The 1996 Brune {\it et al.} experiment on vacuum Rabi oscillation is analyzed
by means of alternative models of atom-reservoir interaction. Agreement with
experimental Rabi oscillation data can be obtained if one defines jump
operators in the dressed-state basis, and takes into account thermal
fluctuations between dressed states belonging to the same manifold. Such
low-frequency transitions could be ignored in a closed cavity, but the cavity
employed in the experiment was open, which justifies our assumption. The cavity
quality factor corresponding to the data is Q=3.31â‹…1010, whereas Q
reported in the experiment was Q=7â‹…107. The rate of decoherence arising
from opening of the cavity can be of the same order as an analogous correction
coming from finite time resolution Δt (formally equivalent to
collisional decoherence). Peres-Horodecki separability criterion shows that the
rate at which the atom-field state approaches a separable state is controlled
by fluctuations between dressed states from the same manifold, and not by the
rate of transitions towards the ground state. In consequence, improving the Q
factor we do not improve the coherence properties of the cavity.Comment: typo in eq. (60) corrected; (older comments: 14 figures (1 added),
value of Q improved, a section on the Peres-Horodecki test of separability
added, various small improvements; v3 includes discussion of finite time
resolution, v4 includes microscopic derivation of the master equation