Extraction of an entanglement by repetition of the resonant transmission of an ancilla qubit

A scheme for the extraction of entanglement in two noninteracting qubits (spins) is proposed. The idea is to make use of the resonant transmission of an 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.