  The metrological ability of carefully designed probes can be spoilt by the
presence of noisy processes occurring during their evolution. The noise is
responsible for altering the evolution of the probes in such a way that bear
little or no information on the parameter of interest, hence spoiling the
signal-to-noise ratio of any possible measurement. Here we show an experiment
in which the introduction of an ancilla improves the estimation of an optical
phase in the high-noise regime. The advantage is realised by the coherent
coupling of the probe and the ancilla at the initialisation and the measurement
state, which generate and then select a subset of overall configurations less
affected by the noise process.
