  We describe an interaction mechanism between electron spins in a
vertically-stacked double quantum dot that can be used for controlled two-qubit
operations. This interaction is mediated by excitons confined within, and
delocalized over, the double dot. We show that gates equivalent to the
SQRT-SWAP gate can be obtained in times much less than the exciton relaxation
time and that the negative effects of hole-mixing and spontaneous emission do
not seriously affect these results.
