  Two trapped ions that are kilometers apart can be entangled by the joint
detection of two photons, each coming from one of the ions, in a basis of
entangled states. Such a detection is possible with linear optical elements.
The use of two-photon interference allows entanglement distribution without
interferometric sensitivity to the path length of the photons. The present
method of creating entangled ions also opens up the possibility of a
loophole-free test of Bell's inequalities.
