Microwave spectroscopy of Josephson-Junctions defined from single layer, bilayer, and trilayer graphene [MEGA-JJ]

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During the last couple of years single layer graphene (SLG) matured to a well characterized Dirac-material, which was set for being the poster child of the next generation solid-state textbooks. However, the very recent discovery of superconductivity in bilayer graphene (BLG), sandwiched under a so-called magic angle, underlines that this particular 2D system still has the power to surprise. The fundamental question we intend to address in this proposal is the application of superconducting SLG and ‘magic-moiré’ BLG as microwave devices. Naturally, there are several possible approaches in order to do this. The path we chose to accomplish this is via microwave spectroscopy of SLG and BLG forming Josephson junctions (JJs). As is well known JJs are essential components of microwave circuits in the quantum realm. Since SLG is not superconducting we will be able to conduct proximity tests with superconducting leads formed by conventional metallic superconductors. This will set the stage for probing the potential application of SLG-JJs for microwave detection, heterodyne mixing, and quantum computing circuits. The fundamentally new features of such SLG-JJs are the Dirac-nature of charge carriers in graphene, i.e., constant charge carrier velocity and hence broadband frequency detection ability, the singularity in the density of states at the charge neutrality point, the high carrier velocity, and the tunable carrier concentration of the graphene layers. All this promises unprecedented single-photon detection in the quantum limit. In extension to this we will also make use of superconductivity in BLG in the microwave regime. From this we expect to obtain Shapiro-signatures in the IV-characteristics, which are an essential tool for deriving the very nature of superconductivity in twisted BLG (labeled in the following as TBG) and testing its application for quantum circuits. Consequently, we will be able to investigate the intricate interplay of superconductivity in TBG moiré lattices via microwave spectroscopy. Inserting a JJ in the TBG will then form a fundamentally new microwave detector with potential for up-scaling without the need of superconducting leads.