  We report the generation of polarization-entangled photons, using a quantum
dot single photon source, linear optics and photodetectors. Two photons created
independently are observed to violate Bell's inequality. The density matrix
describing the polarization state of the postselected photon pairs is also
reconstructed, and agrees well with a simple model predicting the quality of
entanglement from the known parameters of the single photon source. Our scheme
provides a method to generate no more than one entangled photon pair per cycle,
a feature useful to enhance quantum cryptography protocols using entangled
photons.
