Circuit Quantum Electrodynamics in a Magnetic Field

Quantum computers promise to solve certain problems such as quantum chemistry simulations much more efficiently than their classical counterparts. Although it is still unclear what material system will ultimately host large-scale quantum computers, solid-state systems are promising candidates due to their inherent scalability and advanced fabrication techniques that can be adapted from comparable technologies. Crucially, a future quantum computer will
depend on the quality of its most fundamental building block, the quantum bit, or qubit. Qubits, although ideally insensitive to potential noise, are very susceptible to slight changes in their environment. Therefore, they do not only make the building block for quantum computers, but are also precise sensors.

One of the most studied solid-state implementations of a qubit is the transmon, a weakly anharmonic oscillator based on superconducting capacitive and nonlinear inductive elements. Typically, Al-AlOx-Al superconductor-insulator-superconductor Josephson junctions are used for the latter. The interaction of the transmon with the control circuitry, typically superconducting resonators, is described by circuit quantum electrodynamics. In this PhD thesis, a more recently demonstrated type of qubit is further developed and studied in detail using circuit quantum electrodynamics. In these qubits, the Josephson element of the transmon is replaced with indium arsenide nanowires, forming a superconductor-normal metalsuperconductor junction. In addition to the standard flux tunability, these qubits can also be voltage tuned. Due to the compatibility of all the materials used with an applied magnetic field, this type of qubit is a good candidate to be used as a precise and accurate sensor in a magnetic field. The goal of this work is to introduce the in-plane magnetic field as a new tuning knob to the toolbox of circuit quantum electrodynamics.
Advances in material science, especially the epitaxial growth of an aluminum shell directly on the indium arsenide nanowire, have enabled the fabrication of nanowire transmons with state-of-the-art coherence. An understanding of their workings in a zero-field environment is important before applying a magnetic field. Thus, we characterize the noise these qubits are subject to (Chapter 4) and find a strong coupling of charge two-level systems to their Josephson energy next to the expected weakly coupled flux and voltage noise.
Applying a magnetic field reveals that coherence in these qubits can be observed up to 70 mT, substantially above the superconducting gap of bulk aluminum (Chapter 5). Effects limiting the performance include the thick and fully covering aluminum shell, and the alignment and stability of the magnetic field. The use of different nanowires, the installation of a persistent-current vector solenoid and additional magnetic shielding then enables the operation of voltage- and flux-tunable devices in a magnetic field (Chapter 6). This constitutes a good starting point for circuit quantum electrodynamics experiments in a magnetic field, such
as the investigation of the microscopic origin of flux-noise.