All-microwave iSWAP gate with a capacitively shunted double-transmon coupler at zero flux bias

High-fidelity two-qubit gates are essential for scalable quantum computation. We previously demonstrated a fast, high-fidelity CZ gate using a double-transmon coupler (DTC) between two fixed-frequency transmons [1]. Moreover, we introduced a capacitively shunted double-transmon coupler (CSDTC) exhibiting a small residual ZZ interaction around zero flux bias, relaxing the demand for static flux biasing [2]. The two-qubit gates with the DTC and CSDTC, as in many other two-qubit gate schemes widely used, utilize baseband control signals, where low-frequency noise and pulse distortion can be an issue for achieving high gate fidelity and stability. A fast all-microwave-controlled gate with negligible residual interaction, which simplifies control-line wiring and multiplexing and reduces calibration overhead, can be a favorable alternative. In this work, we demonstrate an all-microwave iSWAP gate in the CSDTC device used in Ref. [2]. The gate is activated by an ac modulation of the coupler flux around zero bias at the frequency of half the detuning between data qubits, driving the two-photon parametric transition. The best iSWAP gate achieves 99.9% fidelity, surpassing the CZ-gate fidelity using a baseband net-zero flux-bias pulse. We attribute this improvement to the smaller drive amplitude and reduced excitation of coupler states during the gate.

[1] R. Li et al., Phys. Rev. X 14, 041050 (2024).

[2] R. Li et al., Phys. Rev. Applied. 23, 064069 (2025).