Martingale projections and quantum decoherence

Abstract We introduce so-called super/sub-martingale projections as a family of endomorphisms defined on unions of Polish spaces. Such projections allow us to identify martingales as collections of transformations that relate path-valued random variables to each other under conditional expectations. In this sense, super/sub-martingale projections are random functionals that (i) are boundedness preserving and (ii) satisfy a conditional expectation criterion similar to that of the classical martingale theory. As an application to the theory of open quantum systems, we prove (a) that any system–environment interaction that manifests a supermartingale projection on the density matrix gives rise to decoherence, and (b) that any system–environment interaction that manifests a submartingale projection gives rise an increase in Shannon–Wiener information. It follows (c) that martingale projections in an open quantum system give rise both to quantum decoherence and to information gain.