“…Due to the strong Coulomb coupling and absence of the external electric fields we may restrict the discussion to 'spatially direct' excitonic states, i. e., states with electron-hole pairs residing in the same QD which in these conditions have much lower energy than the 'dissociated' states referring to excitons formed of carriers residing in two different dots. 56,57 These assumptions allow us to treat every QD as a two-level system which may be either empty or contain an exciton and thus describe the set of N QDs as an 2 N -level system, with |0 denoting the ground (or "vacuum") state in which all N dots are empty, single-exciton states |i , corresponding to one exciton localized in the ith QD, biexcitonic states |ij referring to electron-hole pairs residing in the ith and jth QDs, states |ijk with three QDs, ith, jth, and kth (1 i, j, k N ), occupied with an exciton etc.…”