Loss and decoherence at the quantum Hall - superconductor interface

High quality type-II superconducting contacts have recently been developed to a variety of 2D systems, allowing one to explore the superconducting proximity in the quantum Hall (QH) regime. Inducing superconducting correlations into a chiral system has long been viewed as a route for creating exotic topological states and excitations. However, it appears that before these exciting predictions could be realized, one should develop a better understanding of the limitations imposed by the physics of real materials. Here, we perform a systematic study of Andreev conversion at the interface between a superconductor and graphene in the QH regime. We find that the probability of Andreev conversion of electrons to holes follows an unexpected but clear trend: the dependencies on temperature and magnetic field are nearly decoupled. We discuss these trends and the role of the superconducting vortices, whose normal cores could both absorb and dephase the individual electrons in a QH edge. Our study may pave the road to engineering future generation of hybrid devices for exploiting superconductivity proximity in chiral channels

Dynamical Stabilization of Multiplet Supercurrents in Multi-terminal Josephson Junctions

The dynamical properties of multi-terminal Josephson junctions have recently attracted interest, driven by the promise of new insights into synthetic topological phases of matter and Floquet states. This effort has culminated in the discovery of Cooper multiplets, in which the splitting of a Cooper pair is enabled via a series of Andreev reflections that entangle four (or more) electrons. In this text, we show conclusively that multiplet resonances can also emerge as a consequence of the three terminal circuit model. The supercurrent appears due to the correlated phase dynamics at values that correspond to the multiplet condition $nV_1 = -mV_2$ of applied bias. The emergence of multiplet resonances is seen in i) a nanofabricated three-terminal graphene Josephson junction, ii) an analog three terminal Josephson junction circuit, and iii) a circuit simulation. The mechanism which stabilizes the state of the system under those conditions is purely dynamical, and a close analog to Kapitza's inverted pendulum problem. We describe parameter considerations that best optimize the detection of the multiplet lines both for design of future devices. Further, these supercurrents have a classically robust $\cos2\phi$ energy contribution, which can be used to engineer qubits based on higher harmonics