Topological insulator nanowires reveal superconducting effect, bringing topological quantum computing closer to reality

Physicists at the University of Cologne have taken an important step forward in the pursuit of topological quantum computing by demonstrating the first-ever observation of Crossed Andreev Reflection (CAR) in topological insulator (TI) nanowires.

This finding, published under the title “Long-range crossed Andreev reflection in topological insulator nanowires proximitized by a superconductor” in Nature Physics, deepens our understanding of superconducting effects in these materials, which is essential for realizing robust quantum bits (qubits) based on Majorana zero-modes in the TI platform—a major goal of the Cluster of Excellence Matter and Light for Quantum Computing (ML4Q).

Quantum computing promises to revolutionize information processing, but current qubit technologies struggle with maintaining stability and error correction. One of the most promising approaches to overcoming these limitations is the use of topological superconductors, which can host special quantum states called Majorana zero-modes.

These exotic states have been theoretically predicted to provide an inherently stable foundation for quantum computation, immune to many common sources of error. The experimental observation of these states, however, remains controversial despite many optimistic claims.

In this latest study, Junya Feng, postdoctoral fellow in the Topological Matter Laboratory Cologne (TMLC), led by Professor Dr. Yoichi Ando explored topological insulator (TI) nanowires—materials that, when paired with a conventional superconductor, are predicted to support topological superconductivity more easily than other materials.

The team successfully observed Crossed Andreev Reflection (CAR), a rare quantum effect where an electron injected at one terminal of a nanowire “pairs up” with another distant electron, forming a superconducting Cooper pair. This nonlocal effect is a key signature of the long-range superconducting correlations that are a prerequisite for Majorana-based qubits.

“This is the first study to explore how Andreev physics works in topological insulator nanowires when they are coupled to superconductors. Understanding this is crucial for the robust creation of Majorana zero-modes in the TI platform,” said Professor Ando.

“This breakthrough was enabled by Junya’s innovative fabrication approach: etching high-quality nanowires from exfoliated topological insulator flakes. This method produced much cleaner structures than previous techniques, making them ideal for quantum experiments.

“Thanks to this progress, we are now performing experiments that were previously only possible with conventional semiconductor nanowires. We have also decided to focus entirely on this new fabrication method, as it has led to a series of exciting results. With these advances, the ML4Q Cluster is moving a step closer to making a topological qubit a reality.”

The ability to reliably induce and control superconducting correlations in TI nanowires is a crucial step toward engineering Majorana-based qubits based on the TI platform. Next steps will focus on directly observing and controlling Majorana zero-modes in these systems—an essential milestone toward fault-tolerant quantum computing.

To achieve these results, the Cologne group collaborated with theorists at the University of Basel who helped understand the peculiar way the Andreev physics works in TI nanowires.