  We study the exact evolution of two non-interacting qubits, initially in a
Bell state, in the presence of an environment, modeled by a kicked Ising spin
chain. Dynamics of this model range from integrable to chaotic and we can
handle numerics for a large number of qubits. We find that the entanglement (as
measured by concurrence) of the two qubits has a close relation to the purity
of the pair, and follows an analytic relation derived for Werner states. As a
collateral result we find that an integrable environment causes quadratic decay
of concurrence, while a chaotic environment causes linear decay.
