Formation and control of a correlated exciton two-qubit system in a coupled quantum dot

We have proposed the basic device structures for an all-optical controlled two-qubit quantum logic gate using two excitons confined in InAs/GaAs coupled quantum dots (CQDs). A two-qubit quantum logic gate is constructed using a correlated two-exciton system. This simplest two-qubit system involves four states such as the crystal ground states, two distinguishable exciton states, and a correlated-exciton-molecule state consisting of two excitons. The formation of this correlated-exciton-molecule state, which consists of two different excitons in a CQD, is the most important and difficult technology. In this study, we demonstrate the formation of the four states involving this correlated-exciton-molecule state and confirm two-qubit exciton system in CQDs. Moreover, we show that a correlated exciton molecule is created by employing a cascade process.