The multimode nature of spacetime entanglement in QFT

TL;DR

This paper shows that the vacuum state of a free scalar quantum field in flat spacetime contains widespread multimode entanglement across causally disconnected regions, which is essential for entanglement harvesting.

Contribution

It reveals the pervasive nature of multimode entanglement in quantum field theory and its operational relevance, extending understanding beyond pairwise mode entanglement.

Findings

Multimode entanglement exists in the vacuum state across causally disconnected regions.

Entanglement between individual modes is sparse and requires careful selection.

Multimode entanglement is operationally extractable and fundamental to entanglement harvesting.

Abstract

We demonstrate the presence of multimode entanglement in the vacuum state of a free, massless scalar quantum field in four-dimensional flat spacetime between two sets of field modes, each contained within a spacetime region that is causally disconnected from the other. This is true despite the fact that entanglement between pairs of individual field modes is sparse and appears only when the two individual modes are carefully selected. Our results reveal that, while entanglement between individual modes is limited, bipartite multimode entanglement in quantum field theory is ubiquitous. We further argue that such multimode entanglement is operationally extractable, and it forms the basis of the entanglement commonly discussed in entanglement harvesting protocols.