Fast qubit reset of a low frequency fluxonium circuit*

The improvement of the lifetime and coherence time of superconducting circuits is a critical step towards the development of quantum processing of information. Recent results demonstrated coherence times and lifetimes above 100 microseconds on the first transition of fluxonium circuits biased at half flux quantum.These timescales can be improved tenfold by decreasing the transition frequencies below 500 MHz, without compromising on the gate time. The drawback is that the qubit is thermally excited at thermal equilibrium in a dilution refrigerator. Fast and efficient initialization processes are therefore crucial to follow that road.
We present and characterize two initialization schemes on a low frequency fluxonium circuit. First, we use optical pumping to convert circuit excitations into cavity ones in a process similar to the one demonstrated for transmons. Second, using quantum-limited amplifiers, we use single-shot measurements to herald on the initial qubit state.