  The statistics of electron transport in a tunnel junction is affected by
fluctuations of its voltage bias, which modulates the probability for electrons
to cross the junction. We exploit this phenomenon to provide a direct
measurement of quantum vacuum fluctuations in the microwave domain of a
resistor placed at ultra-low temperature: we show that the amplified vacuum
fluctuations are correlated with the noise generated by the junction to
contribute to a third moment of detected voltage fluctuations. This experiment
constitutes an absolute measurement of noise, not affected by the unavoidable
noise added by the detection setup.
