Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: On the applicability of quantum-optical concepts in strong-coupling
  nanophotonics   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.

6
The categories of this paper are: astro-ph
33
The categories of this paper are: plasm-ph
36
The categories of this paper are: cmp-lg
17
The categories of this paper are: q-fin
2
The categories of this paper are: math
23
The categories of this paper are: solv-int
15
The categories of this paper are: nucl-ex
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The categories of this paper are: eess
22
The categories of this paper are: chao-dyn
34
The categories of this paper are: chem-ph
24
The categories of this paper are: dg-ga
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
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The categories of this paper are: hep-th
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The categories of this paper are: hep-ex
35
The categories of this paper are: mtrl-th