逼近 Andreev 能级与微波谐振腔之间的超强耦合区
Approaching the ultrastrong coupling regime between an Andreev level and a microwave resonator
AI总结：
本文提出在InAs-Al纳米线中制备嵌入高动能电感NbTiN谐振腔的弱连接器件，通过光谱测量揭示了Andreev束缚态的对跃迁与单准粒子跃迁，实现了最高968 MHz的耦合强度，逼近超强耦合区。
摘要：
半导体纳米线中形成的 Josephson 结承载 Andreev 束缚态，并作为通过静电门控调控 Andreev 量子比特的物理平台。由于 Andreev 束缚态被限制在纳米尺度的弱连接中，它通过流经弱连接的依赖于态的超导电流与 circuit-QED 架构耦合。因此，增加该耦合强度是该架构面临的关键挑战。在此，我们展示了一种弱连接的制备与微波表征，该弱连接定义在 InAs-Al（核-半壳）纳米线中，并嵌入到超导环路中，环路中的集总元件谐振腔由具有高动能电感的 NbTiN 薄膜图案化而成。我们研究了具有不同弱连接长度的多个器件，并进行了光谱测量，揭示了由自旋轨道分裂的 Andreev 束缚态引起的对跃迁和单准粒子跃迁。我们的方法提供了紧凑的几何结构，以及在 8 GHz 谐振频率下超过 12 kΩ 的谐振腔阻抗，这促进了系统中的强耦合。对于对跃迁，实验观察到的能级分裂表明与 Andreev 能级的耦合达到 490 MHz。我们应用了一个微扰模型，该模型与实验数据吻合良好，并提取出最大耦合为 968 MHz。此外，我们表明，与具有更高透射率的 Andreev 能级的耦合甚至更强。另外，单准粒子跃迁的光谱测量揭示了自旋轨道分裂的 Andreev 束缚态，其提取的自旋-光子耦合为 77 MHz。
英文摘要：
Josephson junctions formed in semiconductor nanowires host Andreev bound states and serve as a physical platform to realize Andreev qubits tuned by electrostatic gating. With the Andreev bound state being confined to the nanoscale weak link, it couples to a circuit-QED architecture via the state-dependent supercurrent flowing through the weak link. Thus, increasing this coupling strength is a crucial challenge for this architecture. Here, we demonstrate the fabrication and microwave characterization of a weak link which is defined in an InAs-Al (core-half shell) nanowire and embedded in a superconducting loop with a lumped-element resonator patterned from a thin NbTiN film with high kinetic inductance. We investigated several devices with various weak link lengths and performed spectroscopy that revealed pair transitions and single-quasiparticle transitions arising from spin-orbit split Andreev bound states. Our approach offers a compact geometry and a large resonator impedance above 12 k$Ω$ at a resonator frequency of 8 GHz, which facilitates large coupling in the system. For the pair transitions, the experimentally observed energy level splitting demonstrates the coupling to an Andreev level of 490 MHz. We apply a perturbative model that shows good agreement with the experimental data and extract the maximum coupling of 968~MHz. Moreover, we show that the coupling is even stronger to an Andreev level with a higher transmission. In addition, spectroscopy of single-quasiparticle transitions reveals spin-orbit split Andreev bound states with the extracted spin-photon coupling of 77 MHz.
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