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Granular aluminium nanojunction fluxonium qubit

Nature Materials volume 22, pages 194–199 (2023)Cite this article

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Abstract

Mesoscopic Josephson junctions, consisting of overlapping superconducting electrodes separated by a nanometre-thin oxide layer, provide a precious source of nonlinearity for superconducting quantum circuits. Here we show that in a fluxonium qubit, the role of the Josephson junction can also be played by a lithographically defined, self-structured granular aluminium nanojunction: a superconductor–insulator–superconductor Josephson junction obtained in a single-layer, zero-angle evaporation. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from that of a standard fluxonium. Remarkably, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically large granular aluminium nanojunction charging energy in the range of tens of gigahertz, comparable to its Josephson energy. We measure coherence times in the microsecond range and we observe spontaneous jumps of the value of the Josephson energy on timescales from milliseconds to days, which offers a powerful diagnostics tool for microscopic defects in superconducting materials.

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Fig. 1: The gralmonium, a single-layer grAl fluxonium circuit.
Fig. 2: Gralmonium spectroscopy versus external flux.
Fig. 3: Time domain characterization of the gralmonium at half-flux bias.

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All relevant data are available from the corresponding authors upon reasonable request.

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Acknowledgements

We are grateful to U. Vool for fruitful discussions, and we acknowledge technical support from S. Diewald, A. Eberhardt, M. K. Gamer, A. Lukashenko and L. Radtke. Funding is 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 European Union’s Horizon 2020 programme under grant no. 899561 (AVaQus). P.P. and I.M.P. acknowledge support from the German Ministry of Education and Research within the QUANTERA project SiUCs (FKZ, 13N15209). D.R., S.G., P.W. and W.W. acknowledge support from the European Research Council advanced grant MoQuOS (no. 741276). Facility use was supported by the Karlsruhe Nano Micro Facility and KIT Nanostructure Service Laboratory. We acknowledge qKit for providing a convenient measurement software framework.

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Author notes

  1. These authors contributed equally: D. Rieger and S. Günzler.

Authors and Affiliations

  1. Physikalisches Institut, Karlsruhe Institute of Technology, Karlsruhe, Germany

    D. Rieger, S. Günzler, M. Spiecker, P. Paluch, P. Winkel, W. Wernsdorfer & I. M. Pop

  2. Institute for Quantum Materials and Technologies, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany

    S. Günzler, P. Paluch, P. Winkel, W. Wernsdorfer & I. M. Pop

  3. Institute of Microstructure Technology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, Germany

    L. Hahn, J. K. Hohmann & A. Bacher

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Contributions

D.R., S.G., M.S. and I.M.P. conceived and designed the experiment. D.R. and S.G. performed the microwave simulation. P.P., L.H., J.K.H. and A.B. contributed to the device fabrication. D.R., S.G., M.S. and P.W. participated in the measurements. D.R. and S.G. analysed the data. D.R. and S.G. led the paper writing, and M.S., P.P., P.W., J.K.H., W.W. and I.M.P. contributed to the text. W.W. and I.M.P. supervised the project.

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Correspondence to D. Rieger or I. M. Pop.

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Rieger, D., Günzler, S., Spiecker, M. et al. Granular aluminium nanojunction fluxonium qubit. Nat. Mater. 22, 194–199 (2023). https://doi.org/10.1038/s41563-022-01417-9

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