Causal signal transmission by quantum fields. I. Response of the   harmonic oscillator
  It is shown that response properties of a quantum harmonic oscillator are in essence those of a classical oscillator, and that, paradoxical as it may be, these classical properties underlie all quantum dynamical properties of the system. The results are extended to non-interacting bosonic fields, both neutral and charged. 
L.I. Plimak and S. Stenholm