Entanglement-enhanced probing of a delicate material system

Quantum metrology¹ uses entanglement^2,3,4,5 and other quantum effects⁶ to improve the sensitivity of demanding measurements^7,8,9. 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 applications^6,11,12,13. 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 effects^14,15,16. The experiment opens the way to ultra-gentle probing of single atoms¹⁷, single molecules¹⁸, quantum gases¹⁹ and living cells²⁰.