Entangled photons can be generated "on demand" in a novel scheme involving unitary time reordering of the photons emitted in a radiative decay cascade. The scheme yields polarization entangled photon pairs, even though prior to reordering the emitted photons carry significant "which path information" and their polarizations are unentangled. This shows that quantum chronology can be manipulated in a way that is lossless and deterministic (unitary). The theory can, in principle, be tested and applied to the biexciton cascade in semiconductor quantum dots.
@article{arxiv.0710.2308,
title = {Entanglement on demand through time reordering},
author = {J. E. Avron and G. Bisker and D. Gershoni and N. H. Lindner and E. A. Meirom and R. J. Warburton},
journal= {arXiv preprint arXiv:0710.2308},
year = {2009}
}