We present a hybrid semiconductor-based superconducting qubit device that remains coherent at magnetic fields up to 1 T. The qubit transition frequency exhibits periodic oscillations with the magnetic field, consistent with interference effects due to the magnetic flux threading the cross section of the proximitized semiconductor nanowire junction. As the induced superconductivity revives, additional coherent modes emerge at high magnetic fields, which we attribute to the interaction of the qubit and low-energy Andreev states.

@en

Majorana bound states (MBSs) are novel particles predicted to be created when superconductor/semiconductor hybrid structures with strong spin-orbit coupling are subjected to strong magnetic fields. Expected to exhibit non-Abelian exchange statistics, they could form the basis of a new kind of quantum computer that is inherently protected from environmental noise, a common problem that has frustrated other quantum computing platforms. The current techniques used to measure these particles are highly sensitive, having provided the best evidence yet for their existence, but they are intrinsically too slow to form the basis of a useful quantum computer. To remedy this, this thesis integrates exotic materials into high frequency superconducting circuits that have been engineered to be resilient to strong magnetic fields, creating hybrid devices that potentially allow for fast and precise measurement and control of MBSs and their properties.

Several proposals to demonstrate the novel exchange statistics of MBSs use a specific type of superconducting qubit, the `transmon', for fast readout of the state of the MBSs. Problematically, the strong magnetic fields required to induce MBSs would destroy the superconductivity traditional transmons rely on, preventing them from operating as intended. To resolve this, the key constituent components of the transmon, the superconducting resonator and the Josephson junction have been engineered separately to become resilient to strong magnetic fields.

Chapter 4 explores how nanofabrication techniques and careful consideration of the properties of thin superconducting films can be used to engineer superconducting co-planar waveguide resonators that remain operational in strong parallel magnetic fields of \SI{6}{\tesla} and perpendicular magnetic fields of \SI{20}{\milli \tesla}, an order of magnitude greater than previously reported. Building on the results of Chapter 4, Chapter 5 utilises a graphene based Josephson junction, where the monoatomic thickness of the graphene provides an inherent protection against parallel magnetic fields, allowing us to demonstrate operation of a transmon circuit at a parallel magnetic field of \SI{1}{\tesla}.

Advances in nanowire material growth intended to improve the signatures of MBS are used in Chapter 6 to create a low power, highly coherent on-chip microwave source. With broad potential applications in superconducting circuits, it demonstrates a platform well suited for the detection of unique radiation that MBSs are predicted to emit. The thesis is concluded by Chapter 7, which describes the engineering and development of a nanowire based transmon qubit capable of measuring key properties of MBSs in the qubit's energy spectrum.
@en

Superconducting coplanar-waveguide resonators that can operate in strong magnetic fields are important tools for a variety of high-frequency superconducting devices. Magnetic fields degrade resonator performance by creating Abrikosov vortices that cause resistive losses and frequency fluctuations or suppress the superconductivity entirely. To mitigate these effects, we investigate lithographically defined artificial defects in resonators fabricated from Nb-Ti-N superconducting films. We show that by controlling the vortex dynamics, the quality factor of resonators in perpendicular magnetic fields can be greatly enhanced. Coupled with the restriction of the device geometry to enhance the superconductors critical field, we demonstrate stable resonances that retain quality factors ≃105 at the single-photon power level in perpendicular magnetic fields up to B⊥ ≃20mT and parallel magnetic fields up to B⥠≃6T. We demonstrate the effectiveness of this technique for hybrid systems by integrating an In-Sb nanowire into a field-resilient superconducting resonator and use it to perform fast charge readout of a gate-defined double quantum dot at B=1T.

@en

Circuit quantum electrodynamics has proven to be a powerful tool to probe mesoscopic effects in hybrid systems and is used in several quantum computing (QC) proposals that require a transmon qubit able to operate in strong magnetic fields. To address this we integrate monolayer graphene Josephson junctions into microwave frequency superconducting circuits to create graphene based transmons. Using dispersive microwave spectroscopy we resolve graphene's characteristic band dispersion and observe coherent electronic interference effects confirming the ballistic nature of our graphene Josephson junctions. We show that the monoatomic thickness of graphene renders the device insensitive to an applied magnetic field, allowing us to perform energy level spectroscopy of the circuit in a parallel magnetic field of 1 T, an order of magnitude higher than previous studies. These results establish graphene based superconducting circuits as a promising platform for QC and the study of mesoscopic quantum effects that appear in strong magnetic fields.

@en