  We study the evolution of entanglement after a global quench in a
one-dimensional quantum system with a localized impurity. For systems described
by a conformal field theory, the entanglement entropy between the two regions
separated by the defect grows linearly in time. Introducing the notion of
boundary twist fields, we show how the slope of this growth can be related to
the effective central charge that emerges in the study of ground-state entropy
in the presence of the defect. On the other hand, we also consider a particular
lattice realization of the quench in a free-fermion chain with a conformal
defect. Starting from a gapped initial state, we obtain the slope via a
quasiparticle ansatz and observe small discrepancies between the field theory
and lattice results, which persist even in the limit of a vanishing gap.
