Double-Transmon Coupler II: High-Fidelity CPHASE and Parametric Gates*

As explained in the previous talk (Double-Transmon Coupler I), we proposed a new coupler, the double-transmon coupler (DTC), that can eliminate the ZZ coupling of the highly detuned qubits. As numerically shown in Ref. [1], we can achieve a fast and high-fidelity CPHASE gate by controlling the DC magnetic flux in the loop of the DTC. In this talk, we first show another important application of the DTC: a parametric gate by applying the AC magnetic flux to the loop of the DTC. The CPHASE gate in Ref. [1] and the parametric gate in this talk are implemented by utilizing ZZ and transverse coupling, respectively. To use both gates interchangeably with only flux control of the DTC, we tune various parameters for not only ZZ but also transverse coupling. As a result of tuning the circuit design parameters and the flux waveform, we successfully implemented CPHASE and parametric gates only by applying the DC and AC magnetic fluxes, respectively. We show numerical results that both gates can be implemented with high-fidelity for highly detuned, fixed-frequency qubits. We expect that a high-performance quantum computer with the DTC can be realized by using these two kinds of gates for different purposes.

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