  We report on the spectral analysis and the local measurement of intensity
correlations of microwave fields using ultra cold quantum gases. The
fluctuations of the electromagnetic field induce spin flips in a magnetically
trapped quantum gas and generate a multi-mode atomlaser. The output of the
atomlaser is measured with high temporal resolution on the single atom level,
from which the spectrum and intensity correlations of the generating microwave
field are reconstructed. We give a theoretical description of the atomlaser
output and its correlations in response to resonant microwave fields and verify
the model with measurements on an atom chip. The measurement technique is
applicable for the local analysis of classical and quantum noise of
electromagnetic fields, for example on chips, in the vicinity of quantum
electronic circuits.
