Conditional operation of a spin qubit
  We report coherent operation of a singlet-triplet qubit controlled by the arrangement of two electrons in an adjacent double quantum dot. The system we investigate consists of two pairs of capacitively coupled double quantum dots fabricated by electrostatic gates on the surface of a GaAs heterostructure. We extract the strength of the capacitive coupling between qubit and double quantum dot and show that the present geometry allows fast conditional gate operation, opening pathways to multi-qubit control and implementation of quantum algorithms with spin qubits. 
I. van Weperen, B. D. Armstrong, E. A. Laird, J. Medford, C. M.   Marcus, M. P. Hanson, and A. C. Gossard
cond-mat.mes-hall quant-ph