  We have investigated few-body states in vertically stacked quantum dots. Due
to small inter-dot tunneling rate, the coupling in our system is in a
previously unexplored regime where electron-hole exchange is the dominant spin
interaction. By tuning the gate bias, we are able to turn this coupling off and
study a complementary regime where total electron spin is a good quantum
number. The use of differential transmission allows us to obtain unambiguous
signatures of the interplay between electron and hole spin interactions. Small
tunnel coupling also enables us to demonstrate all-optical charge sensing,
where conditional exciton energy shift in one dot identifies the charging state
of the coupled partner.
