Turning a molecule into a coherent two-level quantum system . D. Wang, H. Kelkar, D. Martin-Cano, D. Rattenbacher, A. Shkarin, T. Utikal, S. Götzinger and V. Sandoghdar in Nature Phys. 15:483 (2019).  What the paper says!?

This starts the era of resonant strong light-matter coupling of a single molecule in a cavity.

A focus is given to vibrational modes of the molecules as detrimental for quantum application (this wasn't the case in earlier works).

The Authors use an open, tunable and laterally scannable Fabry–Perot microcavity to enhance light-matter coupling of a single molecule and make it closer to an ideal two-level system. Cross-polarized reflection is used as an equivalent to transmission (cf. Figs. 2a and 2b)

It indeed improves figures of merit, claims «saturation with less than 0.5 photons» (sic; overall the text is poorly typeset and proofread) and also excites the molecule with another molecule from another lab.

The extinction spectroscopy approach is still used but not referenced:

The presence of an emitter inside a cavity modifies the interference of the fields that result from the reflections between its mirrors.

A first part is the analysis of strong coupling of their system, mainly through transmission. There the analysis is fairly weak and naive (for a 2019 paper!), for instance, it ignores the impact of the effective quantum state in the resulting splitting. This is particularly important since they appear to be at the frontier of weak coupling: «our experiment is situated right at the onset of strong coupling. This also seems clear from their Fig. 3b. The anticrossing in transmission (Fig. 3d) looks the Fano-looking interference in weak coupling.

Their analysis of the phase shift is more interesting:

Much more interesting, though, and innovative as compared to their previous works, is their analysis of two-photon correlations. They claim «non-classical generation of few-photons super-bunched light» from the observation of~$g^{(2)}\gg 1$ (being 21, which is among the largest photon bunchings reported so far for a single emitter^34,35).

The analysis is very partial (landscapes of correlations should be plotted instead) and comments quite superficial, e.g.,

an efficient coupling between laser light and a molecule can result in highly non-trivial statistics of the emerging photons²⁹.

But, still, in the wake of their earlier extinction work in intensity, they get the correct basic fact that it's related to interferences, and quote at least some of the appropriate literature. Their interpretation remains, however, if not faulty, vague and inaccurate: