  Quantum metrology uses entanglement and other quantum effects to improve the
sensitivity of demanding measurements. Probing of delicate systems demands high
sensitivity from limited probe energy and has motivated the field's key
benchmark-the standard quantum limit. Here we report the first
entanglement-enhanced measurement of a delicate material system. We
non-destructively probe an atomic spin ensemble by means of near-resonant
Faraday rotation, a measurement that is limited by probe-induced scattering in
quantum-memory and spin-squeezing applications. We use narrowband,
atom-resonant NOON states to beat the standard quantum limit of sensitivity by
more than five standard deviations, both on a per-photon and per-damage basis.
This demonstrates quantum enhancement with fully realistic loss and noise,
including variable-loss effects. The experiment opens the way to ultra-gentle
probing of single atoms, single molecules, quantum gases and living cells.
