Nearly all spectroscopic measurements deals with the measurements of the average properties of the material. As an example, the reflectivity of a material is simply defined by the ratio between the number of photons which are reflected by the sample divided by the number of those arriving on it. The interest in measuring mostly average properties is the main drive of the standard scientific practice of repeating the measurements a lot of times so that the error made in one single measurements is averaged out by the repetition of the measurements. In this context the noise which determines fluctuation of the repeated measurements have always been considered as an impediment to a good quality measurements which needs to be mitigated by careful experimentalists.
The approach of repeated measurements is employed conspicuously in pump-probe experiments which are the prime way to study condensed matter out of its equilibrium state. In standards optical pump-probe experiments, ultrashort pulses are always used in pairs. The pump triggers the dynamical response and the probe is used to detect changes in the optical properties of the sample.
Image: Schematic view of the pump-probe set-up used for the experiments. The intensity of every single probe pulse was separately acquired with low-electronic-noise detectors for every pump-probe delay.