Majorana zero modes have been proposed as building blocks of intrinsically fault-tolerant quantum computers. Currently, externally applied magnetic fields are necessary to induce Majorana zero modes in carefully engineered systems. Topological qubit state readout is realized by parity sensing of Majorana islands. However, fast and high fidelity conventional cQED parity readout is incompatible with the magnetic fields needed for Majorana physics. In this thesis, magnetic field insensitive microwave CPW resonators with artificial Abrikosov vortex pinning sites have tested and implemented in a graphene based transmon qubit. It has been shown that these artificial pinning sites reliably trap vortices and are able to retain their zero field Qi ~ 105 up to perpendicular fields of 35 mT. By application of these resonators, we have described the successful continuous wave qubit spectroscopy of a graphene transmon qubit at B|| = 1 T with a minimal linewidth of 166 MHz and demonstrated manipulation of the qubit frequency between 3.2-7 GHz with electric field. This is the first ever measured superconducting qubit that shows these properties at a magnetic field of 1 T.

For quantum computing with superconducting qubits, fabricating transmon qubits with increased coherence time is crucial, especially in view of multi-qubit quantum processors. The fabrication process, geometries and materials used to fabricate a transmon qubit are similar to the fabrication of CPW resonators. However, due to shorter fabrication time and straightforward measurement scheme of the latter, finding a way to increase the internal quality factor Qi in a CPW resonator is a quick way to gain insight into methods to increase the coherence time of a transmon qubit. The main goal of this research is to characterize the Qi at cryogenic temperature and single photon level of CPW resonators that are fabricated with novel surface treatments and etching techniques. The highest Qi is observed for resonators etched using Deep Reactive Ion Etching and HDMS surface treatment, which reached ∼106 at single photon level. This is comparable to the highest Qi reported in literature so far.

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