Theory and Concepts of Polariton Condensates. F. P. Laussy Chapter in the Springer book entitled Short and Long Range Quantum Atomic Platforms — Theoretical and Experimental Developments (provisional title), edited by P. G. Kevrekidis, C. L. Hung, and S. I. Mistakidis.  What the paper says!?

Polaritons are described as WISI (Weakly-Interacting, Strongly-Interfering) particles, with an emphasis on their weak (not strong!) interactions, to the extent that often those can even be assumed zero altogether, as well as on their strong interferences, but with specificities such as non-linear dispersion (momentum-dependent velocity) or their dissipative character. The other focus of the text is on quantum polaritonics, insisting that this should rely on quantum observables (not GPE effects or analogies drawn from the single-particle picture) and that polaritons remain, to date, essentially classical objects, despite their unanimous embrace as light-matter quantum superpositions. Much of the text articulate tensions, if not controversies, on such aspects:

Instead of the usual consensual review-style presentation, we take here the approach of contrasting various interpretations of what polariton condensation is about, including whether this qualifies as BEC or as a laser, whether this is a quantum or nonlinear classical phenomenon, or how theories of greatly-varying sophistication, manage to describe the dynamics of polariton condensation, in particular regarding the role of interactions and nonlinearities.

It some some historical footing—reminding for instance how Weisbuch didn't think his polaritons were polaritons initially, because they were not propagating—as well as conceptual footing—in this case correcting the picture from Yamamoto's mirror image of the polaritons as a propagating particle indeed, in the virtual 3D space. This result in many citations, such as Hopfield's attributing the success of his polariton paper «to the existence of lasers, the polariton condensate, and modern photonics», noting that polaritons are now very much into neural networks, and thus have caught up with Hopfield.