Experimental physicists at the University of Cologne have demonstrated that it is feasible to induce superconducting effects in certain materials that are distinguished by their peculiar edge-only electrical characteristics. This finding offers a fresh perspective on complex quantum states, which may be essential for creating reliable and effective quantum computers. Their work has been published in the journal Nature Physics.
Image Credit: University of Cologne
In some materials, a phenomenon known as superconductivity causes electricity to flow through them without any resistance. The quantum anomalous Hall effect, which similarly results in zero resistance, differs in that it only occurs at the edges rather than spreading throughout.
According to theory, superconductivity and the quantum anomalous Hall effect will produce particles known as Majorana fermions that are topologically protected and have the potential to transform future technology like quantum computers completely. One way to accomplish this combination is to induce superconductivity in the edge of a resistance-free quantum anomalous Hall insulator.
The realization of topologically protected “flying qubits,” or quantum bits, depends on the chiral Majorana edge state that results from this process. Majorana fermions are a particular kind of Majorana fermions.
Anjana Uday, a final-year doctoral researcher in the group of Professor Dr. Yoichi Ando and the Study's First Author explained, “For this study, we used thin films of the quantum anomalous Hall insulator contacted by a superconducting Niobium electrode and tried to induce chiral Majorana states at its edges.”
Anjana Uday said, “After five years of hard work, we were finally able to achieve this goal: When we inject an electron into one terminal of the insulator material, it reflects at another terminal, not as an electron but as a hole, which is essentially a phantom of an electron with opposite charge. We call this phenomenon crossed Andreev reflection, and it enables us to detect the induced superconductivity in the topological edge state.”
This experiment has been tried by many groups in the past ten 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 film deposition of the quantum anomalous Hall insulator, every step of device fabrication, as well as ultra-low-temperature measurements, are all done in the same lab. This is not possible elsewhere.
Gertjan Lippertz, Study Co-First Author and Lippertz Postdoctoral Fellow, University of Cologne
The Cologne group made these discoveries in conjunction with colleagues from Forschungszentrum Jülich, the University of Basel, and KU Leuven. The latter provided theoretical support within the collaborative Cluster of Excellence Matter and Light for Quantum Computing (ML4Q).
The Cluster has been instrumental in providing the collaborative framework and resources necessary for this breakthrough.
Yoichi Ando, Professor, Department of Experimental Physics, University of Cologne
Ando is also a spokesperson of ML4Q.
This discovery opens up numerous directions for future investigation. The next stages involve experiments to explicitly validate the appearance of chiral Majorana fermions and clarify their unusual nature.
Gaining knowledge of and using chiral Majorana edge states and topological superconductivity could transform quantum computing by enabling stable qubits that are less prone to information loss and decoherence.
The platform presented in this work provides a viable route to accomplishing these objectives, and it may eventually result in more reliable and scalable quantum computers.
Journal Reference:
Uday, A., et al. (2024) Induced superconducting correlations in a quantum anomalous Hall insulator. Nature Physics. doi.org/10.1038/s41567-024-02574-1.