The reduced density matrix of a spatial subsystem can be written as the exponential of the entanglement (modular) Hamiltonian, hence this operator contains a lot of information about the entanglement of the corresponding spatial bipartition. For some particular models, states and bipartitions, this operator is local, but in general it is expected to be non-local. We discuss the entanglement Hamiltonians of an interval for three different free quantum field theories in one spatial dimension, in their ground state: the massless Dirac field either on the half line or on the strip with an inhomogeneous background (the rainbow model), and the massive scalar field on the half line with Robin boundary conditions.
