Design of gates for quantum computation: the three-spin XOR gate in   terms of two-spin interactions
  We propose to design multispin quantum gates in which the input and output two-state systems (spins) are not necessarily identical. We describe the motivations for such studies and then derive an explicit general two-spin interaction Hamiltonian which accomplishes the quantum XOR gate function for a system of three spins: two input and one output. 
Dima Mozyrsky, Vladimir Privman, Steven P. Hotaling