What Quantity is Measured in an Excess Noise Experiment?

Consider a measurement in which the current coming out of a mesoscopic sample is filtered around a frequency Omega > 0, then amplfied, measured, squared, then this process is repeated many times and the results are averaged. The final result of such a procedure is called the current fluctuations, the power spectrum, or the currrent noise, at frequency Omega. Often, two such measurements are performed on the same system: the first while it is driven out of equilibrium (e.g., by applying a DC voltage [1] or electromagnetic radiation [2], [3] to it) and the second in equilibrium (the voltage source is turned off, i.e., the power supply becomes a short). The excess noise is defined as the difference in the noise between the first and the second measurement. The present work analyzes what quantity one should calculate in order to predict excess noise.