  The lack of structural symmetry which usually characterizes semiconductor
quantum dots lifts the energetic degeneracy of the bright excitonic states and
hampers severely their use as high fidelity sources of entangled photons. We
demonstrate experimentally and theoretically that it is always possible to
restore the excitonic degeneracy by the simultaneous application of large
strain and electric fields, despite the fact that this possibility has
fundamentally been doubted. This is achieved by using one external perturbation
to align the polarization of the exciton emission along the axis of the second
perturbation, which then erases completely the energy splitting of the states.
This result, which holds for any QD structure, highlights the potential of
combining complementary external fields to create artificial atoms meeting the
stringent requirements posed by scalable semiconductor-based
quantum-technology.
