Tantalum Airbridges for Superconducting Qubit Chips

Short answer: Airbridges are small bridges that let one superconducting circuit line cross another, and researchers have proposed making them from tantalum. A 2024 arXiv preprint, later published in npj Quantum Information in 2025, reports a lift-off fabrication method and a 13-qubit processor with a median T1 above 100 microseconds. These are the authors' laboratory results.

What problem does an airbridge solve?

In a planar superconducting chip, control lines, ground planes and coupling elements sometimes need to cross each other. The paper characterizes tantalum airbridges used as control line jumpers, ground plane crossovers and coupling elements. The authors motivate tantalum by its exceptional resistance to both acid and alkali and, in the preprint, by its long coherent lifetime in superconducting qubits. They also describe a benefit of building coplanar waveguides, qubits and airbridges all on a tantalum film platform with unified fabrication.

How is the tantalum airbridge made?

The authors propose a lift-off method for airbridges with separate or fully-capped structures. An aluminium film is introduced as a barrier layer to separate two layers of photoresist, and is then etched away before the tantalum film is deposited. The preprint adds that the chip is cleaned with piranha solution to remove residual photoresist. The authors present this as a new method, and its value rests on their own characterization.

What did the measurements show?

According to the preprint abstract, the airbridges exhibit excellent connectivity and minimal capacitive loss, effectively suppress microwave and flux crosstalk, and offer high freedom of coupling. To check overall adaptability, the authors present a 13-qubit tunable-coupling processor with median T1 above 100 microseconds. They report a median single-qubit gate fidelity of 99.95 percent for isolated randomized benchmarking, dropping slightly to 99.94 percent for simultaneous benchmarking. The published version adds a result on a separate two-qubit chip, where airbridge coupling reached a controlled-Z gate fidelity surpassing 99.2 percent. The authors suggest this may facilitate scalable quantum computation and quantum error correction with entirely tantalum elements, which is a stated possibility and not a demonstrated outcome.

Preprint and journal versions

The arXiv text is a preprint and the npj Quantum Information article is the peer-reviewed version, so the published abstract is the better guide to what the authors claim after review. The two abstracts differ in wording and in some details, such as the two-qubit gate result appearing in the journal abstract. This post is based on abstracts and visible summary text and has not reviewed the full papers. It makes no claim about products or deployed processors.

What this does not say

The abstracts do not compare tantalum airbridges against other airbridge materials in a way this post can verify, and they do not report performance beyond the devices the authors built. A median T1 on one processor is a single data point on one fabrication run.

FAQ

Why use tantalum?

The authors cite its resistance to acid and alkali. The preprint also cites its long coherent lifetime in superconducting qubits.

What is the fabrication trick?

A lift-off method in which an aluminium barrier layer separates two photoresist layers and is etched away before the tantalum is deposited.

What qubit performance was reported?

A 13-qubit processor with median T1 above 100 microseconds, and median single-qubit gate fidelity of about 99.95 percent for isolated benchmarking.

Was a two-qubit gate shown?

The journal abstract reports a controlled-Z gate fidelity surpassing 99.2 percent on a separate two-qubit chip using airbridge coupling.