  We show how the input-output formalism for cascaded quantum systems combined
with the quantum trajectory approach yields a compact and physically intuitive
description of single photons propagating through a coupled cavity array. As a
new application we obtain the time-dependent spectrum of such a single photon,
which directly reflects the fact that only certain frequency components of
single-photon wavepackets are trapped inside the cavities and hence are delayed
in time. We include in our description the actual generation of the single
photon, by assuming we have a single emitter in one of the resonators.
