Physicists of the Budapest University of Technology and Economics (BME) and the University of Basel discovered a surprising result on the extension of Yu-Shiba-Rusinov states. Their joint studies appeared in Nature Communications.

Recently several promising novel qubit concepts have been put forward that are based on low-energy states, appearing in nanoscaled devices that are proximitized by a superconductor (SC). There are several flavours of these states, known as Andreev bound state (ABS), Yu-Shiba-Rusinov (YSR) state and Majorana bound state (MBS). It was predicted that one can build a protected quantum state with an array of YSR states that overlapp. However, the spatial extension was found to be short, so that one could only realize such an object by a very delicate technique of depositing individual ferromagnetic atoms next to each other.

The MTA-BME Momentum Nanoelectronics Research group and the Nano- and Quantum-Electronics group of the University of Basel studied an alternative realization of YSR states, when instead of an atom a quantum dot (QD) is attached to a superconductor surface. The QD which acts as an artificial atom. Measurements of the extension of such a YSR state resulted in a surprising outcome. The size of these states are significantly larger than for real ferromagnetic atoms, their dimensions is in the range of 50-200nm! This opens a way to engineer a YSR chains.

This work was done in collaboration with the Department of Theoretical Physics at BME.  Our joint publication appeared in Nature Communications.


Measurement of the spatial extension of YSR state By coupling an artificial atom (QD) to a superconductor (SC), a Yu-Shiba-Rusinov state (blue), located between the QD and the SC, forms. The YSR state is detected by current measurement (IT) with a weakly coupled tunnel probe attached to the other side of the SC. Despite the width of 200nm of the SC, the YSR state was observed by the tunnel probe.
Large extendion of Yu-Shiba-Rusinov state