Entanglement-enhanced probing of a delicate material system

Probing of delicate systems demands high sensitivity from limited probe energy. Quantum optical techniques to beat the standard quantum limit (SQL) offer a practical advantage for these measurements. Here we report the first entanglement-enhanced measurement of a delicate material system [1]. We non-destructively probe an atomic spin ensemble by near-resonant Faraday rotation, a measurement that is limited by probe-induced scattering in quantum memory and spin-squeezing applications [2]. We use narrowband, atom-resonant NooN states to beat the standard quantum limit of sensitivity, both on a per-photon and a per-damage basis.