The widely discussed applications in quantum information and quantum cryptography require radiation sources capable of producing a fixed number of photons. This paper reviews the work performed in our laboratory using the one-atom maser or micromaser to produce these fields on demand. In order to achieve this goal the maser is operating under the condition of the so-called trapping states. In this situation the field of the micromaser stabilises to a photon number state. Recently, we have also succeeded in determining the Wigner function of a single-photon state. Presently measurements of the phase diffusion of the one-atom maser field are underway, whereby the case will be studied in detail when the field approaches a number state. The measurements will also give information on the decoherence of the maser field.
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