Generation of long-range entanglement in a macroscopic spin singlet

We report the generation of long-range entanglement in a macroscopic spin singlet (MSS) [1,2] via collective quantum non-demolition (QND) measurement [3] a global entanglement method predicted [4] to produce entanglement at all length scales. In a cold 87Rb spin ensemble of up to 2 × 106 atoms, we generate a MSS, entangling at least half of the atoms. Using a gradient field to convert singlets to triplets, we detect the decay of entanglement in the MSS via spin noise spectroscopy [4] consistent with a mean entanglement length comparable to the size of the atom cloud (~ 4mm), three orders of magnitude larger than previously detected in atomic spin systems [5].