Voltage Control of Exchange Coupling in Phosphorus Doped Silicon
  Motivated by applications to quantum computer architectures we study the change in the exchange interaction between neighbouring phosphorus donor electrons in silicon due to the application of voltage biases to surface control electrodes. These voltage biases create electro-static fields within the crystal substrate, perturbing the states of the donor electrons and thus altering the strength of the exchange interaction between them. We find that control gates of this kind can be used to either enhance, or reduce the strength of the interaction, by an amount that depends both on the magnitude and orientation of the donor separation. 
C.J. Wellard, L.C.L Hollenberg, L.M. Kettle, H.-S.Goan