Fragile states are better for quantum metrology

In quantum metrology, quantum probe states are capable of estimating unknown physical parameters to precisions beyond classical limits. What qualities do such states possess? Here we relate the performance of a probe state at estimating a parameter $\phi$ -- as quantified by the quantum Fisher information -- to the amount of purity it loses when $\phi$ undergoes statistical fluctuation. This indicates that the better a state is for estimating $\phi$, the more it decoheres when $\phi$ is subject to noise.