airbridges. Further improvements in qubit coherence performance using this fabrication process are also discussed.