Universal Fast-Flux Control of a Coherent, Low-Frequency Qubit

The \textit{heavy-fluxonium} circuit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing time at the half-flux frustration point. However, suppressed charge matrix elements low transition frequency have made it challenging perform fast single-qubit gates using standard protocols. We report on new protocols reset, coherent control, readout, that allow high-quality operation of qubit with 14 MHz frequency, an order magnitude lower in energy than ambient thermal scale. utilize higher levels fluxonium initialize $97$\% fidelity, corresponding cooling $190~\mathrm{\mu K}$. realize high-fidelity control universal set single-cycle flux gates, which are comprised directly synthesizable pulses, while plasmon-assisted readout used measurements. On $T_1, T_{2e}\sim$~300~$\mathrm{\mu s}$, we $20-60$~ns average gate fidelity $99.8\%$ as characterized by randomized benchmarking.