Multiplexed double-transmon coupler scheme in scalable superconducting quantum processor

Abstract

Precise control of superconducting qubits is essential for advancing both quantum simulation and quantum error correction. Recently, transmon qubit systems employing the single-transmon coupler (STC) scheme have demonstrated high-fidelity single- and two-qubit gate operations by dynamically tuning the effective coupling between qubits. However, the integration of STCs increases the number of control lines, thereby posing a significant bottleneck for chip routing and scalability. To address this challenge, we propose a robust control line multiplexing scheme based on a double-transmon coupler (DTC) architecture, which enables shared coupler control lines to substantially reduce wiring complexity. Moreover, we experimentally verify that this multiplexed configuration efficiently suppresses undesirable static $ZZ$ coupling while maintaining accurate control over two-qubit gate operations. We further demonstrate the feasibility of the architecture through two distinct gate implementations: a fast coupler $Z$-control-based CZ gate and a parametric iSWAP gate. To validate the practical applicability of this multiplexing approach in quantum circuits, we prepare Bell and three-qubit GHZ states using the proposed scheme with fidelity exceeding 99% and 96%, respectively. This multiplexed DTC architecture offers significant potential to minimize wiring overhead in two-dimensional qubit arrays, thereby greatly enhancing the scalability of superconducting quantum processors.