Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit

• URL: http://arxiv.org/abs/2202.01776v2
• Date: Mon, 5 Sep 2022 09:37:36 GMT
• Title: Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit
• Authors: D. Rieger, S. G\"unzler, M. Spiecker, P. Paluch, P. Winkel, L. Hahn, J. K. Hohmann, A. Bacher, W. Wernsdorfer, I. M. Pop
• Abstract summary: We show that in a fluxonium qubit the role of the Josephson junction can be played by a lithographically defined, self-structured granular aluminum nano-junction. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from the one of a standard fluxonium qubit.
• Score: 0.0
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Mesoscopic Josephson junctions (JJs), consisting of overlapping superconducting electrodes separated by a nanometer thin oxide layer, provide a precious source of nonlinearity for superconducting quantum circuits and are at the heart of state-of-the-art qubits, such as the transmon and fluxonium. Here, we show that in a fluxonium qubit the role of the JJ can also be played by a lithographically defined, self-structured granular aluminum (grAl) nano-junction: a superconductor-insulator-superconductor (SIS) JJ obtained in a single layer, zero-angle evaporation. The measured spectrum of the resulting qubit, which we nickname gralmonium, is indistinguishable from the one of a standard fluxonium qubit. Remarkably, the lack of a mesoscopic parallel plate capacitor gives rise to an intrinsically large grAl nano-junction charging energy in the range of tens of $\mathrm{GHz}$, comparable to its Josephson energy $E_\mathrm{J}$. We measure average energy relaxation times of $T_1=10\,\mathrm{\mu s}$ and Hahn echo coherence times of $T_2^\text{echo}=9\,\mathrm{\mu s}$. The exponential sensitivity of the gralmonium to the $E_\text{J}$ of the grAl nano-junction provides a highly susceptible detector. Indeed, we observe spontaneous jumps of the value of $E_\text{J}$ on timescales from milliseconds to days, which offer a powerful diagnostics tool for microscopic defects in superconducting materials.

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