University of Cologne experiment supports topological quantum computing theory

Physicists at the University of Cologne observed highly transmissive Andreev states in a topological-insulator Josephson junction, a system proposed for studying topological superconductivity. Using tunneling spectroscopy while varying the phase difference between two superconductors, they found states consistent with theoretical predictions across a broad range of charge-carrier densities. Under an applied magnetic field, the states also became concentrated in regions expected for Josephson vortices on the topological-insulator surface. The findings support the theoretical description of superconductivity and spin-momentum locking in this system, but do not prove the predicted energy-gap closing and reopening or a topological phase transition. The researchers say better energy and time resolution will be needed to test that transition.

Why it matters

The result strengthens the case for using topological-insulator–superconductor hybrids to investigate quantum states that could eventually underpin more disturbance-resistant qubits. However, the experiment remains evidence for the material platform and its Andreev physics, not a demonstration of a working topological qubit or confirmed topological protection.

Key points

  • The experimental work was conducted by the University of Cologne’s Topological Matter Laboratory, with theoretical collaborators at RWTH Aachen University.
  • The device was a Josephson junction combining superconductors with a nonsuperconducting topological-insulator region, and the states were measured using tunneling spectroscopy.
  • Highly transmissive Andreev states were observed across a broad range of charge-carrier densities, consistent with their proposed topological-insulator-surface origin.
  • With a magnetic field applied, the states localized in regions matching theoretical predictions for Josephson vortices.
  • The study was published in Science Advances on October 30, 2026; the possible gap closing and reopening and the associated topological phase transition remain unconfirmed.

Based on reporting by The original announcement and Phys.org Quantum.