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Two-qubit-gate operation in a high-connectivity transmon lattice utilizing a tunable coupling to a shared mode

Tuure Orell1, Hao Hsu2, Joona Andersson3, Jani Tuorila1, Frank Deppe2, and Hsiang-Sheng Ku2

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Phys. Rev. Applied 26, 054021 – Published 6 November, 2026

DOI: https://doi.org/10.1103/5vv7-y2rj

Abstract

Increasing connectivity and decreasing qubit-state delocalization without compromising the speed and accuracy of elementary gate operations are topical challenges in the development of large-scale superconducting quantum computers. In this theoretical work, we study a special honeycomb qubit lattice where each qubit inside a unit cell is coupled to every other one via two dedicated tunable couplers and a common central element. This results in an effective multimode interaction enabling tunable, on-demand, all-to-all connectivity between each qubit pair within the unit cell. We provide a thorough analysis of the unit cell, including a proposal for a novel and efficient conditional-Z gate scheme, which takes advantage of the effective multimode coupling. We develop an experimentally viable pulse protocol for a single-step gate implementation, which considerably improves the gate speed compared to the previous two-qubit-gate realizations suggested for architectures utilizing a center mode. We also show numerical results on how the presence of spectator qubits affects the average two-qubit-gate fidelity and analyze how the multimode coupling structure mitigates the delocalization-induced crosstalk during simultaneous single-qubit gates within the unit cell. We also provide analytical estimates for the errors caused by relaxation and dephasing during a two-qubit-gate operation, including noise terms for the multimode coupling structure. Our multimode coupling architecture results in a good balance between increased connectivity and available parallelism, especially when several interacting unit cells form a quantum-processing unit. We anticipate that the obtained results pave the way toward high-connectivity quantum processors with efficient and low-overhead quantum algorithms.

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Phys. Rev. Applied 26, 054021– Published 6 November, 2026
  • Received 12 March 2026
  • Revised 12 June 2026
  • Accepted 2 October 2026
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