  We consider bipartite entangled states that cannot outperform separable
states in any linear interferometer. Then, we show that these states can still
be more useful metrologically than separable states if several copies of the
state are provided or an ancilla is added to the quantum system. We present a
general method to find the local Hamiltonian for which a given quantum state
performs the best compared to separable states. We obtain analytically the
optimal Hamiltonian for some quantum states with a high symmetry. We show that
all bipartite entangled pure states outperform separable states in metrology.
Some potential applications of the results are also suggested.
