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Whether you’re baking a cake, building a house, or developing a quantum device, the quality of the final product depends significantly on the raw ingredients or materials. Researchers working to improve the performance of superconducting qubits, the basis of quantum computers, have experimented with using different base materials in an effort to increase the qubits’ coherent lifetime.
Coherence time is a measure of how long a qubit retains quantum information, and therefore a primary measure of performance. Recently, scientists have discovered that using tantalum in superconducting qubits makes them perform better, but no one has been able to determine why so far.
Scientists from the Center for Functional Nanomaterials (CFN), National Synchrotron Light Source II (NSLS-II), Co-design Center for Quantum Advantage (C2QA), and Princeton University investigated the key reasons why these qubits work better by decoding the chemical profile of the tantalum.
The results of this work, which were recently published in the journal Advanced science, will provide key insights into the design of even better qubits in the future. CFN and NSLS-II are the United States Department of Energy’s (DOE) Office of Science User Facilities at DOE’s Brookhaven National Laboratory. C2QA is a Brookhaven-led national quantum information science research center, in which Princeton University is a key partner.
Finding the right ingredient
Tantalum is a unique and versatile metal. It is dense, hard and easy to work with. Tantalum also has a high melting point and is resistant to corrosion, making it useful in many commercial applications. Furthermore, tantalum is a superconductor, which means that it has no electrical resistance when cooled to low enough temperatures, and consequently can carry current without any loss of energy.
Tantalum-based superconducting qubits have demonstrated record lifetimes of more than half a millisecond. This is five times longer than the lifetime of qubits made from niobium and aluminum, which are currently employed in large-scale quantum processors.
These properties make tantalum an excellent candidate material for building better qubits. However, the goal of improving superconducting quantum computers has been hampered by a lack of understanding of what limits the lifetime of qubits, a process known as decoherence. Noise and microscopic sources of dielectric loss are generally thought to contribute; however, scientists aren’t sure exactly why or how.
“The work in this paper is one of two parallel studies aiming to address a major challenge in the fabrication of qubits,” explained Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University and head of materials thrust. for C2QA. “No one has proposed a microscopic, atomistic model for leakage that explains all observed behavior and then been able to demonstrate that their model limits a particular device. This requires precise and quantitative measurement techniques, as well as sophisticated data analysis “.
To get a better picture of the source of the qubit decoherence, the Princeton and CFN scientists grew and chemically processed tantalum films on sapphire substrates. They then took these samples to the Spectroscopy Soft and Tender Beamlines (SST-1 and SST-2) at NSLS-II to study the tantalum oxide that formed on the surface using X-ray photoelectron spectroscopy (XPS). XPS uses X-rays to knock electrons out of the sample and provides clues about the chemical properties and electronic state of atoms near the sample surface.
Scientists hypothesized that the thickness and chemical nature of this tantalum oxide layer played a role in determining the coherence of qubits, since tantalum has a thinner oxide layer than the more typically used niobium in qubits.
“We measured these materials at beamlines to better understand what was going on,” explained Andrew Walter, a lead beamline scientist in NSLS-II’s soft X-ray scattering and spectroscopy program. “The tantalum oxide layer was assumed to be quite uniform, but our measurements showed that it’s not uniform at all. It’s always more interesting when you discover an answer you don’t expect, because that’s when you learn something.”
The team found different types of tantalum oxides on the tantalum surface, which raised a new set of questions on the way to making better superconducting qubits. Can these interfaces be modified to improve the overall performance of the device and which modifications would provide the greatest benefits? What types of surface treatments can be used to minimize losses?
It embodies the spirit of co-design
“It was inspiring to see experts from very different backgrounds come together to solve a common problem,” said Mingzhao Liu, materials scientist at CFN and materials subthrust leader at C2QA. “This was a highly collaborative effort, bringing together shared facilities, resources and expertise across all of our facilities. From a materials science perspective, it was exciting to create these samples and be an integral part of this research.” .
Walter said, “A job like this speaks to the way C2QA was built. Electrical engineers at Princeton University contributed a great deal in device management, design, data analysis, and testing. materials at CFN grew and processed samples and materials. My group at NSLS-II characterized these materials and their electronic properties.”
Bringing these specialized groups together has not only allowed the study to proceed smoothly and more efficiently, but has given the scientists an understanding of their work in a broader context. Students and PhDs have been able to gain invaluable experience in different areas and contribute to this research in meaningful ways.
‘Sometimes when materials scientists work with physicists, they hand over their materials and wait to hear about the results,’ said de Leon, ‘but our team was working hand in hand, developing new methods along the way that could be used extensively on the beamline in the future.”
Russell A. McLellan et al, Chemical profiles of oxides on tantalum in state-of-the-art superconducting circuits, Advanced science (2023). DOI: 10.1002/advs.202300921
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