  Classical theory asserts that several electromagnetic waves cannot interact
with matter if they interfere destructively to zero, whereas quantum mechanics
predicts a nontrivial light-matter dynamics even when the average electric
field vanishes. Here we show that in quantum optics classical interference
emerges from collective bright and dark states of light, \textit{i.e.},
entangled superpositions of multi-mode photon-number states. This makes it
possible to explain wave interference using the particle description of light
and the superposition principle for linear systems.
