How ubiquitous is entanglement in quantum field theory?

Understand

It is well known that entanglement is widespread in quantum field theory, in the following sense: every Reeh-Schlieder state contains entanglement between any two spatially separated regions.

• This applies, in particular, to the vacuum of a non-interacting scalar theory in Minkowski spacetime.
• Discussions on entanglement in field theory have focused mainly on subsystems containing infinitely many degrees of freedom -- typically, the field modes that are supported within a compact region of space.
• In this article, we study entanglement in subsystems made of finitely many field degrees of freedom, in a free scalar theory in $D+1$-dimensional Minkowski spacetime.