  We propose the concept of valley coherence and superradiance in the
reciprocal space and show that it leads to an $N$-fold decrease of the bright
exciton radiative lifetime in quantum dots (QDs) of an $N$-valley
semiconductor. Next we explain why, despite this, the exciton radiative
lifetimes in PbX (X = S, Se, Te) QDs, measured from the photoluminescence
decay, are in the microsecond range. We also address peculiarities of the
light-matter interaction in nanostructures made of narrow-gap materials with
strong inter-band coupling.
