  We show that the lifetime-reduced fidelity of a semiconductor quantum
dot-cavity single photon nonlinearity can be restored by polarization pre- and
postselection. This is realized with a polarization degenerate microcavity in
the weak coupling regime, where an output polarizer enables quantum
interference of the two orthogonally polarized transmission amplitudes. This
allows us to transform incident coherent light into a stream of strongly
correlated photons with a second-order correlation function of g2(0)~40, larger
than previous experimental results even in the strong-coupling regime. This
purification technique might also be useful to improve the fidelity of quantum
dot based logic gates.
