  We show that the non-parabolic confinement potential is responsible for the
non-monotonic behavior and sign change of the exciton fine-structure splitting
(FSS) in optically active self-assembled quantum dots. This insight is
important for the theoretical understanding and practical control by electric
fields of the quantum state of the emitted light from a biexciton cascade
recombination process. We find that a hard-wall (box) confinement potential
leads to a FSS that is in better agreement with experimentally measured FSS
than a harmonic potential. We then show that a finite applied electric field
can be used to remove the FSS entirely, thus allowing for the creation of
maximally entangled photons, being vital to the growing field of quantum
communication and quantum key distribution.
