Measurement-induced generation of a macroscopic spin singlet

We report the production of a macroscopic spin singlet (MSS) in an atomic system [1] using collective quantum nondemolition (QND) measurement as a global entanglement generator [2]. Using an unpolarized ensemble of up to $10^6$ cold atomic spins, we observe 3 dB of spin squeezing and detect entanglement with $5\sigma$ statistical significance using a generalized spin- squeezing inequality [3], indicating that at least half the atoms in the sample have formed singlets. [1] N. Behbood {\it et al.}, ``Generation of Macroscopic Singlet States in a Cold Atomic Ensemble,'' {\bf Phys. Rev. Lett.} 113, 093601 (2014). [2] R. J. Sewell {\it et al.}, ``Certified quantum non-demolition measurement of a macroscopic material system,'' {\bf Nat. Photon.} 7, 517 (2013). [3] G. T\'{o}th and M.W. Mitchell, ``Generation of macroscopic singlet states in atomic ensembles,'' {\bf New J. Phys.} 12, 053007 (2010).