Characterizing radiative loss mechanisms in fluxonium qubits

As a highly anharmonic artificial atom with a tunable spectral landscape, the fluxonium qubit is a promising candidate for quantum information processing as well as metrology. Significant progress has been made in understanding loss mechanisms in fluxonium, but a complete picture remains elusive. One challenge is that the frequency and transition matrix elements of the qubit are highly dependent on flux and thus can generate a wide range of different dynamics with the environment. This complexity is amplified in Purcell loss, which becomes an involved problem due to the nontrivial interaction of the readout cavity with unoccupied higher qubit levels, which often move quickly in flux. Hence, a careful treatment of this frequency-dependent behavior is important to understand the relative roles other loss channels play over the range of flux tunability. In this work, we develop a more complete model, which includes the role of higher state dynamics in Purcell loss and validate against experimental data. Better characterizing these dependences will aid in the understanding of loss mechanisms in fluxonium, which can enable the engineering of higher coherence qubits.