Noise mediated mixing in a tunnel junction

The microwave mixer is a key element in the detection of all sort of
signals. It is based on a nonlinear element such as a semiconducting
diode or a Josephson junction, and allows for example the downconversion
of microwave signals to low frequency for digitization. In the quantum
regime, the mixing of vacuum fluctuations with an ac excitation leads to
the generation of squeezed microwaves.

A normal metal tunnel junction with a linear current-voltage cannot be
used to mix signals. It has however been shown to generate squeezed
vacuum thanks to another type of nonlinearity, that of noise vs. bias
voltage. We demonstrate the use of a tunnel junction as a noise mediated
mixer: the junction excited at two frequencies (around 20GHz) generates
a noise which amplitude is modulated at the difference frequency. We
compare our data with theoretical predictions of photon-assisted noise
and observe small deviations that seem to indicate an extra frequency
cutoff possibly reated to the thermalization time of electrons in the
electrodes.