Two processes of photon-pair creation can be arranged such that the paths of the emitted photons are identical. The path information is thereby not erased but rather never born in the first place due to this path identity. In addition to its implications for fundamental physics, this concept has recently led to a series of impactful discoveries in the fields of imaging, spectroscopy, and quantum information science. Here the idea of path identity is presented and a comprehensive review of recent developments is provided. Specifically, the concept of path identity is introduced based on three defining experimental ideas from the early 1990s. The three experiments have in common that they contain two photon-pair sources. The paths of one or both photons from the different sources overlap such that no measurement can recognize from which source they originate. A wide range of noteworthy quantum interference effects (at the single- or two-photon level), such as induced coherence, destructive interference of photon pairs, and entanglement generation, are subsequently described. Progress in the exploration of these ideas has stagnated and has gained momentum again only in the last few years. The focus of the review is the new development in the last few years that modified and generalized the ideas from the early 1990s. These developments are overviewed and explained under the same conceptual umbrella, which will help the community develop new applications and realize the foundational implications of this sleeping beauty.

中文翻译：

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通过路径身份和未检测到的光子实现量子不可区分性

可以安排光子对产生的两个过程，使得发射的光子的路径相同。因此，由于该路径标识，路径信息不会被删除，而是从未生成。除了对基础物理学的影响之外，这个概念最近还在成像、光谱学和量子信息科学领域带来了一系列有影响力的发现。这里提出了路径同一性的想法，并对最近的发展进行了全面的回顾。具体来说，路径同一性的概念是基于 20 世纪 90 年代初的三个定义性实验思想而引入的。这三个实验的共同点是它们包含两个光子对源。来自不同源的一个或两个光子的路径重叠，使得任何测量都无法识别它们源自哪个源。随后描述了一系列值得注意的量子干涉效应（在单光子或双光子水平），例如诱导相干性、光子对的相消干涉和纠缠生成。这些想法的探索进展停滞不前，直到最近几年才再次获得动力。回顾的重点是过去几年的新发展，这些发展修改和概括了 20 世纪 90 年代初的想法。这些发展在同一概念框架下进行概述和解释，这将帮助社区开发新的应用程序并实现这种睡美人的基本含义。