  We show that a properly dc-biased Josephson junction in series with two
microwave resonators of different frequencies emits photon pairs in the
resonators. By measuring auto- and inter-correlations of the power leaking out
of the resonators, we demonstrate two-mode amplitude squeezing below the
classical limit. This non-classical microwave light emission is found to be in
quantitative agreement with our theoretical predictions, up to an emission rate
of 2 billion photon pairs per second.
