Multiparticle entanglement and its experimental detection

We discuss several aspects of multiparticle mixed-state entanglement and it s experimental detection. First we consider entanglement between two partic les which is robust against disposals of other particles. To completely det ect these kinds of entanglement, full knowledge of the multiparticle densit y matrix (or of all reduced density matrices) is required. Then we review t he relation of the separability properties of I-partite splittings of a sta te rho to its multipartite entanglement properties. We show that it suffice s to determine the diagonal matrix elements of rho in a certain basis in or der to detect multiparticle entanglement properties of rho. We apply these observations to analyse two recent experiments, where multiparticle entangl ed states of 3 (4) particles were produced. Finally, we focus on bound enta ngled states (non-separable, non-distillable states) and show that they can be activated by joint actions of the parties. We also provide several exam ples which show the activation of bound entanglement with bound entanglemen t.