Certification of genuine multipartite entanglement in spin ensembles with measurements of total angular momentum

We introduce entanglement witnesses for spin ensembles which detect genuine multipartite entanglement using only measurements of the total angular momentum. States that are missed by most other angular-momentum-based witnesses for spin ensembles, which include Greenberger-Horne-Zeilinger states and certain superpositions of Dicke states, can be effectively detected by our witness. The protocol involves estimating the probability that the total angular momentum is positive along equally-spaced directions on a plane. Alternatively, one could measure along a single direction at different times, under the assumption that the total spins undergoes a uniform precession. Genuine multipartite entanglement is detected when the observed score exceeds a separable bound. Exact analytical expressions for the separable bound are obtained for spin ensembles $j_1\otimes j_2\otimes\dots \otimes j_N$ such that the total spin is a half-integer, and numerical results are reported for the other cases. Finally, we conjecture an expression for the separable bound when the total spin is not known, which is well supported by the numerical results.Comment: 24 pages, 11 figure