  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 corresponds to the
standard limit of phase resolution 1/\sqrt{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.
