The demonstration of degeneracy of the exciton spin states is an important
step towards the production of entangled photons pairs from the biexciton
cascade. We measure the fine structure of exciton and biexciton states for a
large number of single InAs quantum dots in a GaAs matrix; the energetic
splitting of the horizontally and vertically polarised components of the
exciton doublet is shown to decrease as the exciton confinement decreases,
crucially passing through zero and changing sign. Thermal annealing is shown to
reduce the exciton confinement, thereby increasing the number of dots with
splitting close to zero.Comment: 12 pages, 4 figure