Karan, SujoyHuang, HaonanPadurariu, CiprianKubala, BjoernTheiler, AndreasBlack-Schaffer, Annica M.Morras, GonzaloLevy Yeyati, AlfredoCarlos Cuevas, JuanAnkerhold, JoachimKern, KlausAst, Christian R.2022-07-182022-07-182022-07-182022-07-0410.1038/s41567-022-01644-6https://infoscience.epfl.ch/handle/20.500.14299/189393WOS:000820553300001A single spin in a Josephson junction can reverse the flow of the supercurrent by changing the sign of the superconducting phase difference across it. At mesoscopic length scales, these pi-junctions are employed in various applications, such as finding the pairing symmetry of the underlying superconductor, as well as quantum computing. At the atomic scale, the counterpart of a single spin in a superconducting tunnel junction is known as a Yu-Shiba-Rusinov state. Observation of the supercurrent reversal in that setting has so far remained elusive. Here we demonstrate such a 0 to pi transition of a Josephson junction through a Yu-Shiba-Rusinov state as we continuously change the impurity-superconductor coupling. We detect the sign change in the critical current by exploiting a second transport channel as reference in analogy to a superconducting quantum interference device, which provides our scanning tunnelling microscope with the required phase sensitivity. The measured change in the Josephson current is a signature of the quantum phase transition and allows its characterization with high resolution.Continuously changing the coupling between a magnetic impurity and a superconductor allows the observation of the reversal of supercurrent flow at the atomic scale.Physics, MultidisciplinaryPhysicslocal electronic-structurejosephson currentcoulomb-blockadepairing symmetryimpuritystatestext::journal::journal article::research article