We address the problem of measuring the relative angle between two ``quantum axes'' made out of N₁ and N₂ spins. Closed forms of our fidelitylike figure of merit are obtained for an arbitrary number of parallel spins. The asymptotic regimes of large N₁ and/or N₂ are discussed in detail. The extension of the concept ``quantum axis'' to more general situations is addressed. We give optimal strategies when the first quantum axis is made out of parallel spins whereas the second is a general state made out of two spins.
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Bagán et al. (2006) studied this question.
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