Extended Quantum XOR Gate in Terms of Two-Spin Interactions
  Considerations of feasibility of quantum computing lead to the study of multispin quantum gates in which the input and output two-state systems (spins) are not identical. We provide a general discussion of this approach and then propose an explicit 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