  The neutral biexciton cascade of single quantum dots is a promising source of
entangled photon pairs. The character of the entangled state is determined by
the energy difference between the excitonic eigenstates known as fine-structure
splitting (FSS). Here we reduce the magnitude of the FSS by simultaneously
using two independent tuning mechanisms: in-plane magnetic field and vertical
electric field. We observe that there exists a minimum possible FSS in each
quantum dot which is independent of these tuning mechanisms. However, with
simultaneous application of electric and magnetic fields, we show the FSS can
be reduced to its minimum value as the energy of emission is tuned over several
meV with a 5-T magnet.
