From Common White Powder to Quantum Innovation: Unlocking Nearly Noiseless Qubits

Researchers led by UChicago Pritzker School of Molecular Engineering Professor Giulia Gall, together with collaborators in Sweden, used theoretical and computational approaches to discover how defects in simple calcium oxide can produce qubits with a handful of promising properties. Credit: UChicago Pritzker School of Molecular Engineering / Peter Allen, edited

Researchers discovered that bismuth atoms embedded in calcium oxide can function as qubits for quantum computers, providing a low-noise, durable, and inexpensive alternative to current materials. This groundbreaking study highlights its potential to transform quantum computing and telecommunications.

Calcium oxide is an inexpensive, chalky chemical compound frequently used in the manufacturing of cement, plaster, paper, and steel. However, the common material may soon have a more high-tech application.

Scientists used theoretical and computational approaches to discover how tiny, lone atoms of bismuth embedded within solid calcium oxide can act as qubits — the building blocks of quantum computers and quantum communication devices. These qubits were described by University of Chicago Pritzker School of Molecular Engineering researchers and their collaborator in Sweden on June 6 in the scientific journal Nature Communications.

“This system has even better properties than we expected,” said Giulia Galli, Liew Family Professor at Pritzker Molecular Engineering and Chemistry and senior author of the new work. “It has an incredibly low level of noise, can hold information for a long time, and is not made with a fancy, expensive material.”

Calcium oxide, also known as quicklime, is a white, caustic, alkaline chemical compound derived from limestone and other calcium-rich materials. It is primarily used in the production of cement and mortar, as well as in the steel industry, for water treatment, and in the manufacture of glass, ceramics, and paper. When mixed with water, it reacts exothermically to form calcium hydroxide, commonly known as slaked lime.

Advancements in Qubit Development

A quantum bit, or qubit, is the basic unit of information that encodes data in quantum computing. Today, researchers have developed many different types of qubits, which are often composed of tiny point defects within semiconducting materials. Some of the properties of these defects can be used to store pieces of information. However, many existing qubits are incredibly fragile; electronic or magnetic “noise” in their surroundings can change their properties, erasing any information that was encoded within them.

In 2022, a collaboration between scientists in Japan and the groups of David Awschalom and Galli simulated the properties of more than 12,000 materials to discover new potential solids that could contain promising defects acting as qubits. That work turned up calcium oxide as one of a number of materials with the potential to contain qubits that encoded information with very low levels of noise for an especially long period of time.

Discovering New Quantum Materials

“Our previous work told us that if you find the right defects to put within its structure, calcium oxide would be a perfect medium for storing quantum information,” said Nikita Onizhuk, a postdoctoral fellow in the Galli group and one of the authors of the paper. “So our new goal was to find the ideal defect.”

In the new paper, Galli and her colleagues used a series of computational methods that were established over recent years to screen more than 9,000 different defects within calcium oxide for their potential as qubits. The results pointed toward one type of defect — in which an antimony, bismuth or iodine atom is embedded within the usual structure of calcium and oxygen that make up calcium oxide.

“It has an incredibly low level of noise, can hold information for a long time, and is not made with a fancy, expensive material.”
— Prof. Giulia Galli

“We never could have guessed that these exact defects would be so promising,” said Joel Davidsson of Linköping University, the first author of the paper and the main developer of the high-throughput approach used to discover novel spin defects. “The only way to do this was with thorough and unbiased screen procedures.”

Galli’s team then showed through their modeling approaches that the bismuth defect within calcium oxide can theoretically encode data with little noise and for relatively long periods of time (multiple seconds compared to the milliseconds of coherence shown by many qubits). It also has the potential to mesh well with telecommunications devices because of the material’s refractive index and its ability to emit photons of light.

Galli and collaborators are now working with experimental groups who can build the calcium-oxide-based materials and test whether the predictions hold true.

“We’re at the very early stages, but from a fundamental science point of view, we think this material is very promising,” Galli said.

Reference: “Discovery of atomic clock-like spin defects in simple oxides from first principles” by Joel Davidsson, Mykyta Onizhuk, Christian Vorwerk and Giulia Galli, 6 June 2024, Nature Communications.
DOI: 10.1038/s41467-024-49057-8

Funding: This work was supported by the Swedish e-science Research Centre (SeRC), the Knut and Alice Wallenberg Foundation, the Swedish Research Council, a Google PhD Fellowship, and the Air Force Office of Scientific Research.