Implementing and characterizing high-fidelity two-qubit gates with long-distance transmon coupler

High-fidelity two-qubit gates are an essential requirement for any quantum computing application. However, as two-qubit-gate fidelities approach the level of single-qubit-gate fidelities, standard characterization tools, such as interleaved randomized benchmarking, become unreliable. In a device featuring the long-distance transmon coupler, we have achieved a controlled-Z (CZ) gate with (99.81 ± 0.02)% fidelity [1]. To improve the estimate of this fidelity, we utilize iterative interleaved randomized benchmarking [2], which amplifies the CZ gate error and additionally reveals coherent errors. We furthermore compare the fidelity estimate to a detailed CZ gate error-budget. We see that the fidelity is limited by the qubit coherence rather than the coupler coherence, which we attribute to having a floating transmon as a coupler.

[1] F. Marxer et al., arXiv preprint arXiv:2208.09460

[2] S. Sheldon et al., Phys. Rev. A 93, 012301 (2016)