Preparing pure photon number states of the radiation field

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 m ost basic quantum states are those with fixed photon number, usually referr ed to as number (or Fock) states. Although Fock states of vibrational motio n can be observed readily in ion traps', number states of the radiation fie ld are very fragile and difficult to produce and maintain. Single photons i n multi-mode fields have been generated using the technique of photon pairs (2,3). But in order to generate these states in a cavity, the mode in quest ion must have minimal losses; moreover, additional sources of photon number fluctuations, such as the thermal field, must be eliminated. Here we obser ve the build-up of number states in a high-Q cavity, by investigating the i nteraction dynamics of a probe atom with the field. We employ a dynamical m ethod of number state preparation that involves state reduction of highly e xcited atoms in a cavity, with a photon lifetime as high as 0.2 seconds. (T his set-up is usually known as the one-atom maser or 'micromaser'.) Pure st ates containing up to two photons are measured unambiguously.