Mutual information in interacting quantum field theories away from equilibrium

Momentum modes of an interacting quantum field theory (QFT) are generically entangled. On the other hand, the notion of decoupling in effective field theory, whereby degrees of freedom well beyond relevant energies do not play a significant role in the system’s behavior, suggests that well-separated modes must not affect each other’s evolution. It has been shown in previous work that the mutual information between two momentum modes of an interacting scalar QFT in Minkowski spacetime in its ground state indeed decays with mode separation, consistent with decoupling. I will first discuss the advantages of using mutual information over entanglement entropy or general Renyi entropies as a measure of correlations between degrees of freedom with a shared environment. I will next discuss the calculation of mutual information away from equilibrium and present results on time-dependent mutual information for different interacting QFTs initialized in the ground state of the free theory at an arbitrary initial time. I will show that mutual information in this out-of-equilibrium case also decays with mode separation and limits to the equilibrium result at late times. Lastly, I will generalize the results to interacting QFTs in de Sitter spacetime and discuss how background dynamics and the presence of a physical horizon affect decoupling.