The electrical current through voltage biased conductors is not constant: it presents fluctuations around the average value: this is “noise”. Two reasons can be invoked for noise in the current: finite temperature and finite voltage. Finite temperature leads to fluctuations in the occupation of the electronic states, and results in Johnson-Nyquist noise, which is Gaussian. Fluctuations associated with the finite voltage are associated with the stochastic nature of transfer of individual electrons. The finite voltage introduces an asymmetry in the current: more charges travel in the direction imposed by the voltage. The noise is then also asymmetric (not Gaussian), and depends on correlations and interactions between charge carriers.
The calculation of the Full Counting Statistics (FCS) of the number of electrons passing through a conductor is now accessible theoretically for a large variety of conductors. In contrast, whereas measurements of the average noise (the second moment of fluctuations) is routinely performed, higher moments are much more difficult to access, because deviations from Gaussian fluctuations are usually small. Only few experiments have succeeded till now to gain information on the third moment. We have developed an integrated noise detector based on a superconducting tunnel junction (a Josephson junction), which is very sensitive to the third moment of the current fluctuations. Till now, we have only demonstrated its capabilities with measurements of the Poissonian noise from a normal tunnel junction. We are now planning to couple our detector to mesoscopic conductors in which the third moment gives access to information on charge transport: metallic wires, point contacts, etc …