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arXiv · 2505.15551

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

Alain Deville·Yannick Deville
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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.

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Alain Deville, Yannick Deville. 2025-05-21. The Landau-Feynman transiently open quantum system: entanglement and density operators. https://arxiv.org/abs/2505.15551

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