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 `activeʹ spins that evolve under chemical shift or scalar coupling operators and a larger subset of `spectatorʹ spins which are returned to their initial states, as if their motion had been temporarily frozen. This allows us to implement basic one- 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.