The effects of readout photons on the fluxonium qubit

Readout of superconducting qubits is typically performed through a readout resonator dispersively-coupled to the qubit. A standard method for improving readout fidelity is to increase the average number of photons used for readout, which should result in greater separation between the qubit-state-dependent responses of the resonator. In the transmon, however, increasing the number of photons used for readout induces unwanted qubit transitions and increases the qubit relaxation rate. While there has been significant recent interest in the fluxonium due to its high gate fidelities, the effects of readout photon number on Josephson-junction-based fluxonium have largely been unexplored. Here, we experimentally investigate the effect of increased readout photon number on the fluxonium qubit by monitoring the fluxonium state in the presence of varying numbers of readout photons. We observe an overall increase in the qubit relaxation rate with the number of readout photons and observe photon-number-dependent changes in the fluxonium transition rate. We further explore the dependence of these effects on coupling between the readout resonator and the fluxonium, and on the resonator frequency.