Quantum theory of SASE FEL
  We describe a free-electron laser (FEL) in the Self Amplified Spontaneous Emission (SASE) regime quantizing the electron motion and taking into account propagation effects. We demonstrate quantum purification of the SASE spectrum, i.e., in a properly defined quantum regime the spiking behavior disappears and the SASE power spectrum becomes very narrow. 
R. Bonifacio, N. Piovella, and G.R.M. Robb