Service de Physique de l'Etat Condensé

A controversy that has arisen many times over in disparate contexts is whether quantum coherences between eigenstates of additively conserved quantities are fact or fiction. I present a pedagogical introduction to the debate in the form of a hypothetical dialogue between proponents from each of the two camps: a factist and a fictionist. It is argued that a resolution of the debate can be achieved by recognizing that quantum states do not only contain information about the intrinsic properties of a system but about its extrinsic properties as well, that is, about its relation to other systems external to it. Specifically, the coherent quantum state of the factist is the appropriate description of the relation of the system to one reference frame, while the incoherent quantum state of the fictionist is the appropriate description of the relation of the system to another, uncorrelated, reference frame. The two views are alternative but equally valid paradigms of description. This conclusion has implications for a variety of conceptual puzzles including whether it is possible to effectively "lift" superselection rules (it is, and we propose an experiment to demonstrate this for atom number). Time permitting, the connection to relationalism in quantum theory will also be discussed.