Monolithic Superconductor - Semiconductor Quantum Circuits

Quantum computers that can be applied to a variety of practical problems finally will rely on the integration of thousands of qubits, the basic unit of quantum information. Scaling up brings challenges as large numbers of qubits must be isolated, not to interact with their environment and thereby losing the essential but fragile properties of entanglement. Solid-state implementations of physical qubits have intrinsic advantages and remarkable progress has been made using qubits based on hybrid superconductor-semiconductor heterostructures. Their properties are controlled by the band alignment of the two materials and one main challenge is to understand and control the interface and charge carrier transfer between the superconductor and the semiconductor. Aluminum has long proved ideal as a superconducting component, whereas the semiconductor germanium is increasingly becoming the focus of research. This project aims at exploring monolithic superconductor-semiconductor (Al-Ge) hybrid structures and their integration in quantum circuits and novel multi-terminal devices. With the characteristic length of actual hybrid devices being in the few nanometer region, the development of conceptually new devices is becoming increasingly important. We will realize and investigate ultra-scaled Al-Ge heterostructures using thin layers of Ge on insulator substrates by a top-down fabrication approach, which enables multi-terminal quantum devices and circuits on wafer scale. The actual monolithic superconductor-semiconductor hybrid structures will be achieved by a thermal induced Al/Ge substitution process. The quality of the monocrystalline Al layers and the in-situ formed Al-Ge interface are the most interesting aspects of the selective substitution process ensuring the formation of a clean and atomically abrupt interface that has never been in contact with ambient atmosphere. This approach gives a perfect opportunity to realize multi-terminal devices without geometric restrictions. The reproducibility of the Al-Ge substitution process ensures further quantum dots uniformity and homogeneity of the junctions even with near-unity transmission. The functionality will be demonstrated by exploring the essential building blocks, namely Josephson junction field effect transistors, Andreev molecules and multi-terminal Josephson junctions. Freestanding devices will be investigated especially with regard to the above mentioned isolation issues. Overall the proposed monolithic Al-Ge-Al heterostructure devices expand the quantum technology toolbox and provide new avenues for exploring superconductor-semiconductor quantum circuits towards scalable quantum information processing.