  We report an experimental investigation of the role of measurement in quantum
metrology when the states of the probes are mixed. In particular, we
investigated optimized local measurements and general global projective
measurements, involving entangling operations, on noisy Werner states of
polarization entangled photons. We demonstrate experimentally that global
measurement presents an advantage in parameter estimation with respect to the
optimized local strategy. Moreover, the global strategy provides unambiguous
information about the parameter of interest even when the amount of noise is
not well characterized. This shows that the coherence in quantum operations,
such as the Bell-state projection device used in our protocol, can be used to
further boost the quantum advantage in metrology and play a fundamental role in
the design of future quantum measurement devices.
