  We investigate the quantum entanglement content of quasi-particle excitations
in extended many-body systems. We show that such excitations give an additive
contribution to the bi-partite von Neumann and R\'enyi entanglement entropies
that takes a simple, universal form. It is largely independent of the momenta
and masses of the excitations, and of the geometry, dimension and connectedness
of the entanglement region. The result has a natural quantum information
theoretic interpretation as the entanglement of a state where each
quasi-particle is associated with two qubits representing their presence within
and without the entanglement region, taking into account quantum
(in)distinguishability. This applies to any excited state composed of finite
numbers of quasi-particles with finite De Broglie wavelengths or finite
intrinsic correlation length. We derive this result analytically in
one-dimensional massive bosonic and fermionic free field theories and for
simple setups in higher dimensions. We provide numerical evidence for the
harmonic chain and the two-dimensional harmonic lattice in all regimes where
excitations have quasi-particle properties. Finally, we provide supporting
calculations for integrable spin chain models and other situations without
particle production. Our results point to new possibilities for creating
entangled states using many-body quantum systems.
