Gralmonium: Granular Aluminum Nano-Junction Fluxonium Qubit

TL;DR

This paper introduces the gralmonium, a fluxonium qubit using a granular aluminum nano-junction, demonstrating comparable performance to standard fluxonium and revealing its potential as a sensitive defect detector in superconducting circuits.

Contribution

The study presents a novel fluxonium qubit design employing a lithographically defined granular aluminum nano-junction, eliminating the need for a mesoscopic capacitor and enabling high sensitivity to microscopic defects.

Findings

The spectrum of gralmonium matches standard fluxonium.

Achieved energy relaxation times of 10 microseconds.

Observed spontaneous jumps in junction energy indicating defect sensitivity.

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.