Implementation of a multi-mode qubit in coaxial circuit QED: Part 1

Alternative qubit design is an active field of research in superconducting quantum systems. In particular, multi-mode quantum circuits can allow error-protected subspaces, nonlinear coupling for multi-qubit operations, and additional mechanisms for state readout or control. In this talk we introduce a simple multi-mode coaxial transmon qubit - a two-mode qubit with a dipole-like differential mode and a coaxial common mode. We discuss the qubit Hamiltonian, analyze its sensitivity to fabrication imperfections, and discuss the effects of quasi-particle tunnelling in this system. Finally, we consider extensions of the design to multi-qubit settings.