Pulse Sequences for NMR Quantum Computers: How to Manipulate Nuclear   Spins While Freezing the Motion of Coupled Neighbours
  We show how to divide a coupled multi-spin system into a small subset of ``active'' spins that evolve under chemical shift or scalar coupling operators, and a larger subset of ``spectator'' spins which are returned to their initial states, as if their motion had been temporarily frozen. This allows us to implement basic one-qubit and two-qubit operations from which general operations on $N$-qubits can be constructed, suitable for quantum computation. The principles are illustrated by experiments on the three coupled protons of 2,3-dibromopropanoic acid, but the method is applicable to any spin-1/2 nuclei and to systems containing arbitrary numbers of coupled spins. 
N Linden, H Barjat, RJ Carbajo and R Freeman