Multimodal and Ultra High-Q Superconducting Niobium Cavities for Circuit Quantum Electrodynamics

The high coherence afforded by 3D superconducting cavities, combined with strong engineered interactions via circuit quantum electrodynamics (cQED), has made 3D cavities a leading platform for studying quantum phenomena. In this thesis, we will develop a 3D architecture that leverages these techniques to allow for the addressing of quantum states encoded onto individual modes of a multimodal cavity. We will show that by engineering the mode dispersion and electric field, we can achieve qubit-cavity cooperativities in excess of 1 billion, while maintaining coherence times approaching 2ms over 9 modes. In order to scale the number of accessible modes, we will introduce niobium cavities as a platform for reaching higher coherence times. First, we will discuss developing a repeatable technique for producing single-mode 3D niobium cavities with loaded single-photon coherence times in excess of 15ms and internal quality factors of greater than 1.5 billion. Next we will examine the effects that surface processing and chemistry have on the cavity performance. Finally, we will outline the intricacies of implementing cQED in such high-coherence devices, before demonstrating a single-mode quantum memory with energy relaxation times approaching 10ms. By extending cQED to 3D multimodal platforms, and developing techniques that produce coherence times in excess of 10s of milliseconds, we hope to enable new inquiry into the application of 3D cQED for exploring quantum information and quantum optics phenomena.