The non-dissipative nonlinearity of Josephson junctions1 converts macroscopic superconducting circuits into artificial atoms2, enabling some of the best-controlled qubits today3,4. Three fundamental types of superconducting qubit are known5, each reflecting a distinct behaviour of quantum fluctuations in a Cooper pair condensate: single-charge tunnelling (charge qubit6,7), single-flux tunnelling (flux qubit8) and phase oscillations (phase qubit9 or transmon10). Yet, the dual nature of charge and flux suggests that circuit atoms must come in pairs. Here we introduce the missing superconducting qubit, 'blochnium', which exploits a coherent insulating response of a single Josephson junction that emerges from the extension of phase fluctuations beyond 2π (refs. 11-14). Evidence for such an effect has been found in out-of-equilibrium direct-current transport through junctions connected to high-impedance leads15-19, although a full consensus on the existence of extended phase fluctuations is so far absent20-22. We shunt a weak junction with an extremely high inductance-the key technological innovation in our experiment-and measure the radiofrequency excitation spectrum as a function of external magnetic flux through the resulting loop. The insulating character of the junction is manifested by the vanishing flux sensitivity of the qubit transition between the ground state and the first excited state, which recovers rapidly for transitions to higher-energy states. The spectrum agrees with a duality mapping of blochnium onto a transmon, which replaces the external flux by the offset charge and introduces a new collective quasicharge variable instead of the superconducting phase23,24. Our findings may motivate the exploration of macroscopic quantum dynamics in ultrahigh-impedance circuits, with potential applications in quantum computing and metrology.

中文翻译：

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超导准电荷量子位

约瑟夫森结的非耗散非线性将宏观超导电路转换为人造原子 2，从而实现了当今一些最佳控制的量子位 3,4。已知三种基本类型的超导量子位 5，每一种都反映了 Cooper 对凝聚物中量子涨落的不同行为：单电荷隧穿（电荷 qubit6,7）、单通量隧穿（flux qubit8）和相位振荡（phase qubit9 或 transmon10） ）。然而，电荷和通量的双重性质表明电路原子必须成对出现。在这里，我们介绍了缺失的超导量子位“blochnium”，它利用了单个约瑟夫森结的相干绝缘响应，该响应从超过 2π 的相位波动扩展中出现（参考文献 11-14）。已经在通过连接到高阻抗引线的结的非平衡直流传输中发现了这种效应的证据 15-19，尽管到目前为止还没有对扩展相位波动的存在达成完全共识 20-22。我们将一个具有极高电感的弱结分流——这是我们实验中的关键技术创新——并测量射频激励谱作为通过所得回路的外部磁通量的函数。结的绝缘特性表现为基态和第一激发态之间量子位跃迁的通量敏感性消失，该跃迁迅速恢复以跃迁到更高能态。光谱与 blochnium 到 transmon 的二元映射一致，它用抵消电荷代替了外部通量，并引入了一个新的集体准电荷变量，而不是超导相23,24。我们的发现可能会激发对超高阻抗电路中宏观量子动力学的探索，并在量子计算和计量学中具有潜在应用。