Photon Statistics of the Free-Electron Laser Start Up
With the ever more increasing theoretical, as well as practical, knowledge about the free-electron laser (FEL), interest has partially shifted from the stimulated emission regime which is very well understood to the regime of spontaneous emission. The latter is decisive for the buildup of the FEL from the field vacuum to the threshold of lasing. From a theoretical point of view it is challenging because quantum mechanical zero-point fluctuations play a role, so that the well established classical approaches need to be augmented. To what extent this is necessary has been disputed.