Quantum control of spins and excitons in semiconductor quantum dots

摘要

Recent advances in semiconductor nano-optics opened the possibility of quantum manipulation of single excitons confined in quantum dots. We have shown explicitly how these quantum manipulations can be put together to solve a quantum algorithm. The perspective of using excitons as qubits is well within current experimental capabilities, but suffers an intrinsic limitation due to the short radiative lifetime of these objects. We have therefore analysed in sect. 3 the possibility of using excitons to mediate an interaction between charged quantum dots. The optical RKKY mechanism represents a powerful and clean tool for the realization of scalable systems, In this scheme the excitons enter as intermediate states and their short lifetime is not a limitation anymore. A simple experimental setup with two dots of different size and a tunable laser could be used to check the spin entanglement induced by the interaction proposed here. One of the limitations of the exchange between dots mediated by delocalized states is that it is not dot-selective. In the presence of an array of dots, delocalized states create an exchange interaction between all neighboring pair of dots in the system. Different dot sizes can give different effective couplings but all the interactions are switched on at the same time. One possible architecture to overcome this problem for the realization of a scalable system involves the use of near-field optical excitations. The dots in fact can be arranged in a chain separated by distances of the order of the wavelength of light. Then, quasi-delocalized states with sub-wavelength extension can be excited only between the two dots intended to be coupled. This possibility is well within the experimental state-of-the-art capabilities of near-field scanning optical microscopy. In such a system, universal quantum computation can be in principle realized only with the optically controlled exchange interaction, without recurring to single-qubit operation.