Quantum computer stability optimized by new topological superconductor

Penn State researchers have pioneered a cutting-edge topological superconductor that improves the stability of quantum computers — a key limitation of the technology.

The team has developed a novel method to fuse two materials – a monolayer superconductor and a topological insulator – with special electrical properties. The combination provides an optimal platform to study an unusual mode of superconductivity known as topological superconductivity, which could pave the way for topological quantum computers that are much more stable than current technology.

The team’s research, entitled “Crossover from Ising- to Rashba-type supraconductivity in epitaxial Bi₂se₃/Monolayer NbSe₂ heterostructures’, published in natural materials.

Development of a topological superconductor

Superconductors are used in various technologies, including powerful magnets, digital circuits, and imaging. They allow the electric current to pass without resistance. In contrast, topological insulators are thin films, only a few atoms thick, that restrict the movement of electrons, resulting in unique properties. Penn State researchers have now developed a way to pair the two materials.

Cui-Zu Chang, associate professor of physics at Penn State and leader of the research team, commented, “The future of quantum computing depends on a type of material we call a topological superconductor, which can be formed by combining a topological insulator with a superconductors, but the actual process of combining these two materials is challenging.

“In this study, we used a technique called molecular beam epitaxy to synthesize both topological insulator and superconductor films, creating a two-dimensional heterostructure that provides an excellent platform for exploring the phenomenon of topological superconductivity.”

Previous attempts to fuse the two materials have yielded poor results because superconductivity in thin films usually disappears after the topological insulator layer has been grown on top. Experts have been able to apply a topological insulator film to a three-dimensional “bulk” superconductor and retain the properties of both materials. However, applications for topological superconductors, such as low-power chips for smartphones and quantum computers, would need to be two-dimensional.

Researchers overcame these problems to develop a two-dimensional topological superconductor by stacking a bismuth selenide (Bi2Se3) topological insulator film of different thicknesses onto a superconductor film composed of a single-layer niobium diselenide (NbSe2). The team successfully retained the topological and superconducting properties by synthesizing the heterostructures at a very lower temperature.

Hemian Yi, a postdoctoral fellow in the Chang Research Group at Penn State and first author of the paper, explained: “In superconductors, electrons form ‘Cooper pairs’ and can flow without resistance, but a strong magnetic field can break up these pairs.

“The monolayer superconductor film we used is known for its ‘Ising superconductivity’, which means that the Cooper pairs are very robust to in-plane magnetic fields. We would also expect the topological superconducting phase formed in our heterostructures to be robust in this way.”

Solving problems with the stability of quantum computers

The researchers discovered that by adjusting the thickness of the topological layer, the heterostructure shifted from Ising-type superconductivity (where the electron spin is perpendicular to the film) to Rashba-type superconductivity (where the electron spin is parallel to the film). They also observe this phenomenon in their theoretical calculations and simulations.

This heterostructure could be an ideal platform for studying Majorana fermions – an enigmatic particle that would help develop a topological quantum computer that’s more stable than previous versions of the technology.

Change concluded: “This is an excellent platform for exploring topological superconductors and we hope to find evidence of topological superconductivity in our further work. Once we have solid evidence for topological superconductivity and demonstrate Majorana physics, this type of system could be adapted for quantum computing and other applications.”