Cologne Physicists See Quantum States Behaving as Theory Predicts in a Topological Qubit Candidate
The Science Advances experiment supports a 2008 model for how superconductivity meets a topological insulator's surface, though the key phase transition is still unconfirmed.
Physicists at the University of Cologne have observed quantum states that behave just as theory predicts in a material system considered promising for topological quantum computing, an approach meant to make quantum information far less prone to errors. The results, published in Science Advances under the title "Tunneling spectroscopy of topological insulator Josephson junctions," back a long-standing description of how superconductivity interacts with the surface of a topological insulator.
Topological quantum computing relies on special quantum states that are expected to resist local disturbances, the small bumps of noise that wreck the delicate states in today's qubits. Topological insulators are materials that block electric current in their interior but carry it along their surfaces. When one is paired with a conventional superconductor, the surface is predicted to host a special form of superconductivity. If realized, it could support Majorana zero modes, exotic quantum states being investigated as the basis for topological qubits. Producing and controlling them has proved very hard.
To test the prediction, Dr. Jakob Schluck, Ella Nikodem and colleagues in the Topological Matter Laboratory Cologne, led by Professor Yoichi Ando, built a Josephson junction: two superconductors joined by a region that is not superconducting. Using tunneling spectroscopy, they probed the quantum states inside while varying the phase difference between the two superconductors.
They saw highly transmissive Andreev states, which form when electrons and holes interact at a superconducting junction. The states appeared across a range of charge-carrier densities, matching the predicted behavior of electrons on the topological insulator's surface. When the team applied a magnetic field, the states concentrated in specific regions of the junction, again as theory expects. The data agree with the model Liang Fu and Charles Kane proposed in 2008 and with numerical simulations by the group of Professor Fabian Hassler at RWTH Aachen University.
"Understanding how superconductivity interacts with the distinctive electronic properties of topological insulators is essential for assessing their potential for topological quantum computing," Ando said. Nikodem said it was "super exciting to see our experimental results align so well with theoretical predictions," calling the match encouraging for future quantum computing applications.
The researchers are careful about what they have not shown. The central question remains open: whether the observed states undergo the predicted closing and reopening of an energy gap. That change would signal the transition into topological superconductivity, the phase in which certain quantum states may be protected against local disturbances and which is a main reason the approach is seen as promising for robust qubits. In the current experiment the transition has not been confirmed.
"To demonstrate the topological phase transition in this system, we need to improve the experiment's energy and time resolution," Schluck said, calling that the next step on the Ando lab's roadmap. For now, the work gives the field firmer ground. It shows that the basic physics of this material combination behaves as the theory says, which is a necessary check before anyone can claim to have built a protected qubit.
