us to conclude that the Rabi frequency overcomes such intervalley relaxation rates. Remarkably, strong coupling thus allows us to put an upper bounds to those rates, in close relation with [@Holmes].

B: Chiraliton diffusion length
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The diffusion length of chiralitons can be estimated by measuring the extent of their photoluminescence (PL) under a tightly focused excitation. To measure this extent, we excite a part of a WS$_2$ monolayer located above the plasmonic hole array (Fig. S\[fig:S1\](a) and (b)). This measurement is done on a home-built PL microscope, using a $100\times$ microscope objective of 0.9 numerical aperture and exciting the PL with a HeNe laser at $1.96$ eV, slightly below the exciton band-gap. A diffraction-limited spot of $430$ nm half-width is obtained (Fig. S\[fig:S1\](c)) by bringing the sample in the focal plane of the microscope while imaging the laser beam on a cooled CCD camera. The PL is collected by exciting at $10~\mu$W of optical power, and is filtered from the scattered laser light by a high-energy-pass filter. The resulting PL image is shown in Fig. S\[fig:S1\](d), clearly demonstrating the propagating character of the emitting chiralitons. The logarithmic cross-cuts (red curves in (c) and (d)) reveal a propagation length of 