  A scheme for the extraction of entanglement in two noninteracting qubits
(spins) is proposed. The idea is to make use of resonant transmission of
ancilla qubit through the two fixed qubits, controlled by the entanglement in
the scatterers. Repetition of the resonant transmission extracts the singlet
state in the target qubits from their arbitrary given state. Neither the
preparation nor the post-selection of the ancilla spin is required, in contrast
to the previously proposed schemes.
