A Rydberg atom maser operated on a two-photon degenerate atomic transition is discussed. This device combines two interesting features: it is a quantum oscillator working on an intrinsically nonlinear effect and it is a micromaser, with only a few atoms and a few tens of photons at a time in the cavity. The reasons why Rydberg atom-superconducting cavity systems are well adapted for two-photon maser studies are discussed, and the main properties of the device are analyzed by comparing it to Rydberg masers operating on one-photon transitions.>
The development of a single-atom maser or a micromaser allows a detailed study of the atom–field interaction. The situation realized is very close to the ideal case of a single two-level atom interacting with a single-mode quantized field as treated in Section 6.2. In a micromaser a stream of two-level atoms is injected into a superconducting cavity with a high quality factor. The injection rate can be such that only one atom is present inside the resonator at any time. Due to the high quality factor of the cavity, the radiation decay time is much larger than the characteristic time of the atom–field interaction, which is given by the inverse of the single-photon Rabi frequency. Therefore, a field is built up inside the cavity when the mean time between the atoms injected into the cavity is shorter than the cavity decay time. A micromaser, therefore, allows sustained oscillations with less than one atom on the average in the cavity. The realization of a single-atom maser or a micromaser has been made possible due to the enormous progress in the construction of superconducting cavities together with the laser preparation of highly excited atoms called Rydberg atoms. The quality factor of the superconducting cavities is high enough for periodic energy exchanges between atom and cavity field to be observed. The interesting properties of the Rydberg atoms make them ideal for micromasers. In Rydberg atoms the probability of induced transitions between adjacent states becomes very large and scales as n 4 , where n denotes the principle quantum number.
Experimental and theoretical studies of Rydberg masers-quantum-electronic devices which produce and employ microwave radiation from highly excited atoms in a cavity-are reviewed. The experimental results are based mainly on studies performed by S. Haroche's group in Paris and H. Walther's group in Munich. In contradistinction to the usual quantum generators the quantum properties of the electromagnetic field itself play an important role in the analysis of the behavioral dynamics of Rydberg masers. The most striking results obtained in this field are the discovery of quantum revival and the achievement of maser generation on two-photon transitions and Fok states of the electromagnetic field.