Observation of Andreev Reflection Enhanced Shot Noise

We have experimentally investigated the quasiparticle shot noise in NbN/MgO/NbN superconductor-insulator-superconductor tunnel junctions. The observed shot noise is significantly larger than theoretically expected. We attribute this to the occurrence of multiple Andreev reflection processes in pinholes present in the MgO barrier. This mechanism causes the current to flow in large charge quanta (Andreev clusters), with a voltage dependent average value of $m\approx 1+2\Delta /eV$ times the electron charge. Because of this charge enhancement effect, the shot noise is increased by the factor $m$.