  In this paper we dwell on three issues: 1) revisit the relation between
vacuum fluctuations and radiation reaction in atom-field interactions, which
began in the 70s and settled in the 90s with its resolution recorded in
monographs; 2) the fluctuation-dissipation relation (FDR) of the system,
pointing out the differences between that in linear response theory (LRT)
assuming ultra-weak coupling between the system and the bath, and the FDR in an
equilibrated final state, relaxed from the nonequilibrium evolution of an open
quantum system; 3) quantum radiation from an atom interacting with a quantum
field: Begin with vacuum fluctuations in the field acting on the internal
degrees of freedom (idf) of an atom, adding to its dynamics a stochastic
component which engenders quantum radiation whose backreaction causes quantum
dissipation in the idf of the atom. For a stationary atom, we show how the
absence of radiation results from complex cancellations, at a faraway
observation point, of the interference between emitted radiation from the atom
and the local fluctuations in the free field. We point out that the entity
which enters into the duality relation with vacuum fluctuations is not
classical radiation reaction, but quantum dissipation. Finally, regarding Issue
2, for systems with many atoms, the existence of a set of
correlation-propagation relations shows how the correlations between the atoms
are related to the propagation of their mutual influence manifesting in the
quantum field. The CPR is absolutely crucial in keeping the balance of energy
flows between the constituents of the system, and between the system and its
environment. Without the consideration of this additional relation in tether
with the FDR, dynamical self-consistency cannot be sustained. Combination of
these two sets of relations capture the physical essence of the interaction
between an atom and a quantum field.
