Andreev Processes in Graphene-based Multiterminal Josephson Devices

Multi-terminal Josephson junctions (MTJJs) have recently emerged as a versatile platform to emulate artificial Andreev band structures in arbitrary dimensions. These band structures are predicted to host rich topological phenomena, including Weyl nodes, non-trivial Berry phases, and non-zero Chern numbers. A striking feature of the MTTJs is that the emergent superconducting properties are entirely governed by the symmetry of the artificial band structure, which can be tuned using external knobs such as voltage bias, magnetic field, and superconducting phase difference. In this talk, I will present our recent experiments on ballistic graphene MTJJs. I will discuss how the intricate network of Josephson junctions gives rise to multiplet supercurrents, involving coherent transport of two or more Cooper pairs among different electrodes. Furthermore, I will demonstrate how breaking the spatial symmetry of the graphene channel allows us to engineer magnetic-field-free, two-port nonreciprocity in these devices—a unique feature with potential applications in quantum technology. Finally, I will introduce our latest results on voltage-tunable zero-\pi phase transitions in three-terminal Andreev interferometers. I will also show how the conductance of these interferometers serves as a sensitive probe of the nonequilibrium occupation of Andreev levels in ballistic systems.