  The quantum behaviour of the electromagnetic field in mesoscopic elements is
intimately linked to the quantization of the charge. In order to probe
nonclassical aspects of the field in those elements, it is essential that
thermal noise be reduced to the quantum level, i.e. to scales where kT <
h{\nu}. This is easily achieved in dilution refrigerators for frequencies of a
few GHz, i.e. in the microwave domain. Several recent experiments have
highlighted the link between discrete charge transport and discrete photon
emission in simple mesoscopic elements such as a tunnel junction. Photocount
statistics are inferred from the measurement of continuous variables such as
the quadratures of the field.
