I know that we explain the slowing down of light in a dispersive medium classically, by inducing small dipoles in the medium (which holds as long as being far away from absorption bands), and the induced dipoles re-emit radiation of the same wavelength but different phase, as being delayed, which ends up in slowing down of the phase velocity:
$$v_p=\frac{c}{n}$$
However, the group velocity can be faster than $c$ in special dispersive media (fast light). I was reading a pretty nice explanation here https://www.photonics.com/Articles/Fast_Light_Slow_Light_and_Optical_Precursors/a27833. However, the explanation for the group velocity is a bit like information is encoded in a radio signal. Each single wavelength the group consists of, is affected differently by dispersion, and in the end this signal modulation is transmitted faster than $c$. The cited article then points out that special relativity (no info faster than light) is still true, because it can be shown that the front velocity is always $c$ (not $\frac{c}{n}$), so that's the maximum speed information can be transferred (leaving the group velocity being faster than $c$ somehow an apparent effect).
The provided intuitive explanation in the article cited above, is said to be going back to Sommerfeld himself: "When the electromagnetic field first starts to interact with the oscillators, the oscillators cannot immediately act back on the field via the induced polarization, because they have a finite response time. Thus, for a brief moment after the front passes, the dispersive material behaves as if there is nothing there — as if it were vacuum".
And that leaves me wondering what happens if we only emit a single photon. There is no group just one photon. Will it be slowed down and propagate by the phase velocity, because of inducing dipoles acting back on its wave-nature? I always thought so, but after reading the article I tend to think a single photon is always a front and will propagate with the front velocity.
