  We study the phase space structure of exact quantum Wightman functions in
spatially homogeneous, temporally varying systems. In addition to the usual
mass shells, the Wightman functions display additional coherence shells around
zero frequency $k_0=0$, which carry the information of the local quantum
coherence of particle-antiparticle pairs. We find also other structures, which
encode non-local correlations in time, and discuss their role and decoherence.
We give a simple derivation of the cQPA formalism, a set of quantum transport
equations, that can be used to study interacting systems including the local
quantum coherence. We compute quantum currents created by a temporal change in
a particle's mass, comparing the exact Wightman function approach, the cQPA and
the semiclassical methods. We find that the semiclassical approximation, which
is fully encompassed by the cQPA, works surprisingly well even for very sharp
temporal features. This is encouraging for the application of semiclassical
methods in electroweak baryogenesis with strong phase transitions.
