Planar Architecture for Studying a Fluxonium Qubit

The spectral and temporal characteristics of a fluxonium qubit coupled to a coplanar resonator on a chip have been experimentally studied. The system has been implemented as a planar integral electric circuit, where the fluxonium qubit itself consists of a tunnel Josephson junction with a small area shunted by a high inductance of a series of Josephson junctions with larger areas. To analyze the experimental data, an extended model of the fluxonium qubit capacitively coupled to the resonator has been proposed, and the structure of the energy levels has been obtained by full diagonalization of the Hamiltonian of the system. Numerical predictions of the model allow interpreting the results of two-tone spectroscopy obtained at various external magnetic fluxes in a wide frequency range.