Title: Entanglement Entropy in Excited States of the Quantum Lifshitz Model,
Abstract: We investigate the entanglement properties of an infinite class of excited
states in the quantum Lifshitz model (QLM). The presence of a conformal quantum
critical point in the QLM makes it unusually tractable for a model above one
spatial dimension, enabling the ground state entanglement entropy for an
arbitrary domain to be expressed in terms of geometrical and topological
quantities. Here we extend this result to excited states and find that the
entanglement can be naturally written in terms of quantities which we dub
"entanglement propagator amplitudes" (EPAs). EPAs are geometrical probabilities
that we explicitly calculate and interpret. A comparison of lattice and
continuum results demonstrates that EPAs are universal. This work shows that
the QLM is an example of a 2+1d field theory where the universal behavior of
excited-state entanglement may be computed analytically.