Optimized quantum computer stability from a new topological superconductor

Researchers at Penn State have pioneered a state-of-the-art topological superconductor that boosts quantum computer stability – a major limitation of the technology.

The team has pioneered a new method for fusing two materials – a single-layer superconductor and a topological insulator – with special electrical properties. The combination provides an optimal platform for investigating an unusual type of superconductivity known as topological superconductivity, which could pave the way for topological quantum computers that are significantly more stable than conventional technology.

The team’s research, titled “Crossover from Ising- to Rashba-type superconductivity in epitaxial Bi₂Se₃/monolayer NbSe₂ heterostructures”, is published in Materials of Nature.

Topological superconductor development

Superconductors are used in a variety of technologies, including powerful magnets, digital circuits, and imaging. They allow electrical 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 combine 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 superconductor, but the actual process of combining these two materials is challenging.

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

Previous attempts to fuse the two materials have yielded poor results, as superconductivity in thin films usually disappears after the topological insulating layer is grown on top. The experts were able to apply a topological insulating film to a three-dimensional “bulky” superconductor and preserve the properties of both materials. However, applications for topological superconductors, such as low-power chips for smartphones and quantum computers, will need to be 2D.

The researchers overcame these problems to innovate a two-dimensional topological superconductor by stacking a topological insulating film of bismuth selenide (Bi2Se3) with different thicknesses on a superconductor film composed of monolayer niobium diselenide (NbSe2). The team successfully preserved the topological and superconducting properties by synthesizing the heterostructures at a much lower temperature.

Hemian Yi, a postdoctoral scholar in the Chang Research Group at Penn State and first author of the paper, explained: “In superconductors, electrons form ‘Cooper pairs’ and can flow with zero resistance, but a strong magnetic field can break these the pairs.

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

Solving quantum computer stability issues

The researchers found that by varying 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 may be an ideal platform for exploring Majorana fermions—an enigmatic particle that would help develop a topological quantum computer 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 ongoing work. Once we have solid evidence of topological superconductivity and demonstrate Majorana physics, then this type of system could be adapted for quantum computing and other applications.”