Amorphous solids show surprisingly universal behaviour at low temperatures. The prevailing wisdom is that this can be explained by the existence of two-state defects within the material. The so-called standard tunneling model has become the established framework to explain these results, yet it still leaves the central question essentially unanswered - what are these two-level defects? This question has recently taken on a new urgency with the rise of superconducting circuits in quantum computing, circuit quantum electrodynamics, magnetometry, electrometry and metrology. Superconducting circuits made from aluminium or niobium are fundamentally limited by losses due to two-level defects within the amorphous oxide layers encasing them. On the other hand, these circuits also provide a novel and effective method for studying the very defects which limit their operation. We can now go beyond ensemble measurements and probe individual defects - observing the quantum nature of their dynamics and studying their formation, their behaviour as a function of applied field, strain, temperature and other properties. This article reviews the plethora of recent experimental results in this area and discusses the various theoretical models which have been used to describe the observations. In doing so, it summarises the current approaches to solving this fundamentally important problem in solid-state physics.

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理解非晶固体中的两级系统——来自量子电路的见解

无定形固体在低温下表现出惊人的普遍行为。普遍的看法是，这可以通过材料中存在二态缺陷来解释。所谓的标准隧穿模型已经成为解释这些结果的既定框架，但它仍然没有解决核心问题——这两个层次的缺陷是什么？随着超导电路在量子计算、电路量子电动力学、磁力测定、电测量和计量学中的兴起，这个问题最近变得更加紧迫。由于包围它们的非晶氧化物层内的两级缺陷，由铝或铌制成的超导电路从根本上受到损耗的限制。另一方面，这些电路还为研究限制其操作的缺陷提供了一种新颖而有效的方法。我们现在可以超越整体测量并探测单个缺陷——观察它们动力学的量子性质并研究它们的形成，它们的行为作为外加场、应变、温度和其他特性的函数。本文回顾了该领域最近的大量实验结果，并讨论了用于描述观察结果的各种理论模型。在此过程中，它总结了解决固态物理学中这一根本重要问题的当前方法。它们的行为是外加场、应变、温度和其他属性的函数。本文回顾了该领域最近的大量实验结果，并讨论了用于描述观察结果的各种理论模型。在此过程中，它总结了解决固态物理学中这一根本重要问题的当前方法。它们的行为是外加场、应变、温度和其他属性的函数。本文回顾了该领域最近的大量实验结果，并讨论了用于描述观察结果的各种理论模型。在此过程中，它总结了解决固态物理学中这一根本重要问题的当前方法。