Research offers direct insight into tantalum oxidation hindering qubit coherence

Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and DOE’s Pacific Northwest National Laboratory (PNNL) used a combination of scanning transmission electron microscopy (STEM) and computer modeling to take a closer look and better understand tantalum oxide. When this amorphous oxide layer forms on the surface of tantalum – a superconductor with great promise for making the building blocks of a quantum computer’s “qubits” – it can hinder the material’s ability to retain quantum information.

Learning how the oxide forms can provide clues as to why this happens and potentially indicate ways to prevent loss of quantum coherence. The research was recently published in the journal ACS Nano.

The paper builds on previous research by a team at Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University, conducted in the framework of the Co-design Center for Quantum Advantage (C²QA), a national research center in quantum information sciences led by Brookhaven and in which Princeton is a key partner.

“In this work, we used X-ray photoemission spectroscopy at NSLS-II to infer details about the type of oxide that forms on the surface of tantalum when exposed to oxygen in the air ” said Mingzhao Liu, a CFN scientist and one of the lead authors of the study. “But we wanted to better understand the chemistry of this very thin oxide layer by making direct measurements,” he explained.

So in the new study, the team partnered with scientists from Brookhaven’s Department of Condensed Matter Physics and Materials Science (CMPMS) to use advanced STEM techniques that allowed them to directly study the ultra-thin oxide layer. They also worked with PNNL theorists who carried out computer modeling revealing the most likely arrangements and interactions of atoms in the material as they oxidized.

Together, these methods helped the team gain an atomic-level understanding of the ordered crystal lattice of tantalum metal, the amorphous oxide that forms on its surface, as well as intriguing new details about the interface between these layers.

“The key is to understand the interface between the surface oxide layer and the tantalum film, because this interface can have a profound impact on the performance of qubits,” said CMPMS physicist Yimei Zhu, co-author. of the study, echoing the wisdom of Nobel laureate Herbert Kroemer, that “The interface is the device.”

Highlighting that “quantitatively probing a simple interface one to two atomic layers thick poses a formidable challenge,” Zhu noted, “we were also able to directly measure the atomic structures and bonding states of the oxide layer and of the tantalum film. such as identifying those at the interface using advanced electron microscopy techniques developed at Brookhaven.

“Measurements reveal that the interface consists of a ‘suboxide’ layer nestled between the periodically ordered tantalum atoms and the fully disordered amorphous tantalum oxide. Within this suboxide layer, only a few atoms d “oxygen are embedded in the tantalum crystal lattice,” Zhu said. .

Combined structural and chemical measurements provide an extremely detailed perspective on the material. Density functional theory calculations then helped scientists validate and expand on these observations.

“We simulated the effect of gradual surface oxidation by gradually increasing the number of oxygen species on the surface and in the subsurface region,” said Peter Sushko, one of the PNNL theorists.

By evaluating the thermodynamic stability, structure and changes in electronic properties of tantalum films during oxidation, the scientists concluded that although the fully oxidized amorphous layer acts as an insulator, the suboxide layer retains the characteristics of a metal.

“We always thought that if tantalum was oxidized, it would become completely amorphous, without any crystalline order,” Liu said. “But in the suboxide layer, the tantalum sites are still quite ordered.”

With the presence of both fully oxidized tantalum and a suboxide layer, scientists wanted to understand which part is most responsible for the loss of coherence of qubits made of this superconducting material.

“It’s likely that the oxide plays multiple roles,” Liu said.

First, he noted, the fully oxidized amorphous layer contains many lattice defects. In other words, the locations of the atoms are not well defined. Some atoms can adopt different configurations, each with a different energy level. Although these changes are small, each consumes a tiny amount of electrical energy, contributing to the qubit’s energy loss.

“This so-called two-level system loss in an amorphous material results in a parasitic and irreversible loss of quantum coherence, the ability of the material to retain quantum information,” Liu said.

But because the suboxide layer is still crystalline, “it may not be as bad as people thought,” Liu said. Perhaps the more fixed atomic arrangements in this layer will minimize the losses of the two-level system.

Again, he noted, because the underoxide layer has some metallic characteristics, it could cause other problems.

“When you put a normal metal next to a superconductor, it could help break up pairs of electrons that move through the material without resistance,” he noted. “If the pair splits into two electrons again, then you will experience a loss of superconductivity and coherence. And that’s not what you want.”

Future studies may reveal more details and strategies to prevent the loss of superconductivity and quantum coherence in tantalum.