  We introduce measures of quantum coherence as the speed of evolution of a
system under decoherence. That is, coherence is the ability to estimate a
dephasing channel, quantified by the quantum Fisher information. We extend the
analysis to interferometric noise estimation, proving that quantum discord is
the minimum sensitivity to local dephasing. A physically motivated set of free
operations for discord is proposed. The amount of discord created by strictly
incoherent operations is upper-bounded by the initial coherence.
