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Single epitaxially grown CdSe/ZnSe quantum dots have been studied by using photoluminescence spectroscopy with a high spatial resolution. The lifting of the spin degeneracy due to exchange interaction results in a splitting of the exciton ground state, strongly dependent on the symmetry of the quantum dot. By applying a magnetic field in Faraday geometry, the energy splitting as well as the polarization properties of the exciton transition can be varied and remarkably, even at high magnetic field a spin coherence time of about 3 ns is found, which exceeds the recombination lifetime of single excitons significantly. As the biexciton state is a spin singlet, both its fine structure splitting as well as its degree of polarization are shown to be controlled by the final state of recombination, the single exciton state. Besides the discrete energy splitting of optical transitons in single quantum dots, we observe a rather statistical, but strongly correlated energy shift was well as a correlated on-off switching behavior of the exciton and the biexciton emission on a typical time constant of seconds. These effects are related to the influence of charge carriers in the nanoenvironment of the dot and to thermal or Auger-driven carrier escape into trap states in the vicinity of the dot, respectively.
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