DECAY WITHOUT LOSSES - MEAN-PHOTON-NUMBER DECAY AND SHANNON ENTROPY DECAY OF MICROMASER FIELDS VIA CONDITIONAL MEASUREMENTS

We investigate the dynamics of a lossless micromaser when a single-mod e cavity field is resonantly coupled via one-photon transitions to two -level atoms crossing the cavity in a sequence. We analyze the statist ical properties of the cavity field when the atoms after the interacti on are conditionally measured, provided the interaction time is short. Namely, we select only those sequences wherein each atom after the in teraction is measured in its initial state. We assume that the total s equence duration is smaller than the cavity decay time so that cavity losses can be neglected. We show that even though there is no direct t ransfer of the energy from the atoms to the field and vice versa, the mean photon number of the cavity field is reduced due to the condition al measurement. This energy ''decay'' is accompanied by a reduction of the Shannon entropy of the field. We show that there exists an attrac tor state of the cavity field which is the Fock state with the smalles t photon number which contributes to the initial state of the field. F inally, we find the remarkable result that the time evolution of a fie ld initially prepared in a coherent state which is conditionally measu red by the sequence of atoms is exactly the same as the time evolution of a coherent field decaying into a zero-temperature heat bath.