Theoretical proposal from 2011 now realized and covered in New Scientist
In a theoretical proposal from 2011, our group had shown that it is possible to entangle two photons, which do not exist at the same time (R. Wiegner et al., Optics Letters 36, 1512 (2011)). This can be demonstrated for example in a system of two atoms each emitting a single photon which are detected in the far field by two detectors such that the source of the photons is unknown. By registering the two photons at different locations (in this way sampling the spatial photon-photon correlation function) and post-selecting only on those events where the condition of non-existence of the two photons at the same time is fulfilled, it is possible to violate a Bell’s inequality. The violation of this inequality proves the entanglement of the two photons (see R. Wiegner et al., Optics Letters 36, 1512 (2011)). The theoretical proposal has now been implemented in a slightly different scheme by a group around Hagai Eisenberg (see E. Megidish et al., ArXiv: 1209.4191). Moreover, both works have been covered in an article by the journal New Scientist (see New Scientist, October 6, 2012, page 11).