Physicists move one step closer to topological quantum computing

A team of experimental physicists led by the University of Cologne has shown that it is possible to create superconducting effects in special materials known for their unique edge-only electrical properties. This discovery offers a new way to explore advanced quantum states that could be crucial for the development of stable and efficient quantum computers.

Their research, titled “Induced superconducting correlations in a quantum anomalous Hall insulator”, is published in Physics.

Superconductivity is a phenomenon in which electricity flows through certain materials without resistance. The quantum anomalous Hall effect is another phenomenon that also causes zero resistance, but with a twist: it is confined to the edges instead of spreading everywhere.

The theory predicts that a combination of superconductivity and the quantum anomalous Hall effect will lead to the formation of topologically protected particles called Majorana fermions, potentially revolutionizing future technologies such as quantum computers.

Such a combination can be achieved by inducing superconductivity in the edge of a quantum anomalous Hall insulator that is already resistance-free. The resulting chiral Majorana edge state, which is a special type of Majorana fermions, is a key to realizing “flying qubits” (or quantum bits) that are topologically protected.

Anjana Uday, a final year PhD researcher in the group of Professor Dr. Yoichi Ando and the first author of the paper, explains: “In this study, we used thin films of the quantum anomalous Hall insulator, which is in contact with a superconducting niobium electrode, and tried to induce chiral Majorana states at their edges.

“After five years of hard work, we were finally able to achieve this goal: when we inject an electron into one of the ends of the insulator material, it is reflected at the other end, not as an electron, but as a hole. That is in fact a ghost image of an electron with an opposite charge.

“We call this phenomenon crossed Andreev reflection and it allows us to detect the induced superconductivity in the topological edge state.”

Gertjan Lippertz, a postdoctoral researcher in the Ando group and co-first author of the paper, added: “This experiment has been attempted by many groups over the past 10 years since the discovery of the quantum anomalous Hall effect, but no one has succeeded before.

“The key to our success is that the film deposition of the quantum anomalous Hall insulator, every step of device fabrication, and ultra-low temperature measurements are all performed in the same lab. This is not possible anywhere else.”

To achieve these results, the Cologne group collaborated with colleagues from KU Leuven, the University of Basel and Forschungszentrum Jülich. The latter provided theoretical support within the joint Cluster of Excellence Matter and Light for Quantum Computing (ML4Q).

“The Cluster has played an important role in providing the collaborative framework and resources needed for this breakthrough,” said Yoichi Ando, ​​professor of experimental physics at the University of Cologne and spokesperson for ML4Q.

This discovery opens up numerous avenues for future research. Next steps include experiments to directly confirm the emergence of chiral Majorana fermions and to clarify their exotic nature.

Understanding and exploiting topological superconductivity and chiral Majorana edge states could revolutionize quantum computing by providing stable qubits that are less susceptible to decoherence and information loss.

The platform demonstrated in this study offers a promising way to achieve these goals and could potentially lead to more robust and scalable quantum computers.