  Quantum metrology exploits quantum mechanical laws to improve the precision
in estimating technologically relevant parameters such as phase, frequency, or
magnetic fields. Probe states are usually tailored on the particular dynamics
whose parameters are being estimated. Here we consider a novel framework where
quantum estimation is performed in an interferometric configuration, using
bipartite probe states prepared when only the spectrum of the generating
Hamiltonian is known. We introduce a figure of merit for the scheme, given by
the worst case precision over all suitable Hamiltonians, and prove that it
amounts exactly to a computable measure of discord-type quantum correlations
for the input probe. We complement our theoretical results with a metrology
experiment, realized in a highly controllable room-temperature nuclear magnetic
resonance setup, which provides a proof-of-concept demonstration for the
usefulness of discord in sensing applications. Discordant probes are shown to
guarantee a nonzero precision in the estimation procedure for different
generating Hamiltonians, while classically correlated probes are unable to
accomplish the estimation in a worst case setting. This work establishes a
rigorous and direct operational interpretation for general quantum
correlations, shedding light on their potential for quantum technology.
