On dynamical stability of composite quantum particles: when entanglement is enough and when interaction is needed

TL;DR

This paper investigates the conditions under which two quantum particles behave as a single object, emphasizing the roles of entanglement and interaction in their dynamical stability and collective behavior.

Contribution

It clarifies when entanglement alone suffices for composite behavior and when interaction or measurement are necessary, advancing understanding of quantum particle stability.

Findings

Entanglement can induce single-object behavior without interaction.

Interaction or measurement are generally required for complex collective dynamics.

Entanglement alone is insufficient to produce certain collective behaviors due to no-signalling constraints.

Abstract

We consider an evolution of two elementary quantum particles and ask the question: under what conditions such a system behaves as a single object? It is obvious that if the attraction between the particles is stronger than any other force acting on them the whole system behaves as one. However, recent insight from the quantum information theory suggests that in bipartite systems it is not attraction per se that is responsible for the composite nature, but the entanglement between the parts. Since entanglement can be present between the subsystems that interacted in the past, but do not interact anymore, it is natural to ask when such an entangled pair behaves as a single object. We show that there are situations when entanglement is enough to observe single-particle behaviour. However, due to the no-signalling condition, in general an interaction, or a post-selective measurement, is necessary for a complex collective behaviour.