Causal signal transmission by quantum fields. IV: The causal Wick   theorem
  Wick's theorem in the Schwinger-Perel-Keldysh closed-time-loop formalism is written in a form where the place of contractions is taken by the linear response function of the field. This result demonstrates that the physical information supplied by Wick's theorem for operators is propagation of the free field in space and time. 
L.I.Plimak and S.Stenholm
quant-ph cond-mat.stat-mech hep-th math-ph math.MP