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Impurity effects on semiconductor quantum bits in coupled quantum dots

We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum dot systems, where each dot contains one or two electrons and a single charged impurity in the presence of an external magnetic field. Using molecular orbital and configuration interaction method, we calculate the effect of the impurity on the 2-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot 2-electron system. We find that the singlet-triplet exchange splitting between the two lowest energy states, both for the individual dots and the coupled dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). A strong electron-impurity coupling breaks down equality of the two doubly-occupied singlets in the left and the right dot leading to a mixing between different spin singlets. As a result, the maximally entangled qubit states are no longer fully obtained in zero magnetic field case. Moreover, a repulsive impurity results in a triplet-singlet transition as the impurity effective charge increases or/and the impurity position changes. We comment on the impurity effect in spin qubit operations in the double dot system based on our numerical results.