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Quantum dynamics of coherent states is studied within quantum field theoryusing two complementary methods: by organizing the evolution as a Taylor seriesin elapsed time and by perturbative expansion in coupling within theinteraction-picture formalism. One of the important aspects of our analysisconsists in utilizing the operators and the vacuum of interacting theory inconstructing the states, without invoking asymptotic particles. Focusing on acoherent state describing a spatially homogeneous field configuration, it isdemonstrated that both adopted methods successfully account for nonlinearclassical dynamics, giving distinguishable quantum effects. In particular,according to the time-expansion analysis the initial field-acceleration, withwhich the field departs from its initial expectation value, is governed by thetree-level potential with renormalized mass and bare coupling constant. Theinteraction-picture computation, instead, can be manipulated to give thenonlinear dynamics, determined in terms of renormalized coupling and mass.However, it results in a logarithmic initial-time singularity in thefield-acceleration, reminiscent of the similar behaviour encountered withinsemi-classical formalism, for certain choices of the initial state forfluctuations. Within our coherent-state analysis, the above mentionedpeculiarities are artefacts of an expansion: in the first case overinfinitesimal time, while in the second case in the coupling constant. Despitethis, we show that the evolution obtained within the interaction-pictureanalysis is valid for extended period of time. Moreover, on top of the desiredclassical dynamics, it serves us with interesting quantum corrections,previously proposed by Dvali-Gomez-Zell.

On Evolution of Coherent States as Quantum Counterpart of Classical Dynamics