  Nonclassical states are essential for optics-based quantum information
processing, but their fragility limits their utility for practical scenarios in
which loss and noise inevitably degrade, if not destroy, nonclassicality.
Exploiting nonclassical states in quantum metrology yields sensitivity
advantages over all classical schemes delivering the same energy per
measurement interval to the sample being probed. These enhancements, almost
without exception, are severely diminished by quantum decoherence. Here, we
experimentally demonstrate an entanglement-enhanced sensing system that is
resilient to quantum decoherence. We employ entanglement to realize a 20%
signal-to-noise ratio improvement over the optimum classical scheme in an
entanglement-breaking environment plagued by 14 dB of loss and a noise
background 75 dB stronger than the returned probe light. Our result suggests
that advantageous quantum-sensing technology could be developed for practical
situations.
