Photon antibunching in a free-electron laser.

A perturbative solution to the difference-differential equation describing the time evolution of a free-electron laser based on the Bambini-Renieri Hamiltonian is obtained. This solution indicates that the photon number distribution is always sub-Poissonian as long as the perturbation approximation is valid. This photon antibunching becomes much more prominent as more photons are emitted; so it can enhance the coherent properties of the free-electron-laser radiation in a way that is very welcome.