Short answer: Niobium nitride (NbN) is a superconductor with high kinetic inductance, which lets researchers build compact resonators and wires with very high impedance. A 2023 preprint, published in Physical Review Applied, reports NbN resonators with an internal quality factor above 10^5 at impedance above 2 kilohms. A 2018 preprint reported a NbN nanowire with an impedance above the resistance quantum and a quality factor of 2.5 x 10^4.
Why high kinetic inductance?
The 2018 arXiv abstract says a high kinetic inductance disordered superconductor can realize a low-loss, non-dissipative circuit element with an impedance greater than the quantum resistance, about 6.5 kilohms. Such an element, called a superinductor, can produce a quantum circuit in which charge fluctuations are suppressed. The 2023 abstract adds that NbN is a promising material for quantum technology applications because it offers the reproducibility needed for large-scale superconducting circuits.
What did the 2018 nanowire show?
The authors made a superinductor from a 40 nm wide niobium nitride nanowire. It shows a single-photon quality factor of 2.5 x 10^4. By examining loss rates, they report that the dissipation in their nanowire devices can be fully understood within a model of two-level system loss. The paper was later published in Physical Review Applied in 2019, according to the arXiv journal reference.
What did the 2023 resonators show?
The 2023 preprint, later published in Physical Review Applied volume 20, demonstrates NbN thin-film resonators with a one-photon internal quality factor above 10^5 while keeping an impedance larger than 2 kilohms. The footprint is about 50 by 100 square micrometres and the self-Kerr nonlinearity is a few tenths of a hertz. The authors say the quality factors are mostly limited by losses induced by coupling to two-level systems and were maintained for kinetic inductances ranging from tenths to hundreds of picohenries per square. They also report minimal variations in resonator performance across multiple cooldowns over more than nine months. They say the work opens perspectives for compact, high-impedance, high-quality multimode circuits with sizable interactions, which is an outlook and not a demonstrated circuit.
Why does impedance matter for circuit design?
The 2018 abstract ties impedance above the resistance quantum to suppressed charge fluctuations in a quantum circuit, and the 2023 abstract links high impedance to sizable interactions in compact circuits. A resonator with high impedance and a small footprint packs more circuit elements into a given area, which is the motivation the authors give for studying kinetic inductance ranging from tenths to hundreds of picohenries per square. Neither abstract reports a qubit or processor built from these elements, so the link to quantum computing is an outlook.
What to keep in mind
Both papers point to two-level system loss as the main limit on quality factor, so future gains would depend on understanding that loss. The figures come from specific devices on specific films. The second listed source is the journal PDF of the 2023 paper, and its title page was the only part read. This post is based on abstracts and visible summary text and full texts were not checked. It makes no claim about products or processors.
FAQ
What is a superinductor?
A low-loss circuit element with impedance above the resistance quantum, which the 2018 abstract says can suppress charge fluctuations.
How high was the quality factor?
Above 10^5 for the 2023 resonators and 2.5 x 10^4 for the 2018 nanowire.
What limits it?
Both abstracts attribute the losses to two-level systems.
Is it stable across cooldowns?
The 2023 authors report minimal variations over more than nine months of cooldowns.
All field notes