Delayed - Choice Entanglement - Swapping with Vacuum-One Photon Quantum   States
  We report the experimental realization of a recently discovered quantum information protocol by Asher Peres implying an apparent non-local quantum mechanical retrodiction effect. The demonstration is carried out by applying a novel quantum optical method by which each singlet entangled state is physically implemented by a two-dimensional subspace of Fock states of a mode of the electromagnetic field, specifically the space spanned by the vacuum and the one photon state, along lines suggested recently by E. Knill et al., Nature 409, 46 (2001) and by M. Duan et al., Nature 414, 413 (2001). The successful implementation of the new technique is expected to play an important role in modern quantum information and communication and in EPR quantum non-locality studies. 
Fabio Sciarrino, Egilberto Lombardi, and Francesco De Martini