Entanglement branes, modular flow, and extended topological quantum field theory

Entanglement entropy is an important quantity in field theory, but its definition poses some challenges. The naive definition involves an extension of quantum field theory in which one assigns Hilbert spaces to spatial sub-regions. For two-dimensional topological quantum field theory we show that the appropriate extension is the open-closed topological quantum field theory of Moore and Segal. With the addition of one additional axiom characterizing the `entanglement brane' we show how entanglement calculations can be cast in this framework. We use this formalism to calculate modular Hamiltonians, entanglement entropy and negativity in two-dimensional Yang-Mills theory and relate these to singularities in the modular flow. As a byproduct we find that the negativity distinguishes between the `log dim R' edge term and the `Shannon' edge term. We comment on the possible application to understanding the Bekenstein-Hawking entropy in two-dimensional gravity.