Characterizing measurement-induced state transitions in fluxonium qubits

Fast and high-fidelity superconducting qubit readout is crucial for realizing functional quantum processors. A common readout scheme relies on dispersive coupling between a qubit and a cavity. This scheme enables the measurement of a qubit state via probing the cavity frequency. To peform the measurement as fast as possible with sufficiently high fidelity, a large number of photons in the cavity is required. However, measurement-induced state transitions (MIST) can complicate mesurements in this regime. MIST occur when near-degenerate high-order states participate in the dynamics of the coupled qubit-cavity system. This causes the qubit population to leak out of the computational subspace, disrupting the qubit state distribution. We present our preliminary measurements of MIST in a fluxonium qubit, aiming to compare with predictions derived from the coupled fluxonium-cavity Hamiltonian. To this end, we measure deviations in qubit state distributions as a function of the number of photons in the cavity during readout in order to observe the onset of MIST.