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        "quote": "最简单的[<u class=\"concept-term\" data-concept-reference=\"concepts/group-theory/lie-group\" data-concept-identity=\"urn:uuid:8c364498-0e05-4c91-8fd7-1730d6e8656e\" data-concept-route=\"/concept-identities#concept-8c364498-0e05-4c91-8fd7-1730d6e8656e\">李群(Lie group)</u>](../../../../../concepts/group-theory/lie-group)例子是平面的旋转群，其元素由单个参数即旋转角$\\theta$来参数化。将这类群元素识别为复平面中的单位向量 $e^{i\\theta}$，会更方便。该群即[<u class=\"concept-term\" data-concept-reference=\"concepts/group-theory/circle-group\" data-concept-identity=\"urn:uuid:35b13cd0-fcbd-4d42-aa3a-04bf19d2fbb9\" data-concept-route=\"/concept-identities#concept-35b13cd0-fcbd-4d42-aa3a-04bf19d2fbb9\" id=\"first-c205b531a9556fbe5a3d-zh\">圆群(circle group)</u>](../../../../../concepts/group-theory/circle-group)，记作 $U(1)$；这是因为这样的复数可视为 $1\\times1$ 的酉矩阵。我们将在本章中看到[第1](../chapter-01/index#section-1)章所描述的一般图景如何在这个简单情形中实现。状态空间将是 $U(1)$ 群的[<u class=\"concept-term\" data-concept-reference=\"concepts/representation-theory/unitary-representation\" data-concept-identity=\"urn:uuid:8adb7f10-55df-4db3-bbdc-c44765232a11\" data-concept-route=\"/concept-identities#concept-8adb7f10-55df-4db3-bbdc-c44765232a11\">酉表示(unitary representation)</u>](../../../../../concepts/representation-theory/unitary-representation)，并且我们将看到任何这样的表示都分解为一系列一维表示之和。这些一维表示由整数 $q$ 标记；这些整数是自伴算符 $Q$ 的特征值，而 $Q$ 是量子理论中的一个可观测量。",
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        "quote": "人们可能会认为最简单的李群是一维加法群 $\\mathbf{R}$，我们将在[第 10](../chapter-10/index#section-10) 章开始连同其表示一起研究这个群。事实上，加入周期性条件后，便可避开参数趋于 $\\pm\\infty$ 时的行为问题，并得到一个更易于分析的李群——由单位圆上的点组成的“[<u class=\"concept-term\" data-concept-reference=\"concepts/group-theory/circle-group\" data-concept-identity=\"urn:uuid:35b13cd0-fcbd-4d42-aa3a-04bf19d2fbb9\" data-concept-route=\"/concept-identities#concept-35b13cd0-fcbd-4d42-aa3a-04bf19d2fbb9\">圆群(circle group)</u>](../../../../../concepts/group-theory/circle-group)”。每个这样的点由一个角刻画，而群法则就是角的加法。",
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