Chapter 5 of Quantum From The Ground Up covers superconducting qubits: Josephson junctions, the Dolan bridge fabrication technique, and IBM's 1,121-qubit Condor chip running at 99.0 to 99.5 percent two-qubit fidelity. That chapter never asks how the metal underneath those junctions gets onto the chip in the first place. A new paper out of Cornell fixes a problem in that step, and it matters more than another fidelity number would.
A superconducting qubit is a tiny circuit built from metal that, once cooled near absolute zero, behaves like a single quantum object instead of an ordinary wire. The part that makes it a qubit rather than just a very cold wire is the Josephson junction: two superconducting metal layers separated by an insulating gap so thin that electrons tunnel straight through it. Tantalum has become a favorite metal for those layers because tantalum-based qubits hold their quantum state longer than most alternatives. But tantalum only works if its atoms land on the chip in one specific crystal arrangement, called the alpha phase. Getting that arrangement has required heating the substrate past 400°C during deposition. Most semiconductor factories run their fabrication lines with a hard ceiling near that same 400°C, so there was almost no margin between what tantalum needed and what a foundry could tolerate, as Cornell's team describes the manufacturing squeeze.
Cornell's group, led by Assistant Professor Valla Fatemi, builds these tantalum layers by sputtering: a process gas is ionized and fired at a block of tantalum, knocking atoms loose so they land on a silicon wafer and build up a thin film. The standard process gas is argon. Fatemi's team swapped in krypton, a heavier noble gas, and found that the heavier atoms push the tantalum into the alpha phase at temperatures as low as 200°C, half of what argon requires, according to the published results in Nature Materials on August 18, 2026. The resulting films also carried noticeably higher electronic conductivity than tantalum deposited the old way.
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| Krypton ions hit the tantalum target at the top, knock atoms loose, and those atoms travel down to build the alpha-tantalum film on the heated silicon substrate. Note 200°C, half the temperature argon sputtering needs. |
The work builds on an earlier study from Fatemi's lab that sputtered niobium films with argon and mapped out how surface chemistry during deposition shapes final qubit performance. Swapping the metal to tantalum and the gas to krypton let the team apply what that niobium work taught them about controlling film quality. Qubits built from the new tantalum films performed, in Fatemi's own assessment, at the leading edge for the field.
None of this sets a new fidelity record. It removes a bottleneck that sits underneath every fidelity number in the chapter: a qubit design cannot scale to production if the fabrication step it needs falls outside what an ordinary chip factory can actually run.
This result covers one sample film deposited at one lab. Scaling it to production is a separate engineering problem, not a physics problem, and it is the harder one. A foundry running krypton sputtering needs the crystal phase, conductivity, and thickness to stay uniform across every chip on a wafer and across every wafer in a batch, not just in the sample that made it into the paper. It needs the process to hold up next to every other step already running on that line, since real chips stack tantalum with silicon oxide, aluminum wiring, and the Josephson junction itself, each with its own temperature limits and contamination risks. And it needs the equipment: krypton sputtering targets, gas handling, and chamber tuning are not yet standard equipment at most semiconductor fabs the way argon sputtering is. Lowering the temperature ceiling was the physics half of the problem. Building a repeatable, monitored, high-yield process around that lower ceiling, at the volume a real qubit chip production line runs, is the engineering half, yet to be figured out.
What This Changes in the Book
Chapter 5 lists IBM's Condor at 1,121 qubits and 99.0 to 99.5 percent two-qubit fidelity, built with Dolan-bridge Josephson junctions. This post adds a manufacturing footnote to the process. A lower-temperature tantalum deposition route now exists that fits inside standard semiconductor foundry limits. The path to building chips at that quality, at scale, gets wider.
This post will fold into the next edition of Quantum from the Ground Up, due September 1. The current edition is available on the book page.
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