  The concept of entanglement in systems where the particles are
indistinguishable has been the subject of much recent interest and controversy.
In this paper we study the notion of entanglement of particles introduced by
Wiseman and Vaccaro [Phys. Rev. Lett. 91, 097902 (2003)] in several specific
physical systems, including some that occur in condensed matter physics. The
entanglement of particles is relevant when the identical particles are
itinerant and so not distinguished by their position as in spin models. We show
that entanglement of particles can behave differently to other approaches that
have been used previously, such as entanglement of modes (occupation-number
entanglement) and the entanglement in the two-spin reduced density matrix. We
argue that the entanglement of particles is what could actually be measured in
most experimental scenarios and thus its physical significance is clear. This
suggests entanglement of particles may be useful in connecting theoretical and
experimental studies of entanglement in condensed matter systems.
