Deconstructing decoherence

The study of environmentally induced superselection and of the process of decoherence was originally motivated by the search for the emergence of classical behavior out of the quantum substrate, in the macroscopic limit [W. H. Zurek, Phys. Rev. D 24, 1516 (1981); 26, 1862 (1982)]. This limit, and other simplifying assumptions, have allowed the derivation of several simple results characterizing the onset of environmentally induced superselection; but these results are increasingly often regarded as a complete phenomenological characterization of decoherence in any regime. This is not necessarily the case: the examples presented in this paper counteract this impression by violating several of the simple general rules. This is relevant because decoherence is now beginning to be tested experimentally [C. Monroe et al., Science 272, 1131 (1996); M. Brune et al., Phys. Rev. Lett. 77, 4887 (1996)], and one may anticipate that, in at least some of the proposed applications (e.g., quantum computers), only the basic principle of ``monitoring by the environment'' will survive. The phenomenology of decoherence may turn out to be significantly different.