March 12, 2025 -- Physicists at the Universities of Basel and Cologne have taken an important step forward in the pursuit of topological quantum computing: They observed for the first time ever the so-called Crossed Andreev Reflection (CAR) in topological insulator (TI) nanowires. They published their findings in Nature Physics.

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.

The new findings of the team headed by Professor Yoichi Ando from the University of Cologne and Professor Jelena Klinovaja from the University of Basel deepen the understanding of superconducting effects in topological insulator nanowires paired with a conventional superconductor.

Nonlocal effect

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.

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

This breakthrough was enabled by an innovative fabrication approach developed by Dr. Junya Feng in the Ando group at the University of Cologne and the theoretical analysis provided by Dr. Henry Legg in the Klinovaja group at the University of Basel. This work opens the door for using non-local transport as probe of exotic proximity-induced superconductivity also in other hybrid platforms.

Research on topological qubits in Basel

“The theoretical understanding we gained in this work about the CAR processes will also be of crucial importance for our research goals in our WSS Research Center for Molecular Quantum Systems,” says Prof. Jelena Klinovaja. “The work provides a solid stepping stone for topological qubits and we are very excited about this progress.”