CZ gate with a capacitively shunted double-transmon coupler operated with a biased net-zero pulse

The double-transmon coupler (DTC) enables a fast and high-fidelity CZ gate between two fixed-frequency data transmons [1,2]. The coupler consists of two transmons connected via a three-junction flux-bias loop, and its two lowest-energy modes, symmetric and antisymmetric, mediate the ZZ interaction between the data transmons, which can be turned on and off by the flux bias. Here, for a capacitively shunted double-transmon coupler (CSDTC) [3], we introduce an additional shunt capacitor between the two coupler transmons and show that the residual ZZ interaction for the off state can be significantly reduced in a wide range of the flux bias. Consequently, we eliminate the necessity of a static flux bias to cancel a modest remnant flux-bias offset (within ~±0.2Φ₀). We demonstrate CZ-gate operation using a biased net-zero flux pulse, which brings additional robustness to low-frequency flux noise and pulse distortion. The gate fidelities achieved are above 99.97% for single-qubit gates and 99.85% for the 80-ns CZ gate within the range of flux-bias offset mentioned above. Through a comprehensive error budget analysis using multiple randomized benchmarking methods, we identify that the current CZ-gate fidelity is limited by the decoherence through the coupler.