Exact and Approximate Fluxnoium Array Modes

We present an exact solution for the linearized junction array modes of the superconducting qubit fluxonium in the absence of array disorder. This solution holds for arrays of any length and ground capacitance and for both differential and grounded devices. Array mode energies are determined by roots of convex combinations of Chebyshev polynomials, and their spatial profiles are modified plane waves. We also provide a simple, approximate solution which estimates array mode properties over a wide range of circuit parameters and an accompanying Mathematica file that implements both the exact and approximate solutions.