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

在过去的几年里，单层石墨烯(SLG)已经成熟为一种具有良好特性的狄拉克材料，它被定为下一代固态教科书的典范。然而，最近发现了夹在所谓魔角下的双层石墨烯(BLG)的超导电性，这突显了这个特殊的2D系统仍然具有令人惊讶的力量。在这项提议中，我们打算解决的基本问题是超导SLG和‘魔莫尔’BLG作为微波器件的应用。当然，要做到这一点，有几种可能的方法。我们选择的途径是通过形成约瑟夫森结(JJ)的SLG和BLG的微波光谱来实现。众所周知，JJ是量子领域中微波电路的基本组件。由于SLG不是超导体，我们将能够对由传统金属超导体形成的超导引线进行邻近测试。这将为探索SLG-JJ在微波探测、外差混频和量子计算电路中的潜在应用奠定基础。这种SLG-JJ的根本新特征是石墨烯中载流子的狄拉克性质，即恒定的载流子速度和宽带频率检测能力，电荷中性点的态密度的奇异性，高的载流子速度，以及石墨烯层的可调载流子浓度。所有这些都有望在量子极限中实现史无前例的单光子探测。在此基础上，我们还将利用微波条件下BLG的超导电性。由此，我们期望获得IV特征中的Shapiro签名，这是推导扭曲BLG(以下标记为TBG)中超导电性的本质并测试其在量子电路中的应用的重要工具。因此，我们将能够通过微波光谱来研究TBG莫尔晶格中超导电性的复杂相互作用。然后，在TBG中插入JJ将形成一个全新的微波探测器，无需超导导线，就有可能扩大规模。

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.