  We study a practical method to measure the shot noise in real time in
Continuous Variable Quantum Key Distribution (CVQKD) systems. The amount of
secret key that can be extracted from the raw statistics depends strongly on
this quantity since it affects in particular the computation of the excess
noise (i.e. noise in excess of the shot noise) added by an eavesdropper on the
quantum channel. Some powerful quantum hacking attacks relying on faking the
estimated value of the shot noise to hide an intercept and resend strategy were
proposed. Here, we provide experimental evidence that our method can defeat the
saturation attack and the wavelength attack.
