The Landau-Feynman transiently open quantum system: entanglement and density operators

TL;DR

This paper clarifies the Landau-Feynman transient quantum system, emphasizing that entanglement, not mixed states, is the correct concept for understanding its dynamics and addressing ongoing controversies.

Contribution

It clarifies the conceptual framework of the Landau-Feynman situation, highlighting the importance of entanglement over mixed states in this context.

Findings

Entanglement is the key concept in Landau-Feynman systems.

Misinterpretations of mixed states cause ongoing controversies.

The paper resolves conceptual confusions from historical debates.

Abstract

Users of quantum mechanics, both in physics and in the field of quantum information, are familiar with the concept of a statistical mixture as introduced by von Neumann, and with the use of a density operator in that context. A density operator may also be used in another situation, introduced by Landau, with a transient coupling between the two parts of a quantum bipartite system. But more than fifty years after a clarifying work by Feynman on the subject, a confusion still persists about what we call the Landau-Feynman situation. This is specifically testified by the development of controversies around that subject. The aim of this paper is to stress that, when facing the Landau-Feynman situation, the right concept to be used is not the one of a mixed state (or statistical mixture) - be it qualified as proper or improper -, but the one of entanglement.