  At optical frequencies the radiation produced by a source, such as a laser, a
black body or a single photon source, is frequently characterized by analyzing
the temporal correlations of emitted photons using single photon counters. At
microwave frequencies, however, there are no efficient single photon counters
yet. Instead, well developed linear amplifiers allow for efficient measurement
of the amplitude of an electromagnetic field. Here, we demonstrate how the
properties of a microwave single photon source can be characterized using
correlation measurements of the emitted radiation with such detectors. We also
demonstrate the cooling of a thermal field stored in a cavity, an effect which
we detect using a cross-correlation measurement of the radiation emitted at the
two ends of the cavity.
