Analogy between optimal spin estimation and interferometry

A scheme for optimal spin state estimation is considered in analogy with phase detection in interferometry. Recently reported coherent measurements yielding the average fidelity (N + 1)/(N + 2) for N particle system correspond to the standard limit of phase resolution 1/root N. It provides the bound for incoherent measurements when each particle is detected separately and information is used optimally. For specific states, improvement up to the value 1/N is possible in quantum theory. The best results are obtained combining sequentially coherent measurements on fractional groups of particles. (C) 2000 Elsevier Science B.V. All rights resented.