Semiconducting nanowire-based Josephson junctions for qubits

The goal of this thesis is to study the photon-mediated coupling between two distant Andreev pair qubits. Ch. 2 presents the theoretical concepts of Andreev bound states in semiconducting nanowire-based Josephson junctions (NWJJs), followed by the introduction to the implementation of Andreev qubits using full-shell epitaxial InAs/Al nanowires. The control and readout of Andreev qubits presented in this thesis rely on the coupling to a superconducting microwave resonator. Ch. 3 provides an overview of the qubit-resonator coupling and concludes the theory part of this thesis. Ch. 4 presents the DC electrical characterization of full-shell epitaxial InAs/Al nanowire Josephson junctions. The electrical performance of these nanowire devices is evaluated in terms of junction transparencies. The extracted high transparency parameters encourage us to further proceed to fabricate the microwave chip. Ch. 5 thoroughly discusses the design considerations for the superconducting cavity coupler for Andreev qubits. From the experimental requirements, we derive design rules for each relevant electronic component, which are checked using numerical simulation tools. We then combine the knowledge about our full-shell epitaxial InAs/Al nanowire Josephson junctions and superconducting cavity coupler. In Ch. 6, we demonstrate the operation of the cavity coupler as a microwave spectroscopy tool and qubit state detector for individual NWJJ devices. In Ch. 7, we simultaneously couple both Andreev pair qubits to the cavity and demonstrate distant, strong coupling of two Andreev pair qubits. With the demonstration of the capability of our superconducting cavity coupler to entangle two Andreev pair qubits, we close the investigation of qubits formed with epitaxial InAs/Al nanowires. In Ch. 8, we motivate the use of Ge/Si core/shell nanowire Josephson junctions and demonstrate coherent manipulation of a superconducting gatemon qubit, that manifests Josephson supercurrent in these nanowire devices. In Ch. 9, we summarize the important experimental techniques, which are the backbone of the presented thesis, helping the readers to understand and reproduce the experiments. This thesis is concluded in Ch. 10 with a brief outlook of possible future experiments.