Gate-defined quantum devices in twisted bilayer graphene

Two-dimensional materials provide a versatile platform to engineer quantum devices via the vertical stacking of individual (atomically thin) layers on top of each other. In such a (van der Waals) heterostructure, not only the sequence and thickness of the individual layers, but also their relative orientation can be controlled with an unprecedented precision. A small misalignment between two such layers results in the formation of a long-range modulation of the local stacking configuration of the underlying crystal structure. The resulting artificial superlattice has a dramatic influence on the electronic properties of the heterostructure. A prominent example is bilayer graphene with a small twist between both layers. For specific twist angles, the electronic structure of this twisted bilayer graphene (tBLG) exhibits isolated, flat bands, associated with a dramatic reduction of the Fermi velocity. The flat bands in this purely carbon-based material lead to a multitude of strongly correlated phases, including superconductivity and interaction-induced insulating states, which are tunable by electrostatic gating. Despite this tunability, only a small number of advanced quantum devices making use of the variety of the correlated phases, such as tunnel junctions, are reported in literature. This thesis studies advanced gate-defined quantum devices in tBLG in low-temperature transport experiments. To demonstrate the versatility of the van der Waals platform, a first proof-of-principle study examines the interaction of two superlattices within the same heterostructure, and presents a method that enables to identify the presence of these superlattices from transport measurements. The second study presents a tunnelling device consisting of two independent Josephson junctions, realized in the same heterostructure. Both junctions exhibit a tunable superconducting diode effect, the origin of which can be traced back to a combination of the influence of twist angle disorder and the bulk properties of the superconducting phase in tBLG. The results thereby propose a mechanism that does not rely on many-body effects, in contrast to the suggested mechanism in a previous report on the superconducting diode effect in a tBLG junction device. In the third and fourth study, a device geometry consisting of multiple independent gate layers, is introduced. This gating scheme enables to electrostatically define constrictions or (quasi) one-dimensional channels, and to realize the worldwide first gate-defined confinement of charge carriers in two dimensions in tBLG. The study furthermore reveals that such charge confinements can be used as sensors for the gate-tunable Fermi surface of tBLG. The introduced device geometry provides a building block for future quantum devices and circuits in tBLG, with potential applications in superconducting electronics and quantum information technology.