Cologne experiment offers a clue for future topological qubits
· BehindLines newsroom
Quantum states measured in Cologne provide new support for an approach intended to make future quantum computers less sensitive to disturbances. In an announcement published on 2 November, the university describes an experiment whose results match several theoretical predictions. However, the transition being sought towards so-called topological superconductivity still needs to be established. That qualification defines the scope of the finding and is essential to understanding what the measurements show.
Yoichi Ando's team studied a Josephson junction, a device in which two superconductors are connected by a region that is not itself superconducting. The material between them is a topological insulator: its interior blocks electrical current, while its surface conducts it. The researchers explored the states present in this junction using tunnelling spectroscopy, varying the phase difference between the two superconductors.
Their measurements reveal highly transmissive Andreev states, associated with interactions between electrons and holes in the device. These states persist at different charge-carrier densities. Under a magnetic field, they also become concentrated in particular parts of the junction. The observations agree with the model proposed by Liang Fu and Charles Kane in 2008, as well as simulations by collaborators at RWTH Aachen University.
The significance goes beyond identifying unusual states in a material. The topological approach seeks to use properties that might protect quantum information against certain local disturbances. Assessing that prospect requires several stages to be distinguished: observing an expected behaviour, confirming the physical regime that explains it, then demonstrating functions useful for storing or processing information. The announced experiment sheds light on the first of these stages.
The next check concerns the closing and reopening of an energy gap, an expected signature of the change in regime. The team believes it must improve the energy and time resolution of its measurements to confirm this. Presented in Science Advances, the work therefore provides an experimental foundation for further investigation. It leaves open the decisive question of whether information is effectively protected, on which an application to topological qubits would depend.