Tunnelling across superconducting junctions proceeds by a rich variety of processes, which transfer single electrons, Cooper pairs or even larger numbers of electrons by multiple Andreev reflections. Photon-assisted tunnelling combined with the venerable Tien–Gordon model has long been a powerful tool to identify tunnelling processes between superconductors. Here, we probe superconducting tunnel junctions including an impurity-induced Yu–Shiba–Rusinov (YSR) state by exposing a scanning tunnelling microscope with a superconducting tip to microwave radiation. We find that a simple Tien–Gordon description describes tunnelling of single electrons and Cooper pairs into the bare substrate, but breaks down for tunnelling via YSR states by resonant Andreev reflections. We develop an improved theoretical description that is in excellent agreement with the data. Our results establish photon-assisted tunnelling as a powerful tool to analyse tunnelling processes at the atomic scale, which should be particularly informative for unconventional and topological superconductors.

跨超导结的隧穿通过多种过程进行，这些过程通过多次Andreev反射转移单个电子，库珀对或什至更大数量的电子。长期以来，光子辅助隧穿与古老的Tien-Gordon模型相结合一直是识别超导体之间隧穿过程的强大工具。在这里，我们通过将具有超导尖端的扫描隧道显微镜暴露于微波辐射，探查包括杂质诱导的Yu-Shiba-Rusinov（YSR）状态的超导隧道结。我们发现，一个简单的Tien-Gordon描述描述了单电子和库珀对进入裸露衬底的隧穿，但是由于共振的Andreev反射而通过YSR状态发生了隧穿而破裂。我们开发出了与数据非常吻合的改进的理论描述。我们的结果将光子辅助隧穿确立为在原子尺度上分析隧穿过程的有力工具，这对非常规和拓扑超导体尤其有用。