  We show how to measure the structural witnesses proposed in [P. Krammer et
al., Phys. Rev. Lett. 103, 100502 (2009)] for detecting entanglement in a spin
chain using photon scattering. The procedure, moreover, allows one to measure
the two-point correlation function of the spin array. This proposal could be
performed in existing experimental platforms realizing ion chains in Paul traps
or atomic arrays in optical lattices.
