Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit

• Published in: arXiv.org
• Main Authors: Rieger, D, Günzler, S, Spiecker, M, Paluch, P, Winkel, P, Hahn, L, Hohmann, J K, Bacher, A, Wernsdorfer, W, Pop, I M
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 05.09.2022
• Subjects: Aluminum; Josephson junctions; Parallel plates; Physics - Quantum Physics; Physics - Superconductivity; Qubits (quantum computing); Superconductivity
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2202.01776

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.