超导量子比特频率调制的优化

超导量子线路具有灵活的可设计性、良好的可扩展性以及可操控性，因此以 其为基础的超导量子计算是目前主流的量子计算技术路线之一。扩展超导量子线 路的一个重要环节是实现可控的量子比特之间的耦合。超导量子比特之间可以通 过片上电容等器件直接耦合。但在这类耦合方案中，由于器件参数在样品制备时 就已经固定下来，因此缺乏可调性，对量子线路的功能有较大的限制。为此，研 究者开发了多种具有更好可调性的间接耦合方案，当前一种被广泛采用的方式是 利用频率可调的超导量子比特作为耦合器。通过改变耦合器的频率，不但可以对 被耦合的两个量子比特之间的等效耦合强度进行调制，还能实现一些直接耦合方 案不能或难以实现的更为复杂的耦合形式，例如比特之间的红、蓝边带（red、blue sideband）耦合，这些耦合对于特定的量子计算和量子模拟功能非常关键。 本论文研究了利用 Transmon 类型的耦合器实现超导量子比特之间可控的 blue(red) sideband 耦合的可行性。我们首先建立了体系的等效电路模型，然后利 用数值模拟方法寻找和优化对耦合器频率进行调制的方式和参数，并在一维超导 量子比特链样品上进行了实验，实现了可以同时开关且强度独立可调的 blue sideband 和 red sideband 耦合。基于这一技术，在由三个超导量子比特和两个耦合器组 成的体系的态空间中，我们构造了一个四能级演化环路，并通过改变 sideband 驱 动信号来调制环路中的相位，从而观测到 Aharonov-Bohm 干涉现象。这一技术对 于利用超导量子线路开展诸如人造规范场和一些代表性的凝聚态物理模型的量子 模拟具有重要价值。

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