Rooted in quantum optics and benefiting from its well-established foundations, strong coupling in nanophotonics has experienced increasing popularity in recent years. With nanophotonics being an experiment-driven field, the absence of appropriate theoretical methods to describe ground-breaking advances has often emerged as an important issue. To address this problem, the temptation to directly transfer and extend concepts already available from quantum optics is strong, even if a rigorous justification is not always available. In this review we discuss situations where, in our view, this strategy has indeed overstepped its bounds. We focus on exciton-plasmon interactions, and particularly on the idea of calculating the number of excitons involved in the coupling. We analyse how, starting from an unfounded interpretation of the term N/V that appears in theoretical descriptions at different levels of complexity, one might be tempted to make independent assumptions for what the number N and the volume V are, and attempt to calculate them separately. Such an approach can lead to different, often contradictory results, depending on the initial assumptions (e.g. through different treatments of V as the-ambiguous in plasmonics-mode volume). We argue that the source of such contradictions is the question itself-How many excitons are coupled?, which disregards the true nature of the coupled components of the system, has no meaning and often not even any practical importance. If one is interested in validating the quantum nature of the system-which appears to be the motivation driving the pursuit of strong coupling with small N-one could instead focus on quantities such as the photon emission rate or the second-order correlation function. While many of the issues discussed here may appear straightforward to specialists, our target audience is predominantly newcomers to the field, either students or scientists specialised in different disciplines. We have thus tried to minimise the occurrence of proofs and overly-technical details, and instead provide a qualitative discussion of analyses that should be avoided, hoping to facilitate further growth of this promising area.

中文翻译：

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量子光学概念在强耦合纳米光子学中的适用性

植根于量子光学并受益于其完善的基础，纳米光子学中的强耦合近年来越来越受欢迎。由于纳米光子学是一个实验驱动的领域，缺乏适当的理论方法来描述突破性进展往往成为一个重要问题。为了解决这个问题，直接转移和扩展已经从量子光学中可用的概念的诱惑很大，即使并不总是有严格的理由。在本次审查中，我们讨论了在我们看来这种策略确实超出了其界限的情况。我们专注于激子 - 等离子体相互作用，特别是计算耦合中涉及的激子数量的想法。我们分析如何，从出现在不同复杂程度的理论描述中的术语 N/V 的毫无根据的解释开始，人们可能会试图对 N 和体积 V 的数量做出独立假设，并尝试分别计算它们。这种方法可能会导致不同的，通常是相互矛盾的结果，这取决于初始假设（例如，通过将 V 作为等离子模式体积中的歧义的不同处理）。我们认为，这种矛盾的根源在于问题本身——有多少激子耦合？，它无视系统耦合组件的真实性质，没有意义，通常甚至没有任何实际意义。如果有人对验证系统的量子性质感兴趣——这似乎是推动追求与小 N 强耦合的动机，那么可以关注光子发射率或二阶相关函数等数量。虽然这里讨论的许多问题对专家来说可能看起来很简单，但我们的目标受众主要是该领域的新手，无论是学生还是专攻不同学科的科学家。因此，我们试图尽量减少证据和过度技术细节的出现，而是对应该避免的分析进行定性讨论，希望能促进这个有前途的领域的进一步发展。虽然这里讨论的许多问题对专家来说可能看起来很简单，但我们的目标受众主要是该领域的新手，无论是学生还是专攻不同学科的科学家。因此，我们试图尽量减少证据和过度技术细节的出现，而是对应该避免的分析进行定性讨论，希望能促进这个有前途的领域的进一步发展。虽然这里讨论的许多问题对专家来说可能看起来很简单，但我们的目标受众主要是该领域的新手，无论是学生还是专攻不同学科的科学家。因此，我们试图尽量减少证据和过度技术细节的出现，而是对应该避免的分析进行定性讨论，希望能促进这个有前途的领域的进一步发展。