超导环中经典可分量子态之间的相干振荡
Coherent oscillations between classically separable quantum states of a superconducting loop
摘要：
十年前，人们首次观察到超导电路的两个量子态之间的相干振荡，其区别在于金属岛上是否存在单个库珀对。这一结果立即刺激了其他几种超导量子线路的发展，这些线路的行为就像人造原子一样，从而在介观物理学和原子物理学之间架起了一座桥梁。有趣的是，这些电路都没有完全实现现在几乎30岁的建议A。莱格特观察两个状态之间的相干振荡，不同的是存在或不存在一个单一的磁通被困在超导回路中断约瑟夫森隧道结。这种在两个经典可分离状态之间的可逆量子隧穿现象，被称为宏观量子相干性（MQC），被认为是精确测试宏观系统（如电路）是否完全遵循量子力学的关键。在这篇文章中，我们报告的观察，这样的振荡与亚GHz的频率和品质因数大于500。我们通过两项创新实现了这一结果。首先，我们的环具有比Leggett考虑的电感大四个数量级的电感，再加上充电机制中的结，这是以前实验中从未解决过的参数选择。第二，读出与一种新的色散计划，消除了电磁弛豫过程中引起的测量电路（珀塞尔效应）。此外，系统复位到它的基态是自然内置到这个方案，工作，即使过渡能小于温度波动。正如我们所认为的，与预期相反，MQC跃迁可能因此成为提高相干性和读出保真度的超导量子比特的基础。
英文摘要：
Ten years ago, coherent oscillations between two quantum states of a superconducting circuit differing by the presence or absence of a single Cooper pair on a metallic island were observed for the first time. This result immediately stimulated the development of several other types of superconducting quantum circuits behaving as artificial atoms, thus bridging mesoscopic and atomic physics. Interestingly, none of these circuits fully implements the now almost 30 year old proposal of A. J. Leggett to observe coherent oscillations between two states differing by the presence or absence of a single fluxon trapped in the superconducting loop interrupted by a Josephson tunnel junction. This phenomenon of reversible quantum tunneling between two classically separable states, known as Macroscopic Quantum Coherence (MQC), is regarded crucial for precision tests of whether macroscopic systems such as circuits fully obey quantum mechanics. In this article, we report the observation of such oscillations with sub-GHz frequency and quality factor larger than 500. We achieved this result with two innovations. First, our ring has an inductance four orders of magnitude larger than that considered by Leggett, combined with a junction in the charging regime, a parameter choice never addressed in previous experiments. Second, readout is performed with a novel dispersive scheme which eliminates the electromagnetic relaxation process induced by the measurement circuit (Purcell effect). Moreover, the reset of the system to its ground state is naturally built into this scheme, working even if the transition energy is smaller than that of temperature fluctuations. As we argue, the MQC transition could therefore be, contrary to expectations, the basis of a superconducting qubit of improved coherence and readout fidelity.
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