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  • Open Access

Capacitively shunted double-transmon coupler realizing bias-free idling and a high-fidelity CZ gate

Rui Li1,*, Kentaro Kubo2, Yinghao Ho2, Zhiguang Yan1, Shinichi Inoue3, Yasunobu Nakamura1,3,†, and Hayato Goto2,1,‡

  • *Contact author: rui.li.gj@riken.jp
  • †Contact author: yasunobu@ap.t.u-tokyo.ac.jp
  • ‡Contact author: hayato1.goto@toshiba.co.jp
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Phys. Rev. Applied 23, 064069 – Published 30 June, 2025

DOI: https://doi.org/10.1103/l8tq-7sb3

Abstract

A high-fidelity controlled-Z (CZ) gate utilizing a double-transmon coupler (DTC) has recently been demonstrated as a building block for superconducting quantum processors. Like many other kinds of tunable couplers, however, the DTC requires a finite dc current for flux biasing the coupler at the idling point to turn off the coupling, necessitating extra care for wiring and heat-load management. To address this issue, we theoretically propose and experimentally realize a coupling scheme by introducing a shunt capacitance between the two transmons of the DTC at zero-flux bias, which demonstrates high-fidelity CZ-gate performance comparable to the previous DTC. Through a comprehensive error budget analysis using multiple randomized benchmarking methods, we also identify that the current fidelity is limited by the decoherence through the coupler. Moreover, we experimentally demonstrate the wide operational flux range of the capacitively shunted DTC, which solves the challenging issue of remnant flux existing even with careful magnetic shielding.

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Phys. Rev. Applied 23, 064069– Published 30 June, 2025
  • Received 5 March 2025
  • Revised 12 May 2025
  • Accepted 20 May 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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