  Generation and control of non-classical electromagnetic fields is of crucial
importance for quantum information physics. While usual methods for the
production of such fields rely on a non-linearity (of a crystal, a Josephson
junction, etc.), a recent experiment performed on a normal conductor, a tunnel
junction under microwave irradiation, has unveiled an alternative: the use of
electron shot noise in a quantum conductor\cite{PAN_squeezing}. Here we show
that such a device can emit \emph{pairs of microwave photons} of different
frequencies with a rate as high as that of superconducting Josephson
junctions\cite{Flurin}. This results in intensity fluctuations of the photon
field at two different frequencies being correlated below the photon shot
noise,i.e. two-mode amplitude squeezing. Our experiment constitutes a
fundamental step towards the understanding of electronic noise in terms of
quantum optics, and shows that even a normal conductor could be used as a
resource for quantum information processing.
