Abstract
Quantum metrology1 uses entanglement2,3,4,5 and other quantum effects6 to improve the sensitivity of demanding measurements7,8,9. Probing of delicate systems demands high sensitivity from limited probe energy and has motivated the field's key benchmark—the standard quantum limit10. 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 applications6,11,12,13. 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 effects14,15,16. The experiment opens the way to ultra-gentle probing of single atoms17, single molecules18, quantum gases19 and living cells20.
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