Multipartite entanglement that is robust against disposal of particles

We investigate the entanglement properties of multiparticle systems, concentrating on the case where the entanglement is robust against the disposal of particles. Two qubits—belonging to a multipartite system—are entangled in this sense if their reduced density matrix is entangled. We introduce a family of multiqubit states, for which one can choose for any pair of qubits independently whether they should be entangled or not as well as the relative strength of the entanglement, thus providing the possibility to construct all kinds of “entanglement molecules.” For some particular configurations, we also give the maximal amount of entanglement achievable.