Observation of sub-Poissonian photon statistics in a micromaser

We describe the first investigation of the nonclassical radiation field of a micromaser. The maser cavity is cooled to 0. 5 K to exclude thermal photons and has a quality factor 310^10. Velocity-selected Rydberg atoms pump the maser. The photon statistics in the cavity are measured via the statistics of the atoms leaving the cavity in the lower maser level. Taking into account a detection efficiency of 10%, their fluctuations are reduced up to 40% below the Poisson level. This corresponds to photon-number fluctuations in the cavity of up to 70% below the vacuum-state limit.