Physicists from the University of Cologne have for the first time experimentally observed quantum states that behave exactly as theory predicts for one of the most promising materials in the field of quantum computing. The results were published in the journal Science Advances.
The material in question is topological insulators - materials that do not conduct electric current inside but do conduct on the surface. If such a material is joined to an ordinary superconductor, theory says a special form of superconductivity, topological superconductivity, should arise on its surface. It is this that is considered one of the routes to creating so-called Majorana states (quantum states that could theoretically become the basis for error-resistant qubits - quantum bits).
To test the predictions, a group led by Professor Yoichi Ando from the laboratory for topological matter in Cologne studied a structure called a "Josephson junction". This consists of two superconductors connected by a region that does not possess superconductivity. Using tunneling spectroscopy (a method that allows researchers to "look into" the energy states of electrons), the researchers varied the phase difference between the superconductors and monitored the quantum states inside the junction.
The scientists observed so-called Andreev states with high transparency - they arise from the interaction of electrons and holes (quasiparticles that behave like positive charges) at the boundary of the superconductor. These states appeared across a wide range of charge carrier concentrations, which matches the predicted behavior of electrons on the surface of a topological insulator. And when a magnetic field was applied, the states concentrated in certain regions of the junction - again, just as theory predicts.
"Understanding how superconductivity interacts with the unique electronic properties of topological insulators is necessary to assess their potential in topological quantum computing," Ando was quoted as saying in a university press release.
The data obtained are consistent with the theoretical model proposed by Liang Fu and Charles Kane in 2008, as well as with numerical calculations by the group of Professor Fabian Hassler at RWTH Aachen University. "It is very exciting to see that our experiments match theory so well. This gives hope for future applications in quantum computing," added co-author Ella Nikodem.
However, the key question remains open: do the observed states undergo the predicted closing and reopening of the energy gap? Such a jump could signal a transition to topological superconductivity, in which quantum states are protected from local disturbances. It is this protection that makes topological quantum computing so attractive. In the current experiment the transition has not yet been confirmed - more precise measurements are needed for that. "To demonstrate the topological phase transition in this system, we need to improve the energy and time resolution of the experiment. This is the next step in the plans of Ando's laboratory," explained the first author of the work, Jakob Schluck.
Thus, the work confirms the theoretical basis but does not prove that topological protection has already been achieved. Nevertheless, the physicists call the results an important step toward creating quantum computers that are more resistant to errors.