The quantum mechanical description of a radiation field is based on states that are characterized by the number of photons in a particular mode; the most basic quantum states are those with fixed photon number, usually referred to as number (or Fock) states. Although Fock states of vibrational motion can be observed readily in ion traps, number states of the radiation field are very fragile and difficult to produce and maintain. Single photons in multi-mode fields have been generated using the technique of photon pairs. But in order to generate these states in a cavity, the mode in question must have minimal losses; moreover, additional sources of photon number fluctuations, such as the thermal field, must be eliminated. Here we observe the build-up of number states in a high-Q cavity, by investigating the interaction dynamics of a probe atom with the field. We employ a dynamical method of number state preparation that involves state reduction of highly excited atoms in a cavity, with a photon lifetime as high as 0.2 seconds. (This set-up is usually known as the one-atom maser or 'micromaser'.) Pure states containing up to two photons are measured unambiguously.
Abstract. Many applications in quantum information or quantum computing require radiation with a fixed number of photons (called Fock or number states) and this special issue demonstrates the real increase in interest in this area. In this paper, we review three recent papers in which we discussed the creation of photon number states in the micromaser and discuss new results. In the first experiment, Fock states were created in steady state via trapping states. Some new results are presented in this section showing a remarkable improvement of our ability to create these states. We then review a second method of dynamic Fock state creation using state reduction of lower state atoms. In this experiment, we also observed the purity of the Fock state that was created via the observation of Rabi oscillations of a probe atom. Finally, we discuss the results of the third experiment that we performed which was an experimental demonstration of a method of creating Fock states on demand.
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