SQMS researchers discover performance-limiting nanohydrides in superconducting qubits


Written by Steve Koppes

Researchers at the U.S. Department of Energy’s Fermi National Accelerator Laboratory and their collaborators have identified a new type of nanostructural imperfection that impacts superconducting qubits, the building blocks of quantum computers.

Fermilab’s Alexander Romanenko and his colleagues describe the origin of the performance-limiting materials, known as nanohydrides, in a paper posted Aug. 23, 2021, on the e-Print archive.

Identifying the new precipitant and understanding what causes it provides an important step toward finding a solution and improving the qubits’ performance. One of the main opportunities to improve superconducting qubits is through extended coherence times—how long they can store quantum information.

Research conducted in the Superconducting Quantum Materials and Systems Center will lead to increased coherence in superconducting transmon qubits and development of a beyond-state-of-the-art quantum computer. Future applications of quantum computers may include modeling climate change and extreme weather events, developing pharmaceuticals, improving traffic patterns and creating financial models.

Currently, superconducting transmon qubits can maintain coherence from between a few microseconds up to hundreds of microseconds. Nanohydride formation has now been identified as one of the factors that contribute to short coherence times.

“Now we can start to understand how these defects occur,” said Matt Reagor, director of engineering at Rigetti Computing and a co-author of the paper. “We can start to engineer them out of our systems.”

The Fermilab SQMS group discovered the evidence of this nanoscale imperfection during microscopic examinations of quantum materials conducted at ultracold temperatures. The test samples involved were qubits produced by Rigetti Computing, a leading startup building quantum computers with superconductors.

“We want these devices to act as a perfect mirror for single photons to bounce indefinitely—but foreign precipitants can act as traps where the photon can disappear,” said Anna Grassellino, director of the Fermilab-hosted SQMS Center. “Our primary goal with the SQMS Center is to identify these traps and eliminate them from the start.”

The new technological discovery is a variant of Q-disease—a strong degradation of cavity performance, which SRF scientists discovered several decades ago in niobium superconducting radio-frequency cavities built for particle accelerators. In those cavities, bulk inclusion of hydrogen in the niobium gave rise to large hydride precipitants upon cooldown to cryogenic temperatures. High-temperature heat treatments of niobium SRF cavities, also known as degassing, has become a standard way to control this hydride formation.

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