  Correlation between the rectilinear polarisations of the photons emitted from
the biexciton decay in a single quantum dot is investigated in a device which
allows the charge-state of the dot to be controlled. Optimising emission from
the neutral exciton states maximises the operating efficiency of the biexciton
decay. This is important for single dot applications such as a triggered source
of entangled photons. As the bias on the device is reduced correlation between
the two photons is found to fall dramatically as emission from the negatively
charged exciton becomes significant. Lifetime measurements demonstrate that
electronic spin-scattering is the likely cause.
