Scientists have discovered defects that affect the performance of superconducting qubits

Researchers at the Fermi National Accelerator Laboratory of the U.S. Department of Energy and their collaborators have discovered a new type of nanostructure defect in superconducting qubits, which affects superconducting qubits-the cornerstone of superconducting quantum computers .

Alexander Romanenko of Fermilab and his colleagues described the source of performance-limiting materials called nanohydrides in a paper on the arXiv.org website .

Identifying new defects and understanding their causes is an important step in finding solutions and improving the performance of qubits. One of the main opportunities for improving superconducting qubits is to extend the coherence time-how long they can store quantum information.

Research conducted at the Center for Superconducting Quantum Materials and Systems will improve the coherence of superconducting qubits and develop advanced quantum computers. Quantum computers may be used in the future to simulate climate change and extreme weather events, develop drugs, improve traffic patterns, and create financial models.

Currently, superconducting transmon qubits can maintain coherence between a few microseconds to hundreds of microseconds. The formation of nano-hydrides has been identified as one of the factors leading to the short coherence time.

Focused ion beam instrument is a tool for observing the surface of superconducting qubit devices and preparing thin layers of specific areas of the device for transmission electron microscopy analysis.

"Now we can begin to understand how these defects occur," said Matt Reagor, director of engineering at Rigetti Computing and co-author of the paper. "We can start designing them from our system."

Researchers at the Fermilab Center for Superconducting Quantum Materials and Systems (SQMS) found evidence of this nano-scale defect when they microscopically examined quantum materials at ultra-low temperatures. The test samples involved are qubits produced by Rigetti Computing, a startup company that makes superconducting quantum computers.

This new technological discovery is a variant of Q-disease (a severe degradation of cavity performance), which was discovered decades ago by the Fermi Lab SRF team in a niobium superconducting radio frequency cavity built for a particle accelerator. This phenomenon.

In these cavities, a large amount of hydrogen contained in niobium will produce a large amount of hydride deposits when it is cooled to a low temperature. The high temperature heat treatment of the niobium SRF cavity, also known as degassing, has become the standard method for controlling the formation of this hydride.

A few years ago, the Fermilab SRF team discovered a much smaller precipitate called nanohydride, which would form in the niobium SRF cavity even after applying high temperature treatment. These deposits are non-superconducting and will cause the performance of the niobium resonator to decrease.

Romanenko, CTO of Fermilab, said: “The presence of hydrogen affects the niobium in the qubit, just like it does in the niobium SRF cavity. This is why we decided to apply cryomicroscopy for the first time to study superconducting quantum Bits. We need to study the nanostructure of quantum devices at their operating temperature."

When exposed to hydrogen or hydrogen-containing compounds, niobium absorbs them like a sponge. Niobium oxide coatings can only provide limited protection against hydrogen absorption. Grassellino explained: "Usually in the process of manufacturing these devices, we remove these oxides. The exposed surface niobium likes to absorb hydrogen."

Physicists now know that even trace amounts of hydrogen can affect qubits through hydrides (nanoscale). Eliminating the hydrogen in the qubit processing process can help. Nevertheless, trace elements are still present in niobium, just like sugar dissolved in a cup of coffee.

Microscopic measurements of superconducting qubits show that when cooled to a low temperature, nano-scale niobium hydride compound particles will appear in the niobium particles. Compared with niobium, these hydrides have poor superconductivity, so they may cause decoherence of qubits. Figure a shows the diffraction patterns extracted from the niobium hydride precipitates, confirming their presence in the niobium particles. The distribution of these precipitates is detailed in Figures a and c.

Although it is possible to determine which steps in the process will expose niobium to hydrogen and then try to eliminate them. But Romanenko said: "It may not be possible to completely eliminate hydrogen." Romanenko, who specializes in niobium SRF cavities, recently worked on adjusting the accelerator cavity to a 3D device to store quantum information when combined with 2D qubits such as Rigetti.

Researchers at Fermilab have rarely used advanced analytical techniques to reveal this new phenomenon at ultra-low temperatures. Atomic force microscopy, scanning electron microscopy, and secondary ion mass spectrometry all played a role in nano-hydride experiments.

At room temperature, hydrogen exists in the form of gas and flows throughout the niobium metal without any problems. But when it is cooled to 160K (-113.15°C), hydrogen bonds combine with niobium, so the niobium hydride starts to grow. The more hydrogen niobium absorbs, the more nano-hydrides, and vice versa.

Repeated tests have shown that hydrides will form within a few minutes after cooling to a temperature of approximately 80K to 200K. Then as the temperature drops below 77K, the hydrogen almost freezes in place, and the formation of hydrides ceases.

Jaeyel Lee (left) and Zuhawn Sung (right) analyze nano-hydrides in the Materials Science Laboratory of the Applied Physics and Superconducting Technology Department of Fermilab.

Romanenko said: "The normal heating and cooling cycles common in quantum computer operations will further affect the formation of nano-hydrides, but in different ways."

SQMS, the center for superconducting quantum materials and systems, is the five largest national quantum information science research center of the United States Department of Energy. SQMS will work with industry partners to build a quantum computer and new quantum sensors in Fermilab.

link:  [1]https://arxiv.org/abs/2108.10385

        [2]https://news.fnal.gov/2021/12/sqms-researchers-discover-performance-limiting-nanohydrides-in-superconducting-qubits/