Observation of sub-Poisson fluctuations in the intensity of the seventh coherent spontaneous harmonic emitted by a rf linac free-electron laser.

We report the observation of sub-Poisson intensity fluctuations in the coherent spontaneous harmonic radiation generated by an infrared free-electron laser in a photon counting experiment using a well-defined ensemble of electron pulses. These observations constitute the first observation of a nonclassical state of the radiation field generated by a beam of free electrons. The fluctuations observed in the experiment are smaller than those expected from semiclassical radiation theory, and larger than those expected from electron shot noise.