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Granular aluminium as a superconducting material for high-impedance quantum circuits


Superconducting quantum information processing machines are predominantly based on microwave circuits with relatively low characteristic impedance, about 100 Ω, and small anharmonicity, which can limit their coherence and logic gate fidelity1,2. A promising alternative is circuits based on so-called superinductors3,4,5,6, with characteristic impedances exceeding the resistance quantum RQ = 6.4 kΩ. However, previous implementations of superinductors, consisting of mesoscopic Josephson junction arrays7,8, can introduce unintended nonlinearity or parasitic resonant modes in the qubit vicinity, degrading its coherence. Here, we present a fluxonium qubit design based on a granular aluminium superinductor strip9,10,11. We show that granular aluminium can form an effective junction array with high kinetic inductance and be in situ integrated with standard aluminium circuit processing. The measured qubit coherence time \(T_2^ \ast \le 30\,{\upmu}{\mathrm{s}}\) illustrates the potential of granular aluminium for applications ranging from protected qubit designs to quantum-limited amplifiers and detectors.

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We are grateful to A. Bilmes, J. Lisenfeld, C. Smith and M. Wildermuth for insightful discussions, and to J. Ferrero, A. Lukashenko and L. Radtke for technical assistance. Funding was provided by the Alexander von Humboldt Foundation in the framework of a Sofja Kovalevskaja award endowed by the German Federal Ministry of Education and Research, and by the Initiative and Networking Fund of the Helmholtz Association, within the Helmholtz Future Project ‘Scalable solid state quantum computing’. This work has been partially supported by the European Research Council advanced grant MoQuOS (no. 741276). I.T. and A.V.U. acknowledge partial support from the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of the National University of Science and Technology MISIS (contract no. K2-2017-081). Facilities use was supported by the KIT Nanostructure Service Laboratory. We acknowledge qKit for providing a convenient measurement software framework.

Author information

L.G. and M.S. designed and fabricated the samples, and performed the measurements. D.G., S.T.S., I.T., F.V., P.W. and H.R. contributed to the sample fabrication effort. L.G. and M.S. analysed the data with help from N.M., W.W. and A.V.U. L.G., M.S. and I.M.P. led the paper writing, while all other authors contributed to the text. I.M.P. supervised and coordinated the project.

Competing interests

The authors declare no competing interests.

Correspondence to Ioan M. Pop.

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Supplementary Information

Supplementary Figs. 1–6, Notes 1–9 and refs. 1–9

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Fig. 1: Fluxonium qubit built using a grAl superinductor.
Fig. 2: Fluxonium and readout resonator spectroscopy obtained from the measurement of the complex reflection coefficient S11 .
Fig. 3: Quantum coherence of the fluxonium superconducting qubit with a grAl superinductor.