Vacuum Noise is quantum noise related to the zero-point fluctuations of the electric field in the vacuum state. Such fluctuations exist even though the vacuum states has a photon number of zero.
Despite being quite weak, vacuum noise can affect the noise properties or even the essential function of optical devices. For example, parametric generation can be considered as the effect of amplifying vacuum noise to macroscopic power levels in a high Gain optical parametric amplifier. Also, the process of spontaneous emission can (in a semiclassical picture) be interpreted as being stimulated by vacuum noise.