The influence of quantum interference between different decay channels of t
hree-level atoms on various characteristics of matter and radiation is cons
idered for the case of thermodynamic equilibrium of atoms in a photon gas.
It is shown that for rare gases when collisions between atoms are not impor
tant, the direct consequence of such an interference is a spontaneously ind
uced atomic coherence between two upper levels. Therefore, for an accurate
description of such atoms and radiation in thermodynamic equilibrium with e
ach other it is not sufficient to specify only Boltzmann exponents, which c
orrespond to the diagonal elements of the density matrix. The reason for th
is is that the conventional Boltzmann description is applied to infinitely
sharp eigenstates of the unperturbed Hamiltonian. As we show, it becomes es
sential to take into account nonzero width of the;levels if the levels are
relatively close to each other. This produces nonzero nondiagonal elements
of the density matrix that modify the emissivity of the equilibrium medium
and lead to a zero point at a certain frequency as well as to an enhanced i
ntensity in the red wing at the corresponding temperature. It is shown that
the occupation number of the photons is not changed and obeys the equilibr
ium Planck distribution. [S1050-2947(98)06911-X].