隧道结中约瑟夫森谐波的观测
Observation of Josephson Harmonics in Tunnel Junctions
AI总结：
本研究观测到隧道结中的约瑟夫森谐波，表明标准电流-相位关系不准确，通过介观模型纳入谐波后能谱吻合度提升数量级，并证明工程化谐波可降低transmon量子比特的电荷色散误差。
摘要：
开发大规模超导量子处理器的方案必须应对固态器件中普遍存在的众多微观自由度。最先进的超导量子比特采用氧化铝（AlO$_x$）隧道约瑟夫森结作为执行量子操作所需的非线性来源。对这些结的分析通常假设一种理想化的、纯正弦的电流-相位关系。然而，这种关系仅在AlO$_x$势垒中通道透明度极低的极限情况下才成立。在此，我们表明标准的电流-相位关系无法准确描述跨多个样品和实验室的transmon人工原子的能谱。相反，通过非均匀AlO$_x$势垒隧穿的介观模型预测了更高约瑟夫森谐波的百分比级贡献。通过将这些谐波纳入transmon哈密顿量，我们获得了计算与实测能谱之间数量级更好的吻合。约瑟夫森谐波的存在和影响对于开发基于AlO$_x$的量子技术（包括量子计算机和参量放大器）具有重要意义。作为示例，我们展示工程化的约瑟夫森谐波可以将transmon量子比特中的电荷色散及相关误差降低一个数量级，同时保持其非谐性。
英文摘要：
Approaches to developing large-scale superconducting quantum processors must cope with the numerous microscopic degrees of freedom that are ubiquitous in solid-state devices. State-of-the-art superconducting qubits employ aluminum oxide (AlO$_x$) tunnel Josephson junctions as the sources of nonlinearity necessary to perform quantum operations. Analyses of these junctions typically assume an idealized, purely sinusoidal current-phase relation. However, this relation is only expected to hold in the limit of vanishingly low-transparency channels in the AlO$_x$ barrier. Here we show that the standard current-phase relation fails to accurately describe the energy spectra of transmon artificial atoms across various samples and laboratories. Instead, a mesoscopic model of tunneling through an inhomogeneous AlO$_x$ barrier predicts percent-level contributions from higher Josephson harmonics. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The presence and impact of Josephson harmonics has important implications for developing AlO$_x$-based quantum technologies including quantum computers and parametric amplifiers. As an example, we show that engineered Josephson harmonics can reduce the charge dispersion and the associated errors in transmon qubits by an order of magnitude, while preserving their anharmonicity.
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