A 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 π-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 π 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.

约瑟夫森结中的单个自旋可以通过改变超导相位差的符号来反转超电流的流动。在介观长度尺度上，这些 π 结被用于各种应用，例如寻找底层超导体的配对对称性以及量子计算。在原子尺度上，超导隧道结中单个自旋的对应物被称为 Yu-Shiba-Rusinov 状态。迄今为止，在这种情况下观察到超电流逆转仍然难以捉摸。在这里，我们展示了约瑟夫森结通过 Yu-Shiba-Rusinov 状态的这种 0 到 π 跃迁，因为我们不断改变杂质 - 超导体耦合。我们通过利用第二个传输通道作为类似于超导量子干涉装置的参考来检测临界电流的符号变化，这为我们的扫描隧道显微镜提供了所需的相位灵敏度。测量到的约瑟夫森电流变化是量子相变的标志，并允许以高分辨率对其进行表征。