Existence of universal resource and uselessness of too entangled states for quantum metrology

We show (i) the existence of universal resource states for a certain class of linear Hamiltonians and (ii) the uselessness of highly entangled states for quantum metrology of linear Hamiltonians. We also show that random pure states are basically not useful even if we consider more general Hamiltonians. Since random pure states have high entanglement, this result strengthens the uselessness of highly entangled states for quantum metrology.