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Reading out the state of superconducting artificial atoms typically relies ondispersive coupling to a readout resonator. For a given system noisetemperature, increasing the circulating photon number $\bar{n}$ in theresonator enables a shorter measurement time and is therefore expected toreduce readout errors caused by spontaneous atom transitions. However,increasing $\bar{n}$ is generally observed to also increase these transitionrates. Here we present a fluxonium artificial atom in which we measure anoverall flat dependence of the transition rates between its first two states asa function of $\bar{n}$, up to $\bar{n}\approx200$. Despite the fact that weobserve the expected decrease of the dispersive shift with increasing readoutpower, the signal-to-noise ratio continuously improves with increasing$\bar{n}$. Even without the use of a parametric amplifier, at $\bar{n}=74$, wemeasure fidelities of 99% and 93% for feedback-assisted ground and excitedstate preparation, respectively.

Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers