Delayed-choice entanglement swapping with vacuum–one-photon quantum states

We report the experimental realization of a recently discovered quantum-information protocol by Peres implying an apparent nonlocal quantum mechanical retrodiction effect. The demonstration is carried out by a 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 (London) 409, 46 (2001)] and by M. Duan et al. [ibid. 414, 413 (2001)].