Krypton Gas Could Make Quantum Chips Easier to Build

Most quantum computing headlines arrive wrapped in qubit counts and error rates. This one is about a gas. A team at Cornell, led by the assistant professor Valla Fatemi, has found that swapping the argon normally used to make superconducting qubits for krypton lets them lay down a critical component at less than half the usual temperature. The result appeared in Nature Materials on 19 August under the gloriously un-catchy title “Krypton-sputtered tantalum films for scalable high-performance quantum devices.”

Boring name, real consequence. Tantalum has become a darling material for superconducting qubits because it helps them hold a fragile quantum state for comparatively long stretches. The snag is that depositing the good, high-conductivity form of tantalum onto a silicon wafer normally demands temperatures north of 400 degrees Celsius, hot enough to complicate everything else on the chip. Cornell’s krypton method brings that down to about 200 degrees.

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Why the temperature is the whole story

The useful form of tantalum is a particular crystal structure, the body-centred cubic alpha phase. Coaxing it to form on silicon usually means cranking up the heat. Krypton atoms are heavier than argon, so when they are ionised and fired at a tantalum target during sputtering, they transfer more momentum to the metal atoms flying off it. That extra kinetic push helps the desirable crystal lock into place at a far lower temperature.

Lower heat matters because the semiconductor industry, the eventual partner any scalable qubit will need, builds its chips on silicon lines that do not enjoy 400-degree steps. Bring the temperature down and you make superconducting qubits far friendlier to the kind of standard fabrication that turns a laboratory curiosity into something you can manufacture in volume.

Does it actually perform

This is where a clever idea usually dies, and this one did not. The team built transmon qubits, the workhorse design behind machines from the likes of IBM and Google, using the krypton-sputtered films with compact 20-micron capacitor gaps. They measured internal quality factors of up to 16.9 million. Quality factor is a rough gauge of how long a resonator rings before losing its energy; higher is better, and 16.9 million is a genuinely competitive figure rather than a consolation prize for a cooler process.

Keep the champagne on ice

A dose of honesty is in order. This is a materials and manufacturing advance, not a new computer, and it does nothing on its own to add qubits or crush error rates. It is one paper, at laboratory scale, and the road from a promising film to a working fault-tolerant processor is littered with ideas that looked great in Nature and never made it into a fridge.

But the bottlenecks in quantum computing are increasingly about engineering and yield rather than raw physics, and anything that makes the good version of a qubit easier and cheaper to fabricate is worth more than it sounds. A different noble gas is an unglamorous place to find an advantage. It may also be exactly the sort of place the field needs to start looking.

Did you know: Tantalum is named after Tantalus of Greek myth, condemned to stand in water he could never drink, a nod to the metal’s stubborn refusal to absorb acids.

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