Researchers at the University of Cologne have observed quantum states that behave as predicted for a material system investigated as a route to topological quantum computing, an approach in which information would be less sensitive to local disturbances.
What was measured
The team at the Topological Matter Laboratory Cologne, led by Professor Yoichi Ando, studied a Josephson junction, formed by two superconductors connected by a nonsuperconducting region. Using tunneling spectroscopy, the researchers probed the states inside the junction while varying the phase difference between the superconductors.
Topological insulators do not conduct electricity in their interior but conduct along their surfaces. Combined with a conventional superconductor, their surface states are candidates for hosting topological superconductivity, a possible route to Majorana zero modes, states investigated for use in topological qubits.
The work found highly transmissive Andreev states, which form when electrons and holes interact at a superconducting junction. They appeared across a range of charge-carrier densities, in line with the predicted behaviour of electrons at the topological insulator surface, and, under a magnetic field, concentrated in specific regions of the junction, as theory predicts.
Confirmation and the limit of the result
The results agree with the model proposed by Liang Fu and Charles Kane in 2008 and with simulations by Professor Fabian Hassler's group at RWTH Aachen University. The study was published in Science Advances as “Tunneling spectroscopy of topological insulator Josephson junctions”.
One point remains open, however: the transition to topological superconductivity has not been confirmed. Demonstrating it will require more precise measurements of the closing and reopening of the energy gap. According to researcher Jakob Schluck, improving energy and time resolution is the Ando lab's next step.
Fontes
- phys.org: Experiment supports theoretical foundation for potentially more error-resistant quantum computing
- Science Advances (2026), DOI: 10.1126/sciadv.aeg0687
Image credit: Jakob Schluck / Topological Matter Laboratory Cologne, via phys.org.
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