Entanglement swapping: in the beginning, there are two entangled links B–E and C–F. Here BS stands for a loss-less 50:50 beam splitter. Detectors 1 and 2 are photon detectors. Modes f and e stand for output modes of the BS. Alice randomly measures mode f or e and then different output states will be achieved. So entanglement between B and C is obtained after the measurement between E and F.