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Hybrid materials made of magnets and superconductors give rise to fascinating quantum phenomena, which are so sensitive that it is crucial to measure them with minimal interference. Researchers at the University of Hamburg and the University of Illinois Chicago have now demonstrated, both experimentally and theoretically, how these quantum phenomena can be detected and controlled over longer distances using special techniques with a scanning tunneling microscope. Their findings, which could be important for topological quantum computers, were published in the journal Nature Physics.
When a magnetic atom is located in a superconductor, so-called Yu-Shiba-Rusinov quasiparticles are created. Normally, they can only be measured with a high detection probability directly at the location of the atom using the tip of a scanning tunneling microscope. A team led by Dr. Jens Wiebe from the Institute for Nanostructure and Solid State Physics at the University of Hamburg and the University of Illinois Chicago has now succeeded in measuring this quantum state over distances more than twenty times its original extent, thereby minimizing interference caused by the measuring probe.
The researchers confined the magnetic atom inside a corral, that was built using the tip of a scanning tunnelling microscope from 91 silver atoms on the surface of a superconducting silver crystal. They precisely dimensioned this quantum corral so that one of the quantum states of the silver electrons confined within it was exactly at the Fermi energy. For the corral shown in the image, this was a state with four antinodes. The researchers positioned the magnetic atom on the leftmost antinode and determined the spatial distribution of the differential conductance at the energy of the Yu-Shiba-Rusinov quasiparticle as a measure of its local probability density.
“Remarkably, the Yu-Shiba-Rusinov quasiparticle could still be detected even at the rightmost 'belly' of the corral state - the point farthest from the magnetic atom - without its probability of occurrence decreasing noticeably with the distance from the atom”, says Jens Wiebe, who is a researcher in the Cluster of Excellence 'CUI: Advanced Imaging of Matter'. The researchers observed the same effect in simulations involving a tight binding model. Comparing these with measurements revealed that the observed phenomenon is a spatially coherent quantum state consisting of Cooper pairs both within the bulk and on the surface of the silver crystal. Moreover, the team demonstrated that the quantum composition of this Yu-Shiba-Rusinov projection - its particle and hole content - can be manipulated by adjusting the size and shape of the quantum corral.
This technique enables the measurement of the fragile quantum states of magnetic superconductor hybrids using a local probe, while minimizing the disruptive influence of the probe. The researchers now intend to apply this technique to Majorana quasiparticles in the future, as these have great potential for the development of novel topological quantum computers. Additionally, quantum corrals could potentially be used to control the interactions between quasiparticles in multiple magnet-superconductor hybrids.
Dr. Jens Wiebe
Institut for Nanostructure and Solid States Physics
University of Hamburg
E-Mail: jens.wiebe@uni-hamburg.de
K. T. Ton, C. Xu, I. Ioannidis, L. Schneider, T. Posske, R. Wiesendanger, D. K. Morr & J. Wiebe
Non-local detection of coherent Yu-Shiba-Rusinov quantum projections
Nature Physics (2025)
DOI: 10.1038/s41567-025-03109-y
https://www.cui-advanced.uni-hamburg.de/en/research/wissenschaftsnews/25-11-26-q...
A magnetic atom was positioned at the leftmost antinode of a quantum corral (bottom). The red color ...
Source: UHH/Wiebe
Copyright: UHH/Wiebe
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