  Continuous variable entanglement between two modes of a radiation field is
usually studied at optical frequencies. As an important step towards the
observation of entanglement between propagating microwave photons we
demonstrate the experimental state reconstruction of two field modes in the
microwave domain. In particular, we generate two-mode correlated states with a
Josephson parametric amplifier and detect all four quadrature components
simultaneously in a two-channel heterodyne setup using amplitude detectors.
Analyzing two-dimensional phase space histograms for all possible pairs of
quadratures allows us to determine the full covariance matrix and reconstruct
the four-dimensional Wigner function. We demonstrate strong correlations
between the quadrature amplitude noise in the two modes. Under ideal conditions
two-mode squeezing below the standard quantum limit should be observable in
future experiments.
