Pairwise concurrence dynamics:: a four-qubit model

We examine entanglement dynamics via concurrence among four two-state systems labelled A, a, B, b. The four systems are arranged on an addressable `lattice' in such a way that A and a at one location labelled Aa can interact with each other via excitation exchange, and the same for B and b at location Bb. The Aa location is prepared entangled with the Bb location, but their mutual complete isolation prevents interaction in the interval between actions of an external addressing agent. There are six pairwise concurrences on the lattice, and we follow their evolution in the interval between external actions. We show how entanglement evolves and may exhibit the non-analytic effect termed entanglement sudden death (ESD), with periodic recovery. These loss and gain processes may be interpreted as entanglement transfers between the subsystems.