Internal entanglement amplification by external interactions

We propose a scheme to control the level of entanglement between two fixed spin-1/2 systems by interaction with a third particle. For specific designs, entanglement is shown to be pumped into the system from the surroundings even when the spin-spin interaction within the system is small or nonexistent. The effect of the external particle on the system is introduced by including a dynamic spinor in the Hamiltonian. Controlled amplification of the internal entanglement to its maximum value is demonstrated. The possibility of entangling noninteracting spins in a stationary state is also demonstrated by coupling each one of them to a flying qubit in a quantum wire.