  Measuring small separations between two optical sources, either in space or
in time, constitute an important metrological challenge as standard
intensity-only measurements fail for vanishing separations. Contrarily, it has
been established that appropriate coherent mode projections can appraise
arbitrarily small separations with quantum-limited precision. However, the
question of whether the optical coherence brings any metrological advantage to
mode projections is still a point of debate. Here, we elucidate this problem by
experimentally investigating the effect of varying coherence on estimating the
temporal separation between two single-photon pulses. We show that, for an
accurate interpretation, special attention must be paid to properly normalize
the quantum Fisher information to account for the strength of the signal. Our
experiment demonstrates that coherent mode projections are optimal for any
degree of coherence.
