Experimental ancilla-assisted phase estimation in a noisy channel

The metrological ability of carefully designed probes can be wrecked by the presence of noise which alters their evolution, reducing the information on the parameter of interest. This spoils 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 with respect to a standard single-qubit strategy. The advantage originates from the coherent coupling of the probe and the ancilla both at the initialization and measurement stages, generating and selecting a subset of overall configurations less affected by the noise.