Short answer: Granular aluminum is a superconducting material that behaves like a dense array of tiny junctions and has high kinetic inductance, which makes it useful for building superinductors. A 2019 Nature Materials paper used it to make a fluxonium qubit with a coherence time T2* of up to 30 microseconds. A later paper, published in Nature Materials in 2023, uses a granular aluminum nano-junction in place of the usual Josephson junction and reports T1 of 10 microseconds. These are device-physics results from single qubits.
What a superinductor is
The Nature Materials abstract explains that superconducting machines are mostly based on microwave circuits with a characteristic impedance of about 100 ohms and small anharmonicity, which can limit coherence and gate fidelity. An alternative is circuits built on superinductors, with characteristic impedance above the resistance quantum of 6.4 kilohms. Earlier superinductors made of mesoscopic Josephson junction arrays can introduce unintended nonlinearity or parasitic resonant modes near the qubit, degrading its coherence.
The 2019 fluxonium
The authors present a fluxonium qubit based on a granular aluminum superinductor strip. They show that granular aluminum can form an effective junction array with high kinetic inductance and be integrated in situ with standard aluminum circuit processing. The measured qubit coherence time T2* is up to 30 microseconds. The authors say this illustrates the potential for protected qubit designs, quantum-limited amplifiers and detectors. This is an outlook, and the paper does not report those devices. An arXiv preprint of the same work (1809.10646, posted in 2018) carries a nearly identical abstract.
The gralmonium
The arXiv paper 2202.01776 (versions in February and September 2022; the arXiv page lists the journal reference as Nature Materials 22, 194 from 2023) describes a fluxonium in which the junction is a lithographically defined, self-structured granular aluminum nano-junction. It is a single-layer superconductor-insulator-superconductor junction made with a single, zero-angle evaporation. The authors nickname the qubit the gralmonium and say its measured spectrum is indistinguishable from a standard fluxonium.
They say the lack of a mesoscopic parallel-plate capacitor gives an intrinsically large nano-junction charging energy of tens of gigahertz, comparable to the Josephson energy. They measure average energy relaxation times of 10 microseconds and Hahn echo coherence times of 9 microseconds. They also observe spontaneous jumps of the Josephson energy on timescales from milliseconds to days, which they present as a diagnostic tool for microscopic defects in superconducting materials. The coherence values are modest next to other qubit types, and the authors frame the device mainly as a detector and diagnostic.
Limits
All numbers are from the authors' own measurements on small numbers of devices. A coherence time of tens of microseconds does not make this material a competitor to the best transmons in these abstracts, and neither paper makes that claim.
FAQ
What is granular aluminum?
A self-assembling superconducting film that acts like an effective junction array with high kinetic inductance, per the 2019 authors.
What coherence time was measured?
T2* of up to 30 microseconds for the 2019 fluxonium, and T1 of 10 microseconds for the gralmonium.
What is a superinductor?
A circuit element with characteristic impedance above 6.4 kilohms, the resistance quantum.
Why did the gralmonium paper matter?
Its jumps in Josephson energy offer a way to study defects, according to the authors.
Is this used in commercial machines?
These abstracts report no such use.
All field notes