  Photons are the elementary quantum excitations of the electromagnetic field.
Quantization is usually constructed on the basis of an expansion in eigenmodes,
in the form of plane waves. Since they form a basis, other electromagnetic
configurations can be constructed by linear combinations. In this presentation
we discuss a formulation constructed in the general formalism of bosonic Fock
space, in which the quantum excitation can be constructed directly on localized
pulses of arbitrary shape. Although the two formulations are essentially
equivalent, the direct formulation in terms of pulses has some conceptual and
practical advantages, which we illustrate with some examples. The first one is
the passage of a single photon pulse through a beam splitter. The analysis of
this formulation in terms of pulses in Fock space shows that there is no need
to introduce "vacuum fluctuations entering through the unused port", as is
often done in the literature. Another example is the Hong-Ou-Mandel effect. It
is described as a time dependent process in the Schr\"odinger representation in
Fock space. The analysis shows explicitly how the two essential ingredients of
the Hong-Ou-Mandel effect are the same shape of the pulses and the bosonic
nature of photons. This formulation shows that all the phenomena involving
linear quantum optical devices can be described and calculated on the basis of
the time dependent solution of the corresponding classical Maxwell's equations
for pulses, from which the quantum dynamics in Fock space can be immediately
constructed.
