Implementation of a multi-mode qubit in coaxial circuit QED: Part 2

We discuss experimental results on a multi-mode qubit operating within a coaxial circuit QED architecture. This device includes a two-mode coaxial transmon qubit with out-of-plane coupling to a coaxial LC resonator. We discuss the characterisation of devices using two-tone spectroscopy and coherence measurements. We observe a strong cross-Kerr coupling between the differential and common modes of the qubit, which results in a qutrit-like behavior. Finally, we investigate charge sensitivity in such a device, and discuss its potential as a novel probe of charge fluctuations in superconducting quantum systems.