  It is commonly believed that photon polarisation entanglement can only be
obtained via pair creation within the same source or via postselective
measurements on photons that overlapped within their coherence time inside a
linear optics setup. In contrast to this, we show here that polarisation
entanglement can also be produced by distant single photon sources in free
space and without the photons ever having to meet, if the detection of a photon
does not reveal its origin -- the which way information. In the case of two
sources, the entanglement arises under the condition of two emissions in
certain spatial directions and leaves the dipoles in a maximally entangled
state.

Quantum Physics