Advanced Tunable Coupler Design for Superconducting Quantum Computers

Tunable couplers play a crucial role in the implementation of fast and high-fidelity two-qubit gates in superconducting quantum computers. The structure and performance of tunable couplers have evolved year by year. As a promising candidate, the double-transmon coupler (DTC), in which two transmons are connected via Josephson junctions (JJs), has been theoretically proposed [1-3]. The DTC can suppress residual ZZ coupling substantially and achieve a high CZ-gate fidelity of 99.9% experimentally [4]. Furthermore, a capacitively shunted DTC (CSDTC), in which the coupler transmons are connected not only by a JJ but also by a shunt capacitance, has also been proposed [5]. This design can suppress the ZZ coupling between qubits at zero flux and achieve a high CZ-gate fidelity of 99.89% experimentally [5]. Both the DTC and CSDTC have the potential to realize high-fidelity parametric iSWAP gates by modulating the flux at half the qubit detuning frequency [6]. In this work, we introduce a further advanced tunable coupler design based on the CSDTC and show by numerical simulations that this enables even higher performance.