  We show that the standard method of introducing the quantum description of
the electromagnetic field -- by canonical field quantization -- is not the only
one. We have chosen here the relativistic quantum mechanics of the photon as
the starting point. The treatment of photons as elementary particles merges
smoothly with the description in terms of the quantized electromagnetic field
but it also reveals some essential differences. The most striking result is the
appearance of various measures of fidelity for quantum states of photons. These
measures are used to characterize the localization of photons.

Quantum Physics