The heavy-fluxonium circuit is a promising building block for superconducting quantum processors due to its long relaxation and dephasing time at the flux-frustration point. However, the suppressed charge matrix elements and low transition frequency make it challenging to perform fast single-qubit gates using standard protocols. We report on new protocols for reset, fast coherent control, and readout that allow high-quality operation of the qubit with a 14 MHz transition frequency, an order of magnitude lower in energy than the ambient thermal energy scale. We utilize higher levels of the fluxonium to read out the qubit state and to initialize the qubit with 97% fidelity corresponding to cooling it to $190\mu \mathrm{K}$. Instead of using standard microwave pulses, we control the qubit only with fast-flux pulses, generating control fields much larger than the qubit frequency. We develop a universal set of gates based on nonadiabatic Landau-Zener transitions that act in 20–60 ns, less than the single-qubit Larmor period. We measure qubit coherence of $T_1,T_{2e}\sim 300\mu \mathrm{s}$ for a fluxonium in a 2D architecture and realize single-qubit gates with an average gate fidelity of 99.8% as characterized by randomized benchmarking.

中文翻译：

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相干，低频量子位的通用快速通量控制

由于重fluxonium电路在通量受挫点具有较长的弛豫和移相时间，因此它是超导量子处理器的有希望的构建基块。但是，受抑制的电荷矩阵元素和较低的跃迁频率使使用标准协议执行快速单量子位门变得颇具挑战性。我们报告了用于复位，快速相干控制和读出的新协议，这些协议允许以14 MHz的转换频率进行qubit的高质量操作，其能量比环境热能规模低一个数量级。我们利用较高水平的助焊剂读出量子位状态，并以97％的保真度初始化量子位，以使其冷却至$190\mu \mathrm{ķ}$。代替使用标准微波脉冲，我们仅使用快速通量脉冲控制量子位，从而产生比量子位频率大得多的控制场。我们基于非绝热的Landau-Zener跃迁开发了一套通用门，该跃迁的工作时间为20-60 ns，小于单量子位拉莫尔周期。我们测量的量子比特相干性${Ť}_{\mathrm{1个}}，{Ť}_{2Ë}〜300\mu \mathrm{s}$ 应用于2D架构中的助焊剂，并以随机基准测试为特征，实现平均门保真度为99.8％的单量子位门。