The problem of identifying the system and the environment in the phenomenon of decoherence

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Lombardi, Olimpia; Fortin, Sebastian; Castagnino, Mario
conference paper description
According to the environment-induced approach to decoherence (EID), the split of the Universe into the degrees of freedom which are of direct interest to the observer (the system) and the remaining degrees of freedom (the environment) is absolutely essential for decoherence. However, the EID approach offers no general criterion for deciding where to place the “cut” between system and environment: the environment may be “external” (a bath of particles interacting with the system of interest) or “internal” (such as collections of phonons or other internal excitations). The main purpose of this paper is to argue that decoherence is a relative phenomenon, better understood from a closed-system perspective according to which the split of a closed quantum system into an open subsystem and its environment is just a way of selecting a particular space of relevant observables of the whole closed system. In order to support this claim, we shall consider the results obtained in a natural generalization of the simple spin-bath model usually studied in the literature. Our main thesis will lead us to two corollaries. First, the problem of identifying the system that decoheres is actually a pseudo-problem, which vanishes as soon as one acknowledges the relative nature of decoherence. Second, the usually supposed link between decoherence and energy dissipation is misguided. As previously pointed out, energy dissipation and decoherence are different phenomena, and we shall argue for this difference on the basis of the relative nature of decoherence.