Inversion of the exciton fine structure splitting in quantum dots

The fine structure splitting of exciton state was measured for a large number of single InAs quantum dots in GaAs. It is shown to decrease as the exciton confinement decreases, crucially passing through zero and changing sign. Degeneracy of the exciton spin states is an important step to producing entangled photons from the biexciton cascade. Thermal annealing reduces the exciton confinement and thereby increases the number of degenerate dots in a particular sample.