  We propose a new approach to the measurement of a single spin state, based on
nuclear magnetic resonance (NMR) techniques and inspired by the coherent
control over many-body systems envisaged by Quantum Information Processing
(QIP). A single target spin is coupled via the natural magnetic dipolar
interaction to a large ensemble of spins. Applying external radio frequency
(rf) pulses, we can control the evolution of the system so that the spin
ensemble reaches one of two orthogonal states whose collective properties
differ depending on the state of the target spin and are easily measured. We
first describe this measurement process using QIP gates; then we show how
equivalent schemes can be defined in terms of the Hamiltonian of the spin
system and thus implemented under conditions of real control, using well
established NMR techniques. We demonstrate this method with a proof of
principle experiment in ensemble liquid state NMR and simulations for small
spin systems.
