  We use a correlation function analysis of the field quadratures to
characterize both the black body radiation emitted by a 50 Ohm load resistor
and the quantum properties of two types of beam splitters in the microwave
regime. To this end, we first study vacuum fluctuations as a function of
frequency in a Planck spectroscopy experiment and then measure the covariance
matrix of weak thermal states. Our results provide direct experimental evidence
that vacuum fluctuations represent the fundamental minimum quantum noise added
by a beam splitter to any given input signal.
