Indirect spin coupling between semiconductor quantum dots

Full Text:We have investigated theoretically an indirect exchange interaction between spins localized in two semiconductor quantum dots. The interaction is mediated by virtual delocalized carrier excitations in the host material and is analogous to the RKKY interaction between two magnetic impurities. The virtual excitations are driven by an interband off-resonance laser, thus making the interaction optically controllable. This exchange mechanism can be utilized for fast coupling between spins in quantum dots for gates in connection with quantum computation. The proposed scheme has both advantages of ultrafast optical control and very long spin coherence times which are maintained due to the virtual nature of the excitations. The effective coupling between the two localized spins is obtained by calculating the exchange interaction between the localized and continuum electrons. We take into account the effects of hybridization of continuum and dot states and double occupancy in the dot. Applying a generalized canonical transformation to the Hamiltonian up to infinite order, we derive contributions to the exchange interaction arising from correlation and hybridization terms, which are added to the conventional Coulomb potential exchange. The overall effect is modified by the Coulomb interactions between the intermediate virtual electrons and holes. We obtain the dependence of the coupling between two spins in quantum dots on the geometry of the quantum dots and the dimensionality of the system. Among the studied geometries, the largest couplings are found for lateral dots in a two-dimensional quantum well, and vertically stacked quantum dots in a quasi one-dimensional wire