Scaling is now a key challenge in superconducting quantum computing. One solution is to build modular systems in which smaller-scale quantum modules are individually constructed and calibrated, and then assembled into a larger architecture. This, however, requires the development of suitable interconnects. Here, we report low-loss interconnects based on pure aluminium coaxial cables and on-chip impedance transformers featuring quality factors up to 8.1×105, which is comparable to the performance of our transmon qubits fabricated on single-crystal sapphire substrate. We use these interconnects to link five quantum modules with inter-module quantum state transfer and Bell state fidelities up to 99\%. To benchmark the overall performance of the processor, we create maximally-entangled, multi-qubit Greenberger-Horne-Zeilinger (GHZ) states. The generated inter-module four-qubit GHZ state exhibits 92.0\% fidelity. We also entangle up to 12 qubits in a GHZ state with 55.8±1.8% fidelity, which is above the genuine multipartite entanglement threshold of 1/2. These results represent a viable modular approach for large-scale superconducting quantum processors.
@article{arxiv.2302.02751,
title = {Low-loss interconnects for modular superconducting quantum processors},
author = {Jingjing Niu and Libo Zhang and Yang Liu and Jiawei Qiu and Wenhui Huang and Jiaxiang Huang and Hao Jia and Jiawei Liu and Ziyu Tao and Weiwei Wei and Yuxuan Zhou and Wanjing Zou and Yuanzhen Chen and Xiaowei Deng and Xiuhao Deng and Changkang Hu and Ling Hu and Jian Li and Dian Tan and Yuan Xu and Fei Yan and Tongxing Yan and Song Liu and Youpeng Zhong and Andrew N. Cleland and Dapeng Yu},
journal= {arXiv preprint arXiv:2302.02751},
year = {2023}
}