Physicists have moved one step closer to topological quantum computing

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

Their study, titled “Induced Superconducting Correlations in a Quantum Anomalous Hall Insulator,” was published in Natural physics.

Superconductivity is a phenomenon where electricity flows without resistance in certain materials. The quantum anomalous Hall effect is another phenomenon that also causes zero resistance, but with a twist: It is confined to the edges rather than spreading through.

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

Such a combination can be achieved by inducing superconductivity at 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 the key to realizing “flying qubits” (or quantum bits) that are topologically protected.

Anjana Udayová, a final year doctoral student in the group of Professor Dr. Yoichi Ando, ​​first author of the paper, explained: “For this study, we used thin layers of a quantum anomalous Hall insulator contacted with a superconducting niobium electrode and tried to induce chiral Majorana states at its edges.

“After five years of hard work, we finally succeeded in achieving this goal: When we inject an electron into one terminal of the insulating material, it bounces off the other terminal, not as an electron, but as a hole, which is essentially a phantom. an electron with an opposite charge.

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

Gertjan Lippertz, a postdoctoral fellow in Ando’s group and co-first author of the paper, added: “Many groups have tried this experiment in the last 10 years since the discovery of the quantum anomalous Hall effect, but no one has succeeded. in it before.

“The key to our success is that the deposition of the quantum anomalous Hall insulator film, every step of the device fabrication, as well as the ultra-low temperature measurements, all take place in the same laboratory. This is not possible elsewhere.”

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

“The cluster was instrumental in providing the collaborative framework and resources necessary for this breakthrough,” said Yoichi Ando, ​​professor of experimental physics at the University of Cologne and ML4Q spokesperson.

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

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

The platform demonstrated in this study offers a promising route to achieving these goals, which may lead to more robust and scalable quantum computers.