Interacting Few Level Systems at the Atomic Scale

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The scanning tunneling microscope (STM) has played an important role as a supporting technology in the context of sensing and material science with its atomic precision and excellent resolution in energy when operated at milli-Kelvin (mK) temperatures. However, it has until now not been able to directly access the properties of quantum coherence in target few level systems, which can manifest in different ways. We will characterize, theoretically describe, and further develop an STM tip functionalized with a single quantum state well-protected inside a superconducting gap, which is extremely sharp in energy. More specifically, a magnetic impurity is added to the apex of the STM tip to induce a bound in-gap Yu-Shiba-Rusinov (YSR) state. This state is a collective state in a correlated many-body system with a lifetime only limited by interactions with residual environmental degrees of freedom. Our general goal is to understand such an STM tip state coupled to target few level systems, to the superconducting hosts, and their surrounding environment. We want to directly access coherence properties of the resulting many-body quantum system and implement coherent manipulation schemes. Furthermore, we want to explore the unique features of the YSR-STM as means for sensing exotic quantum states. Our team is a well-established collaboration between experiment and theory, with an extensive record of joint publications. It brings together vast expertise in the physics and experimental techniques of mK-STM and theoretical expertise in superconducting quantum transport in mesoscopic devices.