Quantum Nondemolition Dispersive Readout of a Superconducting Artificial Atom Using Large Photon Numbers

Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number n̄ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing n̄ is generally observed to also monotonously increase these transition rates. Here we present a fluxonium artificial atom in which, despite the fact that the measured transition rates show nonmonotonous fluctuations within a factor of 6, for photon numbers up to n̄≈200, the signal-to-noise ratio continuously improves with increasing n̄. Even without the use of a parametric amplifier, at n̄=74, we achieve fidelities of 99% and 93% for feedback-assisted ground and excited state preparations, respectively. At higher n̄, leakage outside the qubit computational space can no longer be neglected and it limits the fidelity of quantum state preparation.