  The radiation generated by a quantum conductor should be correlated with
electrons crossing it. We have measured the correlation between the
fluctuations of the high frequency electromagnetic power and the low frequency
transport in a tunnel junction. We have explored the regimes where
electromagnetic fluctuations correspond to real photons and where they
correspond to vacuum at very low temperature. We deduce from our data the
intrinsic third moment of quantum shot noise, which appears to be frequency
independent.
