Correlation between the rectilinear polarizations 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. Optimizing emission from the neutral exciton states maximizes 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. (c) 2007 American Institute of Physics.