Fast and high-fidelity qubit initialization based on auxiliary energy levels in fluxonium

Fast and high-fidelity qubit initialization is a necessary operation for the implementation of quantum error correction. In circuit quantum electrodynamics, the initialization is realized by implementing a state transfer between the qubit and its readout cavity. However, to ensure the qubit does not predominantly decohere through the cavity, the coupling between them is often kept in the dispersive regime, rendering the microwave-activated state transfer a two-photon process which ultimately limits the initialization speed. Leveraging the flux-tunability and the large anharmonicity of our fluxonium qubit, we circumvent this limitation using a resonant interaction between the cavity and a noncomputational qubit transition at no cost to the nominal qubit coherence. Specifically, we utilize the second-excited energy level of the fluxonium as an auxiliary level to perform the initialization protocol with single-frequency microwave driving. Within hundreds of nanoseconds, we achieve a high-fidelity initialization of the first- and second-excited states of a fluxonium qubit. We also demonstrate that the protocol is suitable for initializing multiple-qubits.