Crosstalk analysis of the double-transmon coupler toward its scalability

Recently, a new type of tunable coupler called the double-transmon coupler (DTC) has been proposed [1,2,3]. It consists of two fixed-frequency transmons coupled via a loop including an additional Josephson junction. It has been numerically demonstrated that this coupler enables fast, high-fidelity two-qubit gate operations in a two-qubit system[1,2]. However, its scalability has remained unclear so far. That is, it is unclear whether the DTC can maintain its high performance even for systems with qubits more than two, where additional parasitic couplings, such as couplings between couplers and between non-adjacent qubits, exist, unlike two-qubit systems. In this study, we numerically evaluate the performance of a system with three qubits coupled via two DTCs. Our results demonstrate that the DTC can almost eliminate ZZ couplings between arbitrary two qubits and also the ZZZ coupling among the three qubits. Moreover, our simulations have shown that high-performance single-qubit and two-qubit gates, namely, a π/2 pulse and a CZ gate,can be implemented successfully. These results indicate the high scalability of the DTC. We will also provide simulation results for the single-transmon coupler (STC) [1,2,4] and discuss the differences between the DTC and the STC.

[1] H. Goto, Phys. Rev. Appl. 18, 034038 (2022).

[2] K. Kubo and H. Goto, Appl. Phys. Lett. 122, 064001 (2023).

[3] D. L. Campbell et al., Phys. Rev. Appl. 19, 064043 (2023).

[4] F. Yan et al., Phys. Rev. Appl. 10, 054062 (2018).